ATV RULES AND STANDARDS

WASTEWATER - WASTE

ATV STANDARD ATV-A 128E

Standards for the Dimensioning and Design of Stormwater Overflows in Combined Wastewater Sewers

April 1992

ISBN 3-934984-17-7

Marketing: Gesellschaft zur Förderung der Abwassertechnik e.V. (GFA) Theodor-Heuss-Allee 17 D-53773 Hennef Postfach 11 65 - 53758 Hennef

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The ATV ad hoc Working Group 1.9.1/1.9.3 in the ATV Specialist Committee 1.9 "Assessment and Treatment of Stormwater Discharge", who prepared this ATV Standard A 128, is made up from the following members: Dr.-Ing. Göttle, Kempten, (Chairman) Prof. Dr.-Ing. Brunner, Karlsruhe Dr.-Ing. Durchschlag, Bochum Dipl.-Ing. Freund, Wiesbaden Prof. Dr.-Ing. Geiger, Essen Dr.-Ing. Gniosdorsch, Frankfurt Dipl.-Ing. Jacobi, Darmstadt Dr.-Ing. Meißner, München Dipl.-Ing. Pawlowski, Berlin Dr.-Ing. Pecher, Erkrath Dipl.-Ing. Schitthelm, Düsseldorf Dr.-Ing. Schmitt, Kaiserslautern Dipl.-Ing. Sperling, Essen Dr.-Ing. Verworn, Hannover Dipl.-Ing. Willems, Essen Prof. Dr.-Ing. Wolf, Kassel

The Standard presented here has been prepared within the framework of the ATV committee work, taking into account the ATV Standard A 400 "Principles for the Preparation of Rules and Standards" in the Rules and Standards Wastewater/Wastes, in the January. 1994 .version. With regard to the application of the Rules and Standards, Para. 1 of Point 5 of A 400 includes the following statement "The Rules and Standards are freely available to everyone. An obligation to apply them can result for reasons of legal regulations, contracts or other legal grounds. Whosoever applies them is responsible for the correct application in specific cases. Through the application of the Rules and Standards no one avoids responsibility for his own actions. However, for the user, prima facie evidence shows that he has taken the necessary care.

All rights, in particular those of translation into other languages, are reserved. No part of this Standard may be reproduced in any form by photocopy, microfilm or any other process or transferred or translated into a language usable in machines, in particular data processing machines, without the written approval of the publisher. Gesellschaft zur Förderung der Abwassertechnik e.V. (GFA), Sankt Augustin 1992 German 0riginal produced by: Druck Carl Weyler KG., Bonn

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Contents 1 Scope of Application and Terms 7 2 Objective of Stormwater Treatment 7 2.1 Principles 7 2.2 Method of Approach 7 3 Requirements on Stormwater Treatment 8 3.1 Technical Requirements for the Normal Case (Normal Requirements) 8 3.2 Advanced Requirements 9 3.3 Overall Consideration of a Lake or River 9 3.3.1 Common Requirements in the Overall Drainage Area 9 3.3.2 Combined Requirements with Different Surface Waters 10 4 Planning Principles 10 4.1 Reduction of the Amount of Wastewater Produced 10 4.1.1 Rainwater Run-off 10 4.1.2 Domestic and Industrial Wastewater Flow 10 4.1.3 Sewer Infiltration Water Flow 11 4.2 Measures for Wastewater Treatment in the Combined Wastewater System 11 4.2.1 Intermediate Storage of Combined Wastewater 11 4.2.2 Redistribution of Rainfall Run-offs 11 4.2.3 Wastewater Treatment Measures 11 4.3 Structures with Overflow 12 4.3.1 Stormwater Overflows 12 4.3.2 Stormwater Tanks with Overflow 13 4.3.2.1 Stormwater Tanks Retaining the First Flush of Stormwater 14 4.3.2.2 Stormwater Tanks with Overflow for Settled Combined Wastewater 14 4.3.2.3 Composite Tanks 14 4.3.2.4 Sewers with Storage Capacity and Overflow 15 4.3.2.5 Main and By-pass Streams 16 4.3.3 Arrangement of Several Stormwater Tanks with Overflow 19 4.3.3.1 Parallel Stormwater Tanks with Overflow 19 4.3.3.2 Series Stormwater Tanks with Overflow 20 4.3.4 Stormwater Holding Tanks 20 5 Planning Scope 21 5.1 Determination of Actual Status 21 5.2 Determination of the Planning Status 21 5.3 Planning Periods 21 5.4 Standard and Variant Investigations 22 6 Calculation Principles 22 6.1 Sizes of Catchment Areas 22 6.1.1 Annual Precipitation hPr 22 6.1.2 Surface Areas ACA and Ais 22 6.1.3 Flow Time tf 23 6.1.4 Mean Terrain slope group SGm 23

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6.2 Discharges 23 6.2.1 Combined Wastewater Discharge to the Sewage Treatment Plant Qcw 23 6.2.2 Dry weather Flow in Daily Average Qdw24 24 6.2.3 Hourly Peak Flow with Dry Weather Flow Qdwx 24 6.2.4 Rainwater Run-off from Separate Areas QrS24 25 6.2.5 Rainwater Run-off Qr24 25 6.2.6 Critical Rainwater Run-off Qrcrit 25 6.2.7 Critical Combined Water Flow Qcrit 25 6.2.8 Mean Rainwater Run-off During Overflow Qro 26 6.3 Discharge Rates 26 6.3.1 Dry Weather Discharge Rate qdw24 26 6.3.2 Rainwater Run-off Rate qr 26 6.4 Dry Weather Concentration cdwc 27 6.5 Mean Mix ratio in Overflow Water m 27 7 Determination of the Necessary Total Storage Volume 27 7.1 Determination of the Permissible Overflow Rate 27 7.1.1 Influence of Heavy Polluters ap 28 7.1.2 Influence of Annual Precipitation ah 28 7.1.3 Influence of Sewer Deposits aa 29 7.1.4 Dimensioning Concentration in the Dry Weather Flow cd 30 7.1.5 Theoretical Overflow Concentration ccc 30 7.1.6 Permissible Annual Overflow Rate eo 30 7.2 Necessary Total Storage Volumes 32 7.3 Accountable Storage Volumes 33 7.4 Minimum Storage Volumes 33 8 Dimensioning of Individual Structures with Overflow 34 8.1 Simplified Distribution Method 34 8.1.1 Approach 34 8.1.2 Scope of Application 34 8.2 Verification Procedures 35 8.2.1. Special Basic Facts 35 8.2.1.1 Precipitation Loading 35 8.2.1.2 Registration of the Sewer Network 35 8.2.1.3 Fictitious Central Tanks 36 8.2.2 Approach 36 8.2.2.1 Preliminary Calculation to Determine Permissible, Model-Dependent,Overflow Loads 36 8.2.2.2 Determination of the Rehabilitation Requirement 37 8.2.2.3 Planning of Measures 37 8.2.2.4 Further Verification Parameters 38 8.2.3 Requirements on Pollution Load Calculation Methods 38 9 Dimensioning of Individual Structures with Overflow 39 9.1 Stormwater Overflow 39 9.2 Stormwater Tanks with Overflow 41

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9.3 Sewer with Storage Capacity and Overflow 42 9.3.1 Sewer with Storage Capacity with Top-end Overflow 42 9.3.2 Sewer with Storage Capacity with Bottom-end Overflow 42 9.4 Stormwater Holding Tanks 42 10 Construction and Operation of Structures With Overflow 43 10.1 Stormwater Overflows 43 10.1.1 General 43 10.1.2 Method of Construction of Stormwater Overflows with Overflow Weirs 44 10.1.3 Method of Construction of Stormwater Overflows with Floor Opening

(Spring Overflow) 45 10.2 Stormwater Tanks with Overflow 46 10.2.1 Method of Construction of Separation Structures and Overflows 46 10.2.2 Method of Construction of Stormwater Tanks with Overflow 47 10.2.3 Method of Construction of Sewers with Storage Volume 48 10.2.4 Method of Construction for Discharges 48 10.3 Maintenance and Operation 49 10.3.1 Maintenance Facilities 49 10.3.2 Cleaning and Flushing Facilities 50 10.3.3 Measurement Facilities 50 10.3.4 Other Records 50 11 Dimensioning Example 50 11.1 Local Situation 50 11.2 Necessary Total Storage Volumes 52 11.2.1 Simplified Distribution Procedure 53 11.2.2 Verification Procedure 53 11.2.2.1 Hydrologic Method 56 11.2.2.2 Hydrodynamic Method 56 11.2.2.3 Presentation of Results 57 11.3 Dimensioning of Stormwater Overflows 58 11.3.1 Stormwater Overflow SO1 in Commercial Area 2 58 11.3.2 Stormwater Overflow SO2 in Sub-area 3 58 12 Terms 59 13 References 60 Appendix 1 64 Notes on Advanced Requirements 64 Protection or Management Need 64 Effects 64 Assessment Criteria 66 Measures 67 Further action 67

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Appendix 2 68 Pollution Load Calculation Methods 68 2. Hydrologic-Empirical Methods 68 3. Deterministic Models 68 3.1 Hydrologic-Deterministic Models 69 3.2 Hydrologic-Hydrodynamic Models 69 4. Characterisation of Pollution Load Calculation Methods 70 Appendix 3 72 Appendix 4 73 Calculation Formulas for Figs. 12 and 13 73 Appendix 5 Discharge Diagram for the Simplified Dimensioning Procedure (Th. Bettmann) 74

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1 Scope of Application and Terms These Standards apply for structures with overflows in the overall system of a combined wastewater sewer system within the catchment area of sewage treatment plants. It replaces the previous ATV Standard A 128 from 1977/1. Structures with overflows in combined systems are structures with an overflow into a lake or river such as, for example, stormwater overflows (SO), stormwater tanks with overflows (STO) and sewers with storage capacity and overflow (SSCO). Stormwater holding tanks (SHT) are dealt with in ATV Standard A 117. Stormwater sedimentation tanks (SST) serve for the treatment of stormwater with separate systems. They are also not dealt with here. Information is given in the ATV Working Report in "Korrespondenz Abwasser" (1980), Vol. 1.

2 Objective of Stormwater Treatment For water management and cost reasons the priority task of the planning of measures for wastewater collection and stormwater treatment is the avoidance of stormwater overflow into the sewer system wherever this is possible. For the remaining discharges, for technical water management and economic reasons, stormwater structures with overflow are located in combined wastewater sewers. With precipitation run-off, high pollutant loads can occur which, with discharge into lakes and rivers, could load these heavily. Although the loadings appear only temporarily these can exceed those from the effluents of sewage treatment plants several times over during rainfall run-off. The task of stormwater treatment is so to limit the rainfall run-off into the sewage treatment plants that there the desired effluent values are maintained and, at the same time, the surge-type loadings of the lakes and rivers remain within acceptable limits. The aim of stormwater treatment must be the best possible reduction of the total emissions from stormwater overflows and sewage treatment plants within the scope of water management requirements. An effective protection of lakes and rivers and of sewage treatment plants from excessive loadings is to be expected if the necessary stormwater treatment takes place according to the criteria of these Standards.

2.1 Principles The objective can be achieved with various formulations - from discharge avoidance to substance retention. Stormwater overflows are fundamentally to be assessed together with the sewage treatment plant for interrelated catchment areas of a section of a lake or river. Requirements on the sewage treatment plant run-offs and on the stormwater overflow installations should be matched in their effectiveness for the lake or river. The regional and network specific quantities precipitation, flow time, gradients, sewage storage capacity, heavy pollutants and areas drained with a separate system have a considerable influence on the overflow quantity and concentration. These are therefore taken into account.

2.2 Method of Approach Having taken into account the given possibilities of discharge reduction or discharge avoidance, both prerelieved and non-prerelieved overflow structures are to be dimensioned for the remaining discharges. The effectiveness of a stormwater treatment here depends not only on the available storage volume but also particularly on the arrangement, design and operation of the installations (Chaps. 4 and 5). Basically, there two procedures available for the dimensioning and verification of the objective of the stormwater treatment: - simplified dimensioning procedure using diagrams (Chap. 8.1), - verification procedure using pollutant load lculations (Chap. 8.2).

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The objective is considered as being met if the requirements of these Standards, with regard to pollutant retention, arrangement, design, dimensioning and method of operation of structures with overflows, are observed.

3 Requirements on Stormwater Treatment For different situations with lakes and rivers these Standards differentiate technical rules for the normal case (normal requirements) and advanced requirements according to the situation of the lake or river. While the normal requirements, which basically have to be met, are emission related advanced requirements are planned on the basis of intramission considerations. In the individual case it is to be decided which requirements are to be fulfilled. If a surface lake or river or a system of surface waters is loaded via several stormwater overflows or sewage treatment plants then it is to be verified both for every individual overflow structure and also for the total system, seen from the aspect of related water management, that the requirements placed are fulfilled.

3.1 Technical Requirements for the Normal Case (Normal Requirements) The technical rules for the normal case (normal requirements) are based on a pure consideration of emission without assessment of the local situation of the lakes and rivers and extensively tie up with the previous ATV Standard A 128. With lakes and rivers without special protection or management requirements their observation is considered as sufficient. Deviations therefrom are to be justified in individual cases. The loading of a surface lake or river through stormwater overflow is determined by the induced pollutants and contaminants, their type, quantity, concentration as well as the duration and frequency of loading. As substitute for these parameters the annual pollutant load of the chemical oxygen demand (COD) is enlisted as general indicator for pollution. Dimensioning and verification criterion is therefore a theoretical, fictitious COD annual load which, in the mean over many years with average conditions, reaches the lake or river through run-off precipitation water. It is made up of the annual load of the immediate overflowed combined wastewater and the theoretical residual load of the stormwater jointly treated in the sewage treatment plant. For the assessment of stormwater installations with overflow further criteria such as, for instance, the annual overflow rate and the overflow frequency and duration can be enlisted. According to the current status of knowledge it is not possible to make predictions on the actual pollutant concentrations of the combined wastewater of individual rain events. For this, the interaction of the many components which contribute to the pollution of the wastewater (eg. material accumulation and erosion on the surface and in the sewer) are too complex. However, basic interrelationships can be formulated in order to describe the essential influences on the annual pollutant load in their tendency. This is done here with a formulation of average pollutant concentrations for rain and dry weather. From this situation a "reference load case" was defined in the Standards for average conditions in Germany for which a certain necessary storage volume in combined sewers is required. With this storage volume it should be ensured that, with average conditions according to the current status of knowledge an effective pollution protection is achieved. Deviations from the reference load case can lead to a reduction or increase of the necessary storage volume. Through the matching of the storage volume to the local conditions the load on lakes and rivers in individual cases is not greater than with average conditions. The reference load case is based, in particular, on the following agreed values: - average annual precipitation 800 mm - COD concentration in rainwater run-off 107 mg/l - COD concentration in dry weather run-off 600 mg/l - COD concentration in the stormwater in the sewage treatment plant effluents 70 mg/l

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Deviations from the reference load case are taken into account as follows: - the mean precipitation over many years has, as locally dependent dimension, considerable influence

on the stormwater pollution and on the overflow activity. Higher precipitation leads, as a rule, to heavier loading of lakes and rivers so that, with dimensioning, it leads to an increase of the necessary storage volume.

- the laid-down COD concentration of 600 mg/l in the dry weather run-off represents a theoretical value which, with the determination of the necessary total storage volume according to Chap. 7, may not be undercut. A lesser pollution of the wastewater than 600 mg/l aids the pollution protection as the necessary total storage volume is therefore not smaller. A higher pollution leads to a storage volume increase (heavy pollution surcharge).

- in order to be able to carry out the dimensioning of the structures with overflow for a long planning period despite the possible annual or long-term change of the effluent value of a sewage treatment plant, the laying down of a theoretical constant effluent value is necessary. A mean COD run-off value of 70 mg/l was laid down under the assumption of a constant residual pollution in the sewage treatment plant effluent with wet weather. Actual measured deviations from this value have no influence on the determination of the necessary storage volume for stormwater treatment.

The necessary total storage volume in the drainage network is determined taking into account the local conditions (Chap. 7). Subsequently it can be distributed according to a simplified procedure (Chap. 8.1). Insofar as the application limitations of the simplified procedure are observed a special verification of the overflowed COD load is not necessary. The COD load serves solely as theoretical guidance value for the application of the dimensioning process. How large this may be, dependent on local conditions, is determined in the respectively applied EDP pollution load model in a preliminary calculation (Chap 8.2). A comparison with measured values can only provide information.

3.2 Advanced Requirements If a particular protection or management requirement exists, further requirements can be placed which are deduced from intramission considerations. They are not dealt with in this Standard. Information is contained on this in Appendix 1.

3.3 Overall Consideration of a Lake or River For the complete assessment of a stormwater installation with overflow a total consideration for an associated stretch of a lake or river or a waters system is necessary. This applies in particular for sections of a lake or river into which several overflows discharge close together. With this, a self-cleaning section is to be considered. The permissible total loading is, in such cases, to be distributed to the individual overflow points taking into account the characteristics of the lake or river and the requirements on the individual installations with overflow. The technical rules for the dimensioning of individual structures, according to Chaps. 9 and 10 are fundamentally to be fulfilled. In order to meet the aim of these standards the load overflow relationship with regard to the lake or river together with the fulfilment of other legally required discharge values must be assessed and optimised (Chap. 4). With the assessment of the stormwater overflow the loading of the lake or river due to the sewage treatment plant is to be included.

3.3.1 Common Requirements in the Overall Drainage Area If all overflow sewers of a drainage area empty into subsidiary and main waters with the same protection or management requirements, then the dimensioning of the stormwater installation with overflow can be carried out uniformly.

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3.3.2 Combined Requirements with Different Surface Waters If several lakes and rivers, with different requirements, which still flow into a common main lake or river are present in one drainage area in closer proximity of the catchment area under consideration, then the requirements of the main lake or river are relevant. Should higher requirements be placed on the subsidiary waters than on the main lake or river then a transfer of the combined waters overflow to the less sensitive lake or river can be considered. For the subsidiary lake or river itself its own protection requirements are relevant. If waters with different requirements are separated by river areas (eg. a river and, independent of this, a lake), then each lake or river must be considered as an entity with an associated level of requirement.

4 Planning Principles The outline conditions described below should give an overview of the different measures for stormwater treatment in the combined system. Run-offs from settled areas, together with the diffuse substance input from agriculture and the atmosphere, essentially determine the quality level of the lakes and rivers. Settled areas load lakes and rivers through - stormwater discharges from separate systems, - overflow from combined systems, - discharges from sewage treatment plant effluents.

4.1 Reduction of the Amount of Wastewater Produced Fundamentally it should be examined whether the amount of wastewater can be reduced as, with this, the necessary investments and operating costs for a stormwater treatment can be diminished. In particular the following measures come into question for the discharge of stormwater, domestic and industrial wastewater and sewer infiltration water.

4.1.1 Rainwater Run-off The amount of stormwater depends primarily on the size of the hard, impervious surfaces connected to the sewer network. Measures for the reduction of the rainwater run-off are, for example: - percolation of non-harmful, polluted rainwater (eg. ATV Standard A 138), - direct discharge of lightly polluted discharge from roof surfaces and traffic surfaces into a lake or

river (the run-off from heavily polluted traffic, industrial or other surfaces is to be discharged, in any case, into the wastewater or combined wastewater sewer network and/or is to be pre-treated),

- avoidance of discharge from non-compacted surfaces, - use of rainwater as tap water. The pollution of rainwater run-off can, furthermore, be reduced through the following measures: - frequent street cleaning (as, inter alia, pollutants are tied to fine particles the effectiveness of street

cleaning using sweeping machines is limited), - removal of the causes for the heavy pollution of discharge surfaces, - street drains with improved pollutant retention (dry and wet gullies), - frequent sewer cleaning, - flushing aids.

4.1.2 Domestic and Industrial Wastewater Flow With measures which aim to reduce the amount of domestic and industrial wastewater the effect on stormwater treatment in the combined wastewater system - eg. due to increased concentrations or sewer deposits - is to be verified. Measures for the reduction of the amount of domestic and industrial wastewater are, for example: - application of water saving techniques,

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- circulation systems in industrial operations.

4.1.3 Sewer Infiltration Water Flow The dimensioning regulations of this Standard assume that the amount of sewer infiltration water is reduced as far as possible. If this is not the case then large storage volumes result from dimensioning according to this Standard. Measures for the reduction of sewer infiltration water are, for example: - replacement of the usually non-permitted connection of land drains and seepage pipelines with

proper drains, - the sealing of leaking sewers and drains, - avoidance of faulty connections, - avoidance of discharges from lakes and rivers into wastewater sewers.

4.2 Measures for Wastewater Treatment in the Combined Wastewater System Stormwater treatment in the combined system can include the following measures: - intermediate storage of combined wastewater and subsequent treatment in a sewage treatment

plant, - redistribution of the rainwater run-off within the sewer network, - wastewater treatment measures before discharge into a lake or river.

4.2.1 Intermediate Storage of Combined Wastewater An intermediate storage of combined wastewater can already be achieved by the holding capacity of the sewers. For constructional and operational reasons, however, it is sensible to retain and use storage spaces specifically. This takes place through: - stormwater tanks with overflows or sewers with storage capacity and overflow, - stormwater holding tanks, (eg. ATV Standard A 117), - deliberate backing-up of sewers (elevated overflow sills, discharge control in accordance with the

details given by ATV Working Group 1.2.4, 1985). Rainwater run-off can also be stored in the interim on surfaces, such as, for example, by back-up on parking areas or flat roofs or by a reduction of the inlets into the sewer network. As, with this, a limitation of the usage is involved, these measures can only be brought into the planning in special cases.

4.2.2 Redistribution of Rainfall Run-offs A redistribution of the rainwater run-off within the sewer network with the aim of using available storage space evenly is, together with a discharge control, advantageous. Objective of a redistribution can also be an evening out of the overflow characteristics of stormwater overflow systems (eg. quantity, duration, frequency). The transfer of rainfall run-off into other sections of lakes or rivers requires an appropriate receiving waters situation.

4.2.3 Wastewater Treatment Measures Wastewater treatment processes presuppose balanced wastewater loadings. Quantity and characteristics of the combined wastewater however vary so heavily that the information on wastewater treatment in sewage treatment plants cannot be transferred without question to a stormwater treatment. Nevertheless the following principles can be used for stormwater treatment: - mechanical treatment through the settling effect as, for example, it is achieved in stormwater tanks

according to this Standard, - high density and highly volatile fluids separators, - centrifugal separators,

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- screens, micro-sieving, precipitation, flocculation, - filtration of overflowed combined wastewater (eg. ground filtration). The retention of floating substances can be achieved basically by floating or fixed scumboards. An improved retention of coarse substance on the surface is possible using screens and sieves. The technical wastewater measures must be subject to regular maintenance and monitoring. With all measures it is to be continuously checked whether a central arrangement at the sewage treatment plant or a decentralised arrangement in the drainage area meets the water protection requirements as well as being economically justifiable. Stormwater tanks in the area of the sewage treatment plant, depending on the local conditions, can also be used as an intermediate storage with accidents involving water harmful substances in the sewer system. In special cases this can also apply for stormwater tanks in the sewer network. Such an usage is to be agreed with the responsible water authority.

4.3 Structures with Overflow The effectiveness of a construction measure for the treatment of stormwater depends, apart from the dimensions of the overflow tanks, on the arrangement of the overflow and storage structures in the sewer network and their design. An overflow following primary treatment (so-called fine overflow), for example, for even loading of the biological stage of the sewage treatment plant, is not permitted. Details on the method of construction are contained in Chap. 10. The upper edge of the overflow sill should, as a rule, lie above the water level of the dimensioned high water level of the lake or river. However, as a minimum, efforts should be made that, with a ten year high water level of the lake or river, the upper sill of the weir is still not overdammed with the relevant rainfall run-off in the overflow sewer.

4.3.1 Stormwater Overflows Stormwater overflows (SO) serve for the reduction of high combined wastewater discharge peaks (Fig. 1). They may be located only where the critical combined wastewater discharge Qcrit can be further conveyed at full height and the stormwater treatment is carried out subsequently in a storage structure further downstream. Efforts should be made to site a structure where combined wastewater to be overflowed shows the least pollution. It is, however, sensible to retain sufficient room for a possible, later, eg. through a modification of the tanks, necessary expansion of the network. If the commercial and industrial wastewater is considerably more polluted than domestic wastewater, or if the heavily polluted discharge with the draining of stormwater tanks with overflow is discharged into a combined wastewater sewer and if the necessary dilution of the combined wastewater in accordance with Chap. 9 can no longer be ensured, then overflow can no longer take place via stormwater overflows.

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Fig. 1: Functional diagram of a stormwater overflow With the discharge above the stormwater overflow of domestic or industrial wastewater from an area drained using the separate system attention is drawn to the minimum dilution of the overflowed combined wastewater, in accordance with Chap. 9.1. Overflows into lake and rivers which at times carry little or no water should be avoided. If this is not possible at least a critical rainfall intensity (i.e. at which overflow will come into action) of 15 l/(s.ha) is to be applied. Emergency overflows are not, in the sense of this Standard, to be seen as stormwater overflows. They are to be taken into account with verification procedures.

4.3.2 Stormwater Tanks with Overflow The selection of the site of a stormwater tank with overflow (STO) is to be made taking into account the water management and economic aspects. The greatest water management effect is achieved if the stormwater tank with overflow is sited below heavily sedimented sewer sections or sub-catchment areas. Reference values for the occurrence of deposits in the sewer network are provided as a diagram in Fig. 2.

Fig. 2: Reference values for the occurrence of sewer deposits

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If the gradient in the sewer network of the associated catchment area lies below the curve in Fig. 2 then one must reckon with deposits. This tendency increases with velocities which become smaller and with dry weather discharges. The relationships of the essential influences on the formation of sewer deposits are emphasised in the inflow value aa in accordance with Chap. 7.1.3.

4.3.2.1 Stormwater Tanks Retaining the First Flush of Stormwater Stormwater tanks retaining the first flush of stormwater (STRFF) are to be arranged if a pronounced flushing surge is to be expected. As a rule, this is the case with small catchment areas with short flow times. They store a combined wastewater flushing surge if this occurs at the start of the discharge event. They are not flowed through by overflow water. The stored contents must then be conveyed to the biological treatment stage of the sewage treatment plant. Stormwater tanks retaining the first flush of stormwater are to be planned essentially for the discharge of non-prerelieved drainage areas if the flow time with computed rain in the sewer network up to the tanks is not more than 15 to 20 mins. If stormwater overflows are arranged in the catchment area above a stormwater tank retaining the first flush of stormwater, then the total flow time in the catchment area of this tank and not only the flow time below the stormwater overflow is to be applied.

4.3.2.2 Stormwater Tanks with Overflow for Settled Combined Wastewater With increasing size of catchment area one must reckon with ever more balanced pollution concentrations without exaggerated flushing surge. In this case stormwater tanks with overflow for settled combined wastewater (STOSC) are to be planned which aim to achieve a mechanical treatment of the combined wastewater. As opposed to stormwater tanks retaining the first flush of stormwater, stormwater tanks with overflow for settled combined wastewater have an overflow structure (OSC), which comes into action once the tank is full and which feeds mechanically treated combined wastewater to the receiving water. As a rule a tank overflow (TO) (see also Chap. 9.2) is placed upstream in series to limit the maximum tank throughflow. Until they are filled STOSC act as a storage space and thereafter act as settling tank with overflow for a partial inflow (as a rule Qcrit) into the receiving waters. At the end of the rainfall event the tank contents must be fed to the biological stage of the sewage treatment plant. Stormwater tanks with overflow for settled combined wastewater are provided if: - the flow time in the sewer network up to the tanks, with computation rain, is more than 15 to 20 mins

or if no further exaggerated flushing surge is to be expected, - these tanks are in upstream series of other structures with overflow (networks prerelieved by SO or

STO), - in exceptional cases the inflow to the tank can be more than the maximum possible throttle

discharge. (Such cases can be locally influenced by varying sewer infiltration water or melting snow).

4.3.2.3 Composite Tanks Composite tanks (CT) are provided if flushing surges (from neighbouring parts of the catchment area) and also discharges with balanced pollution occur. They represent a combination of stormwater tanks retaining the first flush of stormwater and stormwater tanks with overflow for settled combined wastewater and consist of a retention section and a treatment section. The in-coming combined wastewater is first stored in a retention part designed as a stormwater tank retaining the first flush of stormwater. Once it is full the combined wastewater that arrives subsequently flows through the treatment part which is designed as a stormwater tank with overflow for settled combined wastewater. Combined tanks come into consideration for the interface area between stormwater tank retaining the first flush of stormwater and the stormwater tank with overflow for settled combined wastewater or, if flushing surges are to be expected from neighbouring parts of the catchment area, with longer flow times. Composite tanks are dimensioned either as for stormwater tanks retaining the first flush of stormwater or for stormwater tanks with overflow for settled combined wastewater.

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Essential advantages: - retention and treatment effect in one tank, - division of volume between retention and treatment parts selectable, - by subdivision into several chambers the back-up frequency and thus the maintenance resources in

the neighbouring throughflow part are reduced significantly. Essential disadvantages: - smaller treatment effect compared with a pure stormwater tank with overflow for settled combined - stormwater, - structurally and operationally more costly. -

4.3.2.4 Sewers with Storage Capacity and Overflow Sewers with storage capacity and overflow (SSCO) differ in their effect through the position of the overflow structure. Sewers with storage capacity and overflow with top-end overflow (SSCTO) (Fig. 3) function as stormwater tanks retaining the first flush of stormwater. Sewers with storage capacity and overflow with bottom-end overflow (SSCBO) (Fig. 4) function as stormwater tanks with overflow for settled combined wastewater in the main stream without tank overflow. With dimensioning according to this Standard they are basically equivalent to both stormwater tanks retaining the first flush of stormwater and stormwater tanks with overflow for settled combined wastewater.

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Essential advantages: - no structure in addition to the sewer necessary, - emptying with natural gradient. Essential disadvantages: - cannot exclude deposits, - volumes of SSCBO larger than with stormwater tank with overflow for settled combined

wastewater, - with overflow of SSCBO partial washing out of storage volume contents into the lake or river. A sewer with storage capacity and overflow can be arranged if a sufficient drag tension for the avoidance or reduction of deposits is to be ensured or if flushing aids are installed. In addition it is to be ensured that no harmful back-up results. A bottom-end overflow may not come into operation as often as one at the top-end as otherwise, with large rainwater run-offs, there is a danger that the pollution trapped in the storage volume is displaced by the subsequent, less polluted combined wastewater. The storage volumes must therefore be larger than for sewers with storage capacity and overflow with top-end overflow.

4.3.2.5 Main and By-pass Streams Stormwater tanks retaining the first flush of stormwater, stormwater tanks with overflow for settled combined wastewater, composite tanks and sewers with storage capacity and overflow can be arranged in main or in by-pass streams (Figs. 5 - 10). With main streams the discharge fed on to the sewage treatment plant is led through the tank; with a by-pass stream it is fed past the tank. The draining of the tank into the by-pass can take place deliberately using discharge regulation (qualified by-pass).

Fig. 5: Stormwater tank retaining the first flush of stormwater in main stream

Qin

TO

Qo

STREFF

Qd

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Fig. 6: Stormwater tank retaining the first flush of stormwater in by-pass stream

Fig. 7: Stormwater tank with overflow for settled combined wastewater in main stream

Fig. 8: Stormwater tank with overflow for settled combined wastewater in by-pass stream

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Fig. 9: Composite tank in main stream (RP = retention part, TP = treatment part)

Fig. 10: Composite tank in main stream (RP = retention part, TP = treatment part) Whether a stormwater tank with overflow is arranged in the main or by-pass stream depends on the local height and site conditions. An arrangement in the by-pass is always advantageous if small height differences between inflow and outflow exist so that the stormwater tank with overflow must be drained using pumps. Nevertheless, more connecting pipelines than with a main stream flow and an additional separator structure are necessary. The main stream flow is suitable if sufficient height difference is available between inflow and outflow and little freedom exists in the arrangement of the site. It offers operational and design advantages. Stormwater tanks with overflow for settled combined sewage should, if possible, be arranged in by-pass streams as, generally, with this arrangement, combined wastewater is stored and overflows with a somewhat lower pollutant concentration. The cause is that, at the beginning and end of a rainfall event the dry weather discharge mixes with a relatively low rainfall run-off. Through slight dilution this combined wastewater is more heavily polluted. With the by-pass stream it flows past the tank up to the scale of the throttle discharge. Through this, the overall overflowed pollution load, in comparison with stormwater tanks with overflow for settled combined sewage in the main stream, reduces somewhat. If a controlled tank drainage is planned or considered for the future, in which the draining of the stored wastewater to the sewage treatment plant does not take place immediately following the end of rainfall, then the by-pass stream is to be used. Otherwise the stored combined wastewater is continuously mixed with the subsequent dry weather discharge. There is a danger that, with this, more highly concentrated combined wastewater reaches the lake or river with a subsequent rainfall event.

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4.3.3 Arrangement of Several Stormwater Tanks with Overflow With spatially separated sub-drainage areas such as, for example, with sections of communities or neighbouring communities, enclosed drainage areas should be provided with their own stormwater tanks with overflow. With the selection of the site of the tank it is to be investigated whether a technical and economically practical discharge regulation or control is possible. The maximum permissible feed flow of the sewage treatment plant may not be exceeded. With the subdivision of larger catchment areas into several sub- areas the individual stormwater tanks with overflow can be connected in parallel or in series.

4.3.3.1 Parallel Stormwater Tanks with Overflow Here the total catchment area is so arranged that the tanks sited at the end of each sub-area are connected in parallel. With this, stormwater tanks retaining the first flush of stormwater, stormwater tanks with overflow for settled combined sewage and composite tanks are possible in both main and by-pass streams. Tanks connected in parallel (Fig. 11, top) present the more advantageous solution for water pollution control if they can be throttled, by section, according to the permitted sewage treatment plant discharge. Modifications, such as, for example, with whirl separators are only practical for parallel connection as the once separated suspended and floating solids can only in this way reach the sewage treatment plant immediately and not reach further structures with overflow. Essential advantages: - stored combined wastewater reaches the sewage treatment plant completely, - there is no mutual influence between the tanks, - free choice of tank design, - clear hydraulic conditions and simple dimensioning. Essential disadvantages: - often higher construction costs as a result of an integral delivery connector drain to the sewage

treatment plant (transporter sewer or wastewater collector drain).

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Fig. 11: Functional diagrams of parallel and series connected stormwater tanks with overflows

4.3.3.2 Series Stormwater Tanks with Overflow With series stormwater tanks with overflow (Fig. 11, bottom) the upstream separated pollutants again mix with stormwater and, under certain conditions, overflow at the subsequent overflow tank. Basically, the throttle discharge of series overflow tanks should grow in the direction of flow so that the stored contents of an upstream tank can be fed to the sewage treatment plant without leading to an overflow at the next tank. Series connected tanks with controlled overflow presume a very efficient sewer operation and a careful maintenance of the plant components. In individual cases the installation of measurement and control devices is to be examined with regard to the relationship of usage to costs and maintenance. In the simplified dimensioning procedure, without control devices the specific volumes of the downstream tanks are usually larger as a result of the longer flow times. With regard to future development a later modification with control devices should be taken into account in planning.

4.3.4 Stormwater Holding Tanks Stormwater holding tanks (SHT) are to be installed if the sewer network cannot further convey stormwater discharge peaks and an overflow of combined wastewater is not possible. If its run-off discharge rate qr (Chap. 6.3.2) lies above 5 l/(s.ha) then it has no significant influence on the subsequent stormwater overflows. In this case the complete catchment area including the areas above the stormwater tank with overflow is relevant for the dimensioning of overflow structures.

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If the run-off discharge rate qr of individual stormwater tanks with overflow is under 5 l/(s.ha) and above the run-off discharge rate of the sewage treatment plant then this type of tank has a definitely unfavourable effect on subsequent overflows due to its long drainage durations.

5 Planning Scope The planning of stormwater overflow installations takes place based on ATV Standard A 101. Data for the following conditions are to be determined: - actual status, - planning status. For the rehabilitation of existing sewer networks both conditions are to be recorded; with new planning the planning status only. In each case the total catchment area of a sewage treatment plant is to be recorded (eg. general drainage plan). If required, the different planning levels are to be examined for this total network.

5.1 Determination of Actual Status The limitation and description of the catchment area with its water management conditions and constraints belong, in particular, to the clarification of the definition of the problem. In addition, the combined effect of stormwater treatment and sewage treatment plant is to be determined and the efficiency of the sewage treatment plant is to be established. The establishment of the actual status (inter alia number of inhabitants, areas, degree of sealing, dry weather discharge, lakes and rivers) includes the assessment of the efficiency of the existing sewer system and the sewage treatment plant. Measurements of the dry-weather discharge as well as the total annual wastewater inflow of the sewage treatment plant is to be brought into its determination. The dry-weather discharge is to be compared with and supported by the water usage.

5.2 Determination of the Planning Status In order to establish the planning status (planning objective) the following must, in particular, be taken into account: - town development plans, - area usage plans - building plans, - outline plans of other infrastructural installations. The necessary data for the planning of sewer network and sewage treatment plant can be extrapolated from available data sources.

5.3 Planning Periods As a rule, the following are to be used as planning periods: - for the sewer network 50 - 100 years, - for the sewage treatment plant 15 - 25 years.

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As a correct stormwater treatment can only be ensured if the throttle discharges passed on from the stormwater overflow installations can be treated biologically to the full extent in the sewage treatment plant, the stormwater overflow installations must be designed to the expansion status of the associated sewage treatment plant. Therefore, the planning period of the sewage treatment plant of some 15 - 25 years is to be used as the planning period of the stormwater treatment plants. However, it must be verified that the stormwater treatment plants can also fulfil their function fully for the larger planning period of the sewer system. If, for example, larger storage volumes are necessary for this then the required space is to be kept free.

5.4 Standard and Variant Investigations With the assessment of existing facilities, objectives (water management, operational, cost-oriented, etc) are to be formulated based on usage times, usage durations, operational parameters, operational experiences and the therefrom resultant information on operational difficulties. In particular the existing loadings and reserve capacities are to be determined and assessed. Investigations of variants are to be undertaken based on similar criteria as for the standard investigation. However, different, objective dependent variants and alternative investigations are necessary. These should be supplemented by cost investigations for better assessment. The capability for implementation of the planned measures is to be examined. Intermediate conditions with staged expansion are to be examined for the establishment of priorities.

6 Calculation Principles For the complete catchment area of a sewage treatment plant that is drained using the combined system a necessary total storage volume must, in accordance with this Standard, be determined for the intermediate storage of combined wastewater. This applies equally for the simple distribution system (Chap. 8.1) as for the detailed verification system (Chap. 8.2). The necessary initial data and the therefrom derived dimensions are defined and described below.

6.1 Sizes of Catchment Areas

6.1.1 Annual Precipitation hPr The annual overflow duration depends, inter alia, on the annual precipitation hPr (hPr can, for example, be taken from the Year Book of the German Weather Service). With increasing amounts of precipitation the combined wastewater is overflowed longer and therefore must also discharge more wastewater directly into the lake or river. The precipitation hPr in mm is taken into account with the dimensioning (Chap. 7.1.2).

6.1.2 Surface Areas ACA and Ais The catchment area covered or coverable by a sewer system is designated as drainage area ACA. It is subdivided into a hard surface area Ared and an unhardened surface area (ACA less Ared). The hard surface areas Ared are variously defined through different loss statements in EDP precipitation run-off models. The computed part of a catchment area from which the rainfall run-off, after deduction of all losses, completely reaches a combined sewer system is designated the impervious area Ais. )h10/(VQA eff.Prris ⋅= in ha (6.1) with VQr in m

3 = annual rainfall run-off sum in the combined wastewater system,

hPr,eff in mm = effective precipitation (after deduction of losses).

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Separate areas do not count as effective discharge areas, combined areas with precipitation percolation only with their effective discharge part. Outer areas and unhardened surface areas can, in general, be ignored. The impervious surface area Ais is, as a rule, significantly smaller than the hard surface area Ared. So far as no calculations or measurements are available the impervious area of the hard surface is to be set as equal, i.e.: redis AA = (6.2)

6.1.3 Flow Time tf The damping of discharge waves is influenced by the concentration time. The concentration in prerelieved sewer networks can only be determined with considerable effort. As it only slightly influences the relieved annual pollution load it is alternatively replaced by the flow time. This can be calculated from the longest flow path in the sewer network with complete filling or approximated from the time difference between the maximum values of relevant time curves. Flow times from widely separated areas can, with minor significance for the combined wastewater inflow, be ignored

6.1.4 Mean Terrain slope group SGm In accordance with ATV Standard A 118 "Standards for the Hydraulic Calculation of Wastewater, Stormwater and Combined Wastewater Sewers" the inclination of a drainage area is sub-divided into four groups:

Terrain slope group SG Mean Terrain Gradient JT 1 2 3 4

JT < 1% 1% µ JT µ ≤ 4% 4% µ JT µ ≤ 10% JT > 10%

For the overall catchment area of a stormwater overflow installation the mean terrain slope group derives from the equation i,CAii,CAm A/SGA(SG Σ⋅Σ= (6.3) with

i,CAA in ha = overall catchment area of the sub-area i, SGi = terrain slope group (1,2,3 or 4) of sub-area i.

6.2 Discharges

6.2.1 Combined Wastewater Discharge to the Sewage Treatment Plant Qcw The combined wastewater discharge Qcw is made up from the dry weather discharge Qdw together with the rainwater run-off Qr. As a rule Qcw is to be applied with not less than 2Qpx + Qdw24 (see ATV Standard A 131). As Qcw is often determined for a point in time far into the future this value usually deviates from the current combined wastewater discharge to the sewage treatment plant. Here there are two cases to differentiate: - the sewage treatment plant is, for the foreseeable future (8 - 10 years) still in a position to treat

biologically at least 24iwpxcw QQ2Q +≥ , then the stormwater tanks are to be dimensioned to the

actual sewage treatment plant capacity, - should the sewage treatment plant to be expanded in the foreseeable future then the dimensioning

of the stormwater tanks must already take into account the planning status of the future sewage treatment plant.

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With parallel catchment areas the respective throttled discharges from the part areas may be more than 2Qpx + Qiw24 if it is ensured that, at no time, the dimensioning inflow Qcw to the biological stage of the sewage treatment plant is exceeded. This verification can, for example, be carried out with discharge simulation with the planning of the sewer network controls.

6.2.2 Dry weather Flow in Daily Average Qdw24 The theoretical dry weather flow relevant for the individual catchment areas from combined and separate systems is made up from the wastewater discharges from the residential areas, including the small commercial part, Qd, the commercial part Qc , the industrial part Qi and the sewer infiltration water Qiw24: s/linQQQQ 24i24c24d24w ++= (6.4)

24iw24s24dw QQQ += with Qd24 in l/s = I . ws/86400 (daily average value), I = the number of connected inhabitants ws in 1/(I.d) = annual average water consumption per inhabitant and day Qc24 in l/s = daily average commercial wastewater flow calculated from the annual average Qiw24 in l/s = annual average of the sewer infiltration water flow from separate and combined systems with dry weather. With all formulations consideration is to be given that the sum of the dry weather flow of one year, as a rule, must correspond with the annual dry weather flow sum of the inflow to the sewage treatment plant. Particular value is to be placed on the realistic recording of the inhabitants and of the water consumption. Qc24 and Qi24 are to be determined on the basis of available figures taking into account future developments separately. Only if this is not possible and values from comparable areas are not available should one reckon with 0.2 to 0.8 l/(s.ha), according to the water consumption, referred to the respective impervious surface area Ais, for commercial and industrial areas. Sewer infiltration water is to be determined with the same care on the basis of available figures taking into account future development. With this, care is to be taken that all possibilities to reduce the sewer infiltration water have been exhausted. If continuous measurements of the flow are available in the sewage treatment plant then an estimate of the quantity of the sewer infiltration water can be made using the minimum night values with dry weather. Insofar as no measurements are available or can be carried out one may reckon with up to 0.15 l/(s.ha) referred to the impervious area Ai depending on the groundwater conditions and the condition of the sewers in the separate and combined systems. Note: due to the reference to the impervious surface areas and the details for average daily values the figures given deviate from ATV Standard A 118.

6.2.3 Hourly Peak Flow with Dry Weather Flow Qdwx The daily peak Qdwx of the dry weather flow is most accurately obtained from measurements which, however, are available only at the sewage treatment plant (see ATV Standard A 131). The generally relatively high peak values in the catchment sub-areas are more and more flattened on the way to the sewage treatment plant due to the overlapping of the flow curves. Insofar as no measurements are available Qdwx is calculated from the dry weather flow, in the daily average, as follows (in l/s):

24iii

24ccc

24dpx Qb

365a24Q

b365

a24Q

x24Q ⋅⋅+⋅⋅+=

24iwpxdwx QQQ += (6.5)

with the indices c for commercial, i for industrial and Qpx in l/s = daily peak of the wastewater flow (see ATV Standard A 131)

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)s/linQ)s/linQ)s/linQ

24i

24g

24d = domestic and industrial wastewater from Eqn. (6.4),

x in h = hourly duration per day according to ATV Standard A 118, eg. 14, 16 or 18 hr. ac, ai in h = working hours per day (with an 8 hr shift) bc, bi in d = production days per year

6.2.4 Rainwater Run-off from Separate Areas QrS24 With the storage dimensioning one must reckon with rainwater run-off over and above the sewer infiltration water which appears with dry weather due to the barely avoidable inflow of stormwater into the wastewater network of separate systems. If no measurements are available then one must reckon with a supplement of 100 % of the average domestic and industrial wastewater flow QwS24 (the index S stands for separate areas). QwS24 is determined using Eqn. (6.4) on the respective separate area (average daily value). QrS24 = QwS24 (6.6) With larger separate areas (eg. over 10 ha) a rainwater run-off measurement is recommended for the determination of the fundamental planning details.

6.2.5 Rainwater Run-off Qr24 The rainwater run-off Qr24 of the total area derives from the difference between the combined wastewater flow Qcw to the sewage treatment plant, the dry weather flow Qdw24 at midday and the 24-hour average value of the rainwater run-off from separate areas QrS24

24rS24dwcw24r QQQQ −−= in l/s (6.7) In catchment sub-areas the rainwater run-off Qr24 is made up from the combined wastewater flow at the throttle Qt instead of the combined wastewater flow Qcw to the sewage treatment plant 24rS24dwt24r QQQQ −−= in l/s (6.8)

6.2.6 Critical Rainwater Run-off Qrcrit The critical rainwater run-off Qrcrit from a direct catchment area is determined as icritrcrit ArQ ⋅= in l/s (6.9) with rcrit in l/(s.ha) = critical rainfall intensity (see Chap. 9.1). With growing flow time a flattening of the inflow waves occurs. Through this the stormwater overflows, the sum of overflows and thus also the relieved pollution load decrease. This is taken into account with its dimensioning (Chap. 9.1). With the determination of the critical rainfall intensity for stormwater tanks with overflow for settled combined wastewater the reduced influence of the flow time remains unconsidered. It is to be calculated with a constant rcrit.

6.2.7 Critical Combined Water Flow Qcrit The critical combined water flow is the sum of the average daily value of the dry weather flow and the critical run-off discharge of the immediately associated drainage area as well as, if necessary, all immediate top-end throttle discharges from stormwater overflows and stormwater tanks

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i,trcrit24dwcrit QQQQ Σ++= (6.10) with Qdw24 in l/s = dry weather flow at midday from the immediate, intermediate catchment area (6.2.2) Qrcrit in l/s = critical rainwater run-off from the immediate, intermediate catchment area (6.2.6) Σ Qt,i in l/s = sum of all immediate upstream inflowing throttle discharges

6.2.8 Mean Rainwater Run-off During Overflow Qro If one divides the relieved combined water discharge in one year at one overflow structure by the associated total duration of the overflow event then one obtains the average overflow discharge from the structure. At the same time, during the overflow process, the rainfall part Qr24 (6.2.5) discharges through the throttle. Both discharges together give the mean rainwater run-off Qro during all overflow events in the course of one year 24rooro Q)6.3T(VQQ +⋅= in l/s (6.11) with VQo in m

3 = sum of combined water discharge in one year

To in h = summed overflow durations in one year. For stormwater tanks with overflow the mean rainwater run-off Qro during relief overflow can be approximated using the following equation as long as the run-off discharge rate qr (6.3.2) is smaller than 2 l/(s.ha) )Q2.3A0.3(aQ 24rifro +⋅= in l/s (6.12)

)100t/(5050.0a ff ++= for tf µ 30 min (6.13)

885.0af = for tf > 30 min with af = flow time reduction of the rainwater run-off, tf in mm = longest flow time up to the stormwater tank (6.1.3) Ais in ha = impervious surface areas (6.1.2) Qr24 in l/s = rainwater part in the throttle discharge. With run-off discharge intensities over 2 l/(s.ha) the mean rainwater run-off Qro during relief overflow must be determined by a verification procedure and Eqn. (6.11).

6.3 Discharge Rates

6.3.1 Dry Weather Discharge Rate qdw24 The dry weather discharge rate qdw24, referred to the annual mean, derives as quotient of the dry weather flow Qdw24 (6.2.2) and the impervious surface area Ai (6.1.2) is24dw24dw A/Qq = in l/(s) (6.14)

6.3.2 Rainwater Run-off Rate qr The rainwater run-off rate qr, referred to the annual mean, derives from the quotient of the rainwater run-off and the associated impervious surface area i24rr A/Qq = in l/(s.ha) (6.15) Two cases can be differentiated: - current rainwater run-off rate,

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- future rainwater run-off rate. If the current rainwater run-off rate is smaller the future one then it is to be clarified, in individual cases, whether the sewage treatment plant should be expanded, the stormwater tanks dimensioned for a smaller qr or whether the development can, for the time being, be delayed (for this see Chap. 6.2.1, combined wastewater discharge to the sewage treatment plant).

6.4 Dry Weather Concentration cdwc For the calculation of the necessary storage volume in the overall catchment area of a sewage treatment plant the COD concentration in the dry weather flow must be known. It is determined from measurements as annual mean value in the inflow to the primary settling stage. If measurements are available from the outflow of the primary settling stage only, then these values, as a rule, can be multiplied by 1.5. If measurements are not possible then the mean COD concentration is determined from the following equation )QQQQ/()cQcQcQ(c 24iwicdiiccdcdw +++⋅+⋅+⋅= in mg/l (6.16) Here one is concerned with mean daily values which are derived from the annual mean. It should be noted that the concentration ct for the complete dry weather flow, that is including the sewer infiltration water, applies. If, within the catchment area, there are discharges with higher concentrations (eg. heavily polluted commercial wastewater), the respective concentration values must be taken into account with the dimensioning of subsequent overflow structures both in the simplified procedure (Chap. 8.1) and in the verification procedure (Chap. 8.2).

6.5 Mean Mix ratio in Overflow Water m The mean mix ratio between storm and dry weather during all overflow events results from the ratio of the mean rainwater flow during all relief overflow events in one year, including the rainwater flow from separate areas and from the simultaneously inflowing mean dry weather flow 24dw24rSro Q/)QQ(m += (6.17) with Qro in l/s = mean rainwater flow during overflows (6.2.8), QrS24 in l/s = rainwater flow from separate areas (6.2.4), Qdw24 in l/s = dry weather flow as daily mean (6.2.2).

7 Determination of the Necessary Total Storage Volume The determination of the necessary total storage volume takes place for the whole catchment area of a sewage treatment plant above the last overflow structure. The surface areas, discharges, flow times, pollution concentrations and area characteristic values necessary for dimensioning are referred to this point in the network. In place of the dimensioning inflow the actual sewage treatment plant inflow is to be applied for the examination of the actual status.

7.1 Determination of the Permissible Overflow Rate The permissible overflow rate is influenced by several parameters. The mean pollution concentration in the overflowing combined wastewater, which depends on the mix ratio between stormwater and the domestic and industrial wastewater, is decisive. The more heavily polluted the overflowed combined wastewater is the less it may be relieved; this means the greater the necessary storage space will be. For the determination of the permissible overflow rate within the complete catchment area, all values (eg. impervious surface areas, flow times, mean terrain slope group) are necessary for the last throttle whose discharge is delivered completely into the biological treatment stage of the sewage treatment plant, without further overflow.

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If the throttle discharges of overflow structures in parallel catchment sub-areas flow into a common inflow to the sewage treatment plant, without there being a further overflow possibility available, then the storage requirement for each catchment sub-area can be determined separately. The necessary dimensioning parameters are, in this case, to be ascertained for each area at the throttle of the last overflow structure. The condition is that the sum of the throttle discharges from all parallel catchment areas, even with controlled discharges, at no time exceeds the capacity of the biological treatment stage of the associated sewage treatment plant. In this Standard a reference loading case is assumed for the determination of the total storage requirement for combined wastewater. With average conditions the COD concentrations, in accordance with Chap. 3.1, are tprdw c:c:c = 600 : 107 : 70 (7.1)

with the following abbreviations for the 24 hour values from the annual mean cdw in mg/l = COD concentration in dry weather flow cr in mg/l = COD concentration in the running-off rainwater ctp in mg/l = COD concentration in the sewage treatment plant effluent with storm weather. The mean COD concentration in the rainwater flow results from an assumed annual load of 600 kg per hectare of impervious surface area which is flushed with average conditions of 800 mm annual precipitation and a total discharge coefficient of 0.70. The portion of the precipitation which reaches the sewer network is 560 mm (effective precipitation). The respective concentrations arising in the overflowed combined wastewater cco depend essentially upon the dry weather concentration cdw, the rainwater concentration cr, the mix ratio of both parts during the overflow and the sewer deposits. These influences are taken into account as follows.

7.1.1 Influence of Heavy Polluters ap If a mean COD concentration of 600 mg/l is exceeded in the untreated dry weather flow then the necessary storage volume must be enlarged. This is achieved through the heavy polluter surcharge ap which reflects the increase of the pollution concentration ap = 1 for cdw µ 600 mg/l (7.2) ap = cdw/600 for cdw > 600 mg/l with cdw in mg/l = mean COD concentration in the dry weather flow from measurements or from the Eqn. (6.16)

7.1.2 Influence of Annual Precipitation ah The annual relief overflow duration at stormwater tanks with overflow depends on the annual precipitation hPr (hPr can be taken from the Year Book of the German Weather service). With increasing precipitation the combined wastewater is relieved for longer periods and therefore also more domestic and industrial wastewater is discharged straight into the receiving water. In order to keep the thus relieved annual pollution load more or less constant a mathematical dependency of the pollution concentration of the long-term mean annual precipitation is assumed for the determination of the permissible overflow rate. The influencing factor results from the equation ah = hPr /800 - 1 for 600 ≤ hPr ≤ 1000 mm (7.3) ah = - 0.25 for hPr < 600 mm ah = + 0.25 for hPr > 1000 mm with hPr in mm = long-term local annual precipitation. With hPr over 1000 mm or below 600 mm the relationship between annual overflow duration and overflow load, in general, no longer exists. With precipitation areas with more than 1000 mm one is here concerned

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primarily with mountainous areas in which the snow element can no longer be ignored in the annual precipitation. The simulation of thawed water flow cannot currently be formulated as a general rule of technology. Annual precipitation amounts below 600 mm are to be connected with increased pollution concentrations in the rainwater flow so that a further reduction of the necessary storage volume can be excluded through reasons of water pollution control.

7.1.3 Influence of Sewer Deposits aa The current knowledge on all processes which lead to the build-up and removal of sewer deposits are insufficient for a mathematical description. Therefore, only tendencies can be reflected with a surcharge with the dimensioning of the storage volumes for the combined wastewater, assuming the reference loading case. Sewer deposits are, as a minimum, to be expected during the night-time hours in sub-areas of nearly all combined wastewater sewers, primarily in the early sections and stretches with small gradients. The depositing potential in a sewer network depends on the drag tension which occurs with dry weather as well as with storm weather. The smaller the flow and the sewer gradient are the higher, as a rule, is the tendency to sewer deposits. The base gradients in the overall drainage area of the sewage treatment plant are relevant. As replacement, the area determined terrain slope group IGm (6.1.4) is applied. Together with the dry weather discharge rate qdw24 (6.3.1) and the ratio xd from the daily mean Qdw24 (6.2.2) and the daily peak Qdwx (6.2.3) of the dry weather flow dwx24dwd Q/Q24x ⋅= (7.4) the addition for sewer deposits aa can be determined from Fig. 12 or Appx. 4. If, using operational measures, attention is paid that, for example, it can be shown that sewer deposits can be excluded through regular flushing with dry weather flow, then the addition for sewer deposits can be reduced or done away with completely (aa = 0).

Fig. 12: Influence of sewer deposits

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7.1.4 Dimensioning Concentration in the Dry Weather Flow cd The pollution concentration in the dry weather flow has, with average conditions, according to Eqn. (7.1), the value 600 mg/l. In order, with dimensioning, that local conditions can be taken into account, a locally relevant dimensioning concentration is determined from the three significant influence values for heavy polluters, annual precipitation and sewer deposits. )aaa(600c ahpd ++= in mg/l (7.5)

7.1.5 Theoretical Overflow Concentration ccc The combined calculation for the determination of the mean pollution concentration in overflow water takes place with the dimensioning concentration cd from Eqn. (7.5) and the mean mix ratio m according to Eqn. (6.17) )1m/()ccm(c drcc ++⋅= in mg/l (7.6) If the impervious surfaces areas are loaded with a verifiable COD annual pollution load higher than 600 kg/ha then the rainwater discharge concentration cr can be calculated, analogous to the procedure in Chap. 7.1, as 560 mm effective precipitation in the Eqn. (7,6). Annual precipitation amounts deviating from the reference load case are taken into account with the influence factor ah.

7.1.6 Permissible Annual Overflow Rate eo As the unrelieved portion from the rainwater flow sum runs to the sewage treatment plant and loads the lake or river with the concentration ctp, a pollution load balance can be produced together with the mean concentration ccc. From the objective of the stormwater treatment there follows, for average conditions (reference load case) with the given concentrations for the permissible overflow rate:

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Objective equation: rtpo PLPLPL ≤+

rrtpor cVQc)e1(VQ ⋅≤⋅−⋅

with PLo in kg = overflowed annual pollution load from combined wastewater overflows, PLtp in kg = annual pollution load in the stormwater of the sewage treatment plant effluent, PLr in kg = load flushed from the surface through rainwater, VQr in m3 = rainwater discharge sum of an average year, eo = quotient from the overflowed combined wastewater quantity as annual mean and the rainwater discharge sum. Solved with regard to eo there results, after applying the values from Eqn. (7.19) of the determination equation for the permissible overflow rate )70c/(3700e coo −= in % (7.7) With lakes and rivers with a mix ratio MLWQ/Qpx > 100 with MLWQ in l/s = mean low water flow into the lake or river, Qpx in l/s = daily peak of domestic and industrial wastewater flow to the sewage treatment plant the permissible annual overflow rate eo may be increased by a factor which increases linearly from the value 1.0 with MLWQ/Qpx = 100 up to the value 1.2 with MLWQ/Qpx ≥ 1000. The permissible overflow rate eo is a theoretical value which has been derived from the reference load case with hPr = 800 mm. The influence of other annual precipitation amounts is taken into account with the value ah in Eqn. (7.5). The actual overflow rate e in a drainage area deviates more or less according to the local precipitation conditions from the dimensioning value eo.

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7.2 Necessary Total Storage Volumes

Fig. 13: Specific storage volume in dependence on the rainwater run-off rate and the permissible overflow rate In order to be able to maintain the permissible overflow rate there needs to be a certain storage capacity in the sewer network. The specific storage volume VS can be taken from Fig. 13. Also needed for this is the rainwater run-off rate qr (6.3.2) from the dimensioning discharge of the biological treatment stage and from the associated total catchment area. The necessary storage space results as iS AVV ⋅= in m3 (7.8) In Appendix 3 there is a form with which the necessary total storage volume can be determined manually. In addition, in Appx. 4, approximation formulas for the diagram in Fig. 12 (influence of sewer deposits) and Fig. 13 (Specific storage volume). A required specific total storage volume of 40 m3/ha represents, in general for water management and economic reasons, an upper limit. If, from dimensioning in accordance with Chap. 7, a higher value is necessary the reasons for this are to be given and, again, all possibilities according to Chap. 4 are to be exhausted in order to reduce the loading of the lake or river. If the sewage treatment plant can accept a rainwater run-off rate of more than 2 l/(s.ha) or, despite all efforts in accordance with Chap. 4 for the reduction of the loading of the lake or river, a specific storage volume of over 40 m3/ha is necessary, then the scope of application of Fig. 13 is exceeded. The determination of the necessary total storage volume then takes place iteratively with a verification procedure according to Chap. 8.2 in the following manner:

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- selection of the local applicable precipitation loading (Chap. 8.2.1.1), - calculation of the mean stormwater pollution cr from 600 kg/ha COD annual load and the effective

precipitation, - registration of the sewer network (Chap. 8.2.1.1), - first estimate of the necessary total storage volume; precipitation run-off simulation with this volume

as central tank (Chap. 8.2.2.1), - determination of the mean overflow inflow Qro according to Eqn. (6.11) and the mean mix ratio m

according to Eqn. (6.17), - determination of the theoretical overflow concentration ccc according to Eqns. (7.5) and (7.6) ignoring

the influence of the annual precipitation (ah = 0), - determination of the permissible overflow rate eo according to Eqn. (7.7) taking into account the

upper theoretical stormwater pollution cr - comparison of the actual theoretical rate with the permissible overflow rate; if required, iterative

improvement of the necessary total storage volume until both values agree with each other.

7.3 Accountable Storage Volumes In the simplified distribution procedure in accordance with Chap. 8.1 the following storage spaces can be counted on top of the total volume: - stormwater tanks with overflow up to 1.2 qr (6.3.2). On this scale the negative influence (eg.

lengthening of the overflow duration with subsequent overflow tanks) is still to be accepted even without verification process according to Chap. 8.2,

- with stormwater activatable storage volume at sewage treatment plants, eg. in the retention capable primary settling tank with tank overflow into the inflow,

- additional storage space which can be activated through movable weir sills, - static sewer volume above stream of stormwater tanks with overflow - also storage sewer volumes -

under the horizontal in the height of the lowest overflow crest, reduced in accordance with Eqn. (9.6) to the theoretical value of

Vs = (Vstat/Ais)/1.5 in m3/ha (7.9) with Vstat in m

3 = static sewer volume in sewers as a rule from DN 800 or corresponding cross-

section area (water volume below the horizontal in the height of the lowest overflow crest), Ais in ha = impervious surface area of the associated part catchment area (6.1.2). In the verification procedure in accordance with Chap. 8.2 all storage spaces are included, unreduced, into the precipitation simulation model as they are actually available.

7.4 Minimum Storage Volumes In order to be able to achieve sufficient settling effect in stormwater tanks with overflow for settled combined wastewater minimum retention times in the annual mean should be maintained during all relief overflows. For this, the mean rainwater run-off during the overflows, Qro according to Eqn. (6.12), is used for the site of the sewage treatment plant without taking into account the reduction of flow time (af = 1.0). With the rainwater run-off rate qr for the sewage treatment plant there results the specific minimum volume for 20 mins retention time from the mean rainwater run-off during the overflows of Vs,min = 3.60 + 3.84 qr in m3/ha. (7.10) If the combined wastewater discharge Qcw of the sewage treatment plant is more than 2Qdwx, then the rainwater run-off rate qr in Eqn. (7.10) can be limited to the value which results from 2Qdwx: qr = [(48/xd - 1).Qdw24 - QrS24]/Ais in l/(s.ha) (7.11)

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For all stormwater tanks with overflow in the catchment area of the sewage treatment plant the same specific minimum volume is to be applied. Verified sewer storage volume can be calculated on the minimum volume. In this case the inflow sewer is to be treated as a sewer with storage capacity and overflow with bottom-end overflow. In each case the treatment conditions for the stormwater tanks with overflow (Chap. 9.2) or sewers with storage capacity and overflow (Chap 9.3) as well as the requirements on the minimum mix ratio (Chap. 9.2) are to be maintained.

8 Dimensioning of Individual Structures with Overflow The dimensioning of the stormwater overflow installations is completed in three steps: - determination of the necessary total storage volume (Chap. 7), - volume determination of individual stormwater tanks with overflow and sewers with storage capacity

and overflow using a simplified distribution procedure (Chap. 8.1) or verification procedure (Chap. 8.2),

- dimensioning of individual overflow structures according to normal requirements (Chap. 9). If the scope of application of the simplified distribution procedure is exceeded, then it must be verified for the planned measures that the objective of this Standard is being observed. The verification procedure in Chap. 8.2 serves for this purpose. The normal requirements on individual overflow structures (Chap. 9) are to be maintained in any case. Attention is drawn to Appx. 1 for advanced requirements on the rainwater treatment.

8.1 Simplified Distribution Method

8.1.1 Approach The approach with the dimensioning of individual stormwater tanks corresponds with the determination of the necessary total storage volume (Chap. 7). At each stormwater overflow of a combined wastewater network a certain total volume for combined wastewater storage must be available for the upstream catchment area. The dimensioning parameters for the establishment of the permissible overflow rate according to Chap. 7.1 are ascertained respectively for the total catchment area above the tank in question. The associated discharge Qt here corresponds with the throttle discharge of this tank which can be approximated as the mean value of the discharges at the beginning of retention and at the beginning of overflow. After the establishment of the permissible overflow rate the necessary storage requirement according to Chap. 7.2 can be determined for the total catchment area lying upstream. If one removes from this the already available accountable storage volume then one obtains the necessary size of the overflow tank in question. It is to be investigated whether this volume is sufficient to maintain the treatment conditions and that the necessary minimum volume according to Chap. 7.4 is not undercut.

8.1.2 Scope of Application In order to be able to apply the simplified dimensioning using the distribution of the total volume to individual structures the following scope of application must be observed. If this is not possible then a verification procedure in accordance with Chap. 8.2 must be carried out in order to be able to take into account the no longer negligible influence on the overflow which occurs with the exceeding of the scope of application. - The rainwater run-off rate qr (Chap. 6.3.2) of the sewage treatment plant may not exceed 2 l/(s.ha). - The rainwater run-off rate qr for the upstream total catchment area of a stormwater tank with

overflow may not be greater than 1.2 times the rainwater run-off rate of the sewage treatment plant. - There may be, as a maximum, five stormwater tanks with overflow connected in series as, with

more, the inaccuracy of the simplified distribution procedure becomes too great. - Throttle discharges from stormwater overflows must be at least as large as given in this Standard. - The number of stormwater overflows in the catchment area of an overflow tank may not be greater

than five as with more than these the accuracy of the simplified distribution procedure is too great.

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- Stormwater holding tanks within the catchment area in question must show a rainwater run-off rate of at least qr · 5 l/(s.ha). Its volume, in the simplified procedure, does not count towards the necessary total storage volume. It can only be taken into account in the verification procedure (Chap. 8.2).

- The necessary specific storage volume Vs may not exceed 40 m3/ha.

8.2 Verification Procedures

8.2.1. Special Basic Facts Verification procedures are to be employed when the scope of application of the simplified distribution procedure according to Chap. 8.1 can no longer be observed. The determination of the necessary total storage volume according to Chap 7.1 is, however, prerequisite for the maintenance of the normal requirements. Assuming that the total storage volume is a single fictitiously arranged central tank upstream of the sewage treatment plant, the model specific annual COD load, relieved as long-term mean from the total network, is calculated using the verification procedure in a preliminary calculation. In subsequent planning calculations an optimisation of the rainwater treatment measures can be carried out, whereby the previously calculated COD overflow load for the central tank may not be exceeded.

8.2.1.1 Precipitation Loading The verification procedures are to be based on long-term series of rainfall which show the best possible relationship to local conditions. The rainfall series should cover a time period of at least ten years and represent, from statistical aspects, the basic entirety of the local precipitation activity. In general, the actual task setting allows the long-term precipitation series to be replaced by a suitable rainfall series or a suitable rainfall spectrum. With this, it is to be verified that, with the replacement loading, the local overflow behaviour, in comparison with the long-term simulation, can be described accurately. In opposition to this, the description of the precipitation activity is of less significance. Further detail of this is contained in the Report of ATV Working Group 1.9.3 (1989).

8.2.1.2 Registration of the Sewer Network In normal cases, for economic reasons, the long-term simulation can not be carried out on a detailed network (by section). Therefore, as a rule, a coarse network which is derived from the detailed sewer network must be produced for the pollution load calculation. With this, in the first instance, the main collector sewer is taken into account and secondary collection areas with comparable conversion behaviour summarised into sub-areas. The subdivision of the area should cover the available and possible structure sites already in the preliminary calculation. As a rule, the reproduction of the total catchment area of a sewage treatment plant with a single transfer function is not permissible. Separate areas are dealt with as follows: - the dry weather discharge from separate areas, which is made up from the domestic and industrial

wastewater effluent and the sewer infiltration water effluent is treated as single discharge into the combined wastewater network. Both dry weather discharge components should, as far as possible, be derived from measurements (see Chap. 6.2.2),

- the stormwater portion from separate areas which reaches the combined wastewater network via the domestic and industrial wastewater network can be represented in that the rainwater run-off simulates the relevant impervious surface areas of a separate area and a fictitious stormwater overflow (separate element) is added. The throttle discharge of this stormwater overflow is so determined that the rainwater part, as far as possible determined from measurements with rainfall, is transferred onward and discharged into the combined system. If no measurements are available the hourly peak discharge with dry weather Qdwx from the separate area is selected as throttle discharge.

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The hydraulic equivalence of the coarse network and detailed sewer network is to be shown in a suitable manner. For example, in simple cases, the flow time in all catchment sub-areas should be of comparably size or, for example, the overflow volume as a result of annual dimensioning rain should be comparable at all structures for coarse network and fine network.

8.2.1.3 Fictitious Central Tanks The total volume determined according to Chap. 7.2 is arranged centrally in the by-pass, before the sewage treatment plant, as stormwater tank with overflow for settled combined wastewater and without tank overflow structure, sufficiently deep that no sewer volume can be activated (central tank). The throttle discharge of the central tank corresponds with the inflow to the biological treatment stage of the sewage treatment plant.

8.2.2 Approach Dimensioning applying the verification procedure is carried out in the following steps: - preliminary calculation for the determination of the permissible model-specific COD overflow load for

a fictitious central tank, - determination of the rehabilitation requirement, - planning of measures, - verification that the permissible overflow load, determined in the preliminary calculation, is not

exceeded. The same model formulations and precipitation loadings are to be applied for all calculation variants.

8.2.2.1 Preliminary Calculation to Determine Permissible, Model-Dependent, Overflow Loads

The preliminary calculation takes place with the pollution load model used in the verification procedure. Formulations for accumulation (contamination collection on the surface and in the sewer with small drag tension), removal of deposits and settling effect in drains and storage spaces as well as formulations for flushing surges with increased concentrations at the start of rainfall are permitted only with the agreement of the controlling authority. This regulation is necessary until assured information and generally accepted rules are available for the actual processes in the sewer network and storage spaces. For the preliminary calculation the coarse network is to be so modified that the annual combined wastewater quantity arising in the catchment area of the sewage treatment plant is fed to the central tank, completely and without back-up. For this one proceeds as follows. Every overflow is included in the calculation in that the throttle discharge in the preliminary calculation is set so large that even peak discharges can be transported, free of overflow and back-up, in the mainstream through the structures (SO, STO, SHT), or past them in the by-pass, to the central tank. Through this resultant or, in any case, existing overloading of sewer lines is to be removed mathematically by sufficient enlargement of the sewer cross-section. The necessary cross-section enlargements for back-up-free discharge of the annual combined wastewater volume can, for example, be estimated for the annual dimensioning rainfall. The overflow structure at the centrally placed stormwater tank with overflow for settled combined wastewater in the by-pass thus represents, in the preliminary calculation, the sole overflow to the lake or river. The locally dependent influences of separate areas, heavy polluters and the annual precipitation must be observed according to the condition to be investigated in the preliminary calculation. Only the discharge conditions in the network are so corrected that back-up-free discharge to the central tank is guaranteed. The preliminary calculation with the central tank gives the model-dependent COD annual overflow load. This parameter serves as objective parameter with the dimensioning of all planning or optimisation variants. It may no longer be exceeded.

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8.2.2.2 Determination of the Rehabilitation Requirement The determination of the rehabilitation requirement assumes the determination of the available loading of the lake or river through the application of the verification procedure on the actual status (see Chap. 5.1). In this procedural step the characteristics of the catchment area (sewer system and sewage treatment plant) are to be recorded with all characteristic values as are necessary for the application of the total storage determination according to Chap. 7 and the application of the model. The pollution load calculation gives the theoretical loading of the lake or river in the actual status. The predetermined characteristic values are to be documented. With this one is concerned with operands which serve for the comparison with the theoretical permissible loading of the lake or river in accordance with the preliminary calculation (Chap. 8.2.2.1). The comparison of the operands allows the assessment of stormwater treatment measures independent of the applied verification method and independent of the selected parameter formulations.

8.2.2.3 Planning of Measures With the planning of individual measures in the network the rules and notes given in Chap. 4 are to be observed. The lessening effect of alternative measures on the overflow load can be fully accounted for insofar as it is verifiable. For the actual drainage system, that is taking into account all influences which were suppressed in the preliminary calculation, the verification is to be implemented so that the overflowed COD annual load does not exceed the value from the preliminary calculation. In the verification procedure the overflowed COD annual load calculated for sewers with storage capacity and bottom-end overflow without depositing is increased by 15 % compared with stormwater tanks with overflow for settled combined wastewater. Through this global addition a smaller depositing effect in comparison with stormwater tanks with overflow for settled combined wastewater and a higher pollution load in the sewage treatment plant effluent due to longer emptying durations of the volumetrically larger sewers with storage capacity and overflow are taken into account. Should a theoretical depositing effect in the storage spaces be applied, in agreement with the supervisory authority, the percentage figure of the applied depositing effect in the sewers with storage capacity and bottom-end overflow, in comparison with the depositing effect in the stormwater tanks with overflow for settled combined wastewater, must be 10 % lower (eg. 15 % in stormwater tanks with overflow for settled combined wastewater and 5 % in sewers with storage capacity and bottom-end overflow). In this case an increase of the theoretical overflowed COD annual load of 15 %, as described above, is not necessary. Depositing effect is understood to be the reduction of the COD concentration in the overflowed combined wastewater in percent. If alternative measures for the construction of storage volumes in accordance with Chap. 4 are brought into play which lead to a system change in comparison with the initial status which was considered in the preliminary calculation (eg. change of the degree of paving) then an adjusted preliminary calculation, taking into account this system change, is to be carried out. The results of this preliminary calculation are relevant for the assessment of the individual measures. If, in the prognosis data, there are considerable risks with regard to planning safety and associated with serious effects on the stormwater treatment measures, then verifications, including the preliminary calculation also for planned intermediate conditions (eg. rehabilitated actual status) are to be carried out. If, for the planning status, a considerably larger requirement for storage volume is necessary then the measures are to be so planned that the requirements on the actual status are realised for an appropriate transitional period.

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The following verifications are to be carried out for each individual measure: a) observation of the minimum mix ratio according to Chaps. 9.1 and 9.2 b) observation of the wastewater treatment conditions in accordance with Chap. 9 c) verification of the theoretical emptying times d) observation of the minimum volume according to Chap. 7.4 e) verification of the theoretical overflow characteristic figures according to Chap. 11.2.2.3, Table 2. All verifications are to be documented clearly and comprehensibly. The load sum of all individual overflows within the catchment area of a sewage treatment plant may not exceed the permissible, model-dependent COD overflow load from the preliminary calculation. The sum of all individual tank volumes may exceed or fall short of the necessary total storage volume in accordance with Chap. 7.2 insofar as the above-given verifications are met. Should the sum of all individual tank volumes of the planning calculation deviate considerably from the necessary total storage volume in the network then this is to be justified.

8.2.2.4 Further Verification Parameters For the assessment of the water management situation and advanced requirements on the stormwater treatment, further parameters can be determined with the aid of verification procedures, from the analysis of the combined wastewater system taking into account - given planning details (development planning, usages), - local conditions (implementability), - economy (cost effectiveness). These could be, for example: - overflow discharge sum, - overflow frequency, - overflow duration, - overflow load, - overflow concentration, - hydraulic loading of the lake or river with certain frequency, in each case as annual mean value for the total system and for individual structures. The terms given are to be found in more detail in the Report of ATV Working Group 1.9.3 (1989). Using the verification it is to be established whether the theoretical overflow behaviour at the individual structures is in the best possible relationship to the acceptance capacity of the receiving lake or river. Which criterion is to receive a particular priority is to be determined based on the real characteristics of the lake or river. The taking into account of the residual load of the rainwater run-off part, which must also be treated in the sewage treatment plant, can also be significant for the assessment of the conception of the stormwater treatment measures.

8.2.3 Requirements on Pollution Load Calculation Methods For the implementation of verification described in Chaps. 8.2.1 and 8.2.2 there are various pollution load calculation methods available (see Appendix 2): A. Hydrologic - empirical method B. Hydrologic - deterministic models C. Hydrologic - hydrodynamic - deterministic models

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The selection of the suitable method depends essentially on the local overflow conditions and, from these, whether the method can be matched to the local conditions, i.e. can be calibrated using available precipitation discharge and concentration measures. The selected method, the verification parameters and calculation formulations are to be agreed well in time between customer, planner and test authority. The quality of the calculation method cannot fundamentally be assessed on the quality of the applied model module alone. rather it is determined by to what extent area-dependent characteristics can be taken into account and the model parameters of locally measured data can be calibrated for the actual application case. Relevant for the quality of the calculation results is, in addition, the care with the involution of the initial data. The dimensioning of the individual measures with verification procedures is based on a comparison between the theoretical effects of the planning variants and the preliminary calculation. The following belong to the minimum provisions of verification procedures: - sufficient detailed consideration of the catchment area and the drainage system (area sub-division), - consideration of the local precipitation events in the form of statistically derived or measured

precipitation data in accordance with Chap. 8.2.1.1, - simulation of the discharge formulation taking into account past events, - simulation of the discharge concentration for impervious surface areas including the auxiliary

collector sewers in the sub-areas, - taking account of the locally dependent dry weather discharge and its properties at least as daily

mean value, - simulation of the discharge conveyance into the main collector sewers taking into account translation, - simulation of the material transport in the main collector sewers according to the combination

formulation with chronological overlapping of the rainfall and dry weather discharge and the respective load components,

- simulation of the discharge and solids load distribution at all normal types of overflow structure taking into account the pre-filling and balancing of the input, further transported and relieved discharge sums and solids load,

- clear and comprehensible documentation of input data, the model formulations applied, the model parameters and the calculation results.

9 Dimensioning of Individual Structures with Overflow 9.1 Stormwater Overflow In order to avoid an excessively large pollution input into individual sections of the lake or river the stormwater overflows must be designed with a minimum critical rainfall intensity between rcrit = 7.5 and 15 l/(s.ha). The values for the critical rainfall intensity are to be determined dependent upon the flow time from Fig. 14 or from the following equation: rcrit = 15.120/(tf + 120) in l/(s.ha) (9.1) for tf µ 120 min rcrit = 7.5 l/(s.ha) for tf < 120 min with tf in min = longest flow time up to the stormwater overflow from the immediate catchment area without taking into account the flow time in pure transport collector sewers. The discharge to be further transported Qt results from the associated intermediate catchment area and all upstream throttle discharges (Chaps. 6.2.6 and 6.2.7). Should throttle discharges Qt,i from upstream stormwater overflows show, for example for design reasons, higher values than necessary in accordance with Eqn. (6.10), then only the necessary value from Eqn. (6.10) needs to be applied with downstream series stormwater overflows (see example Chap. 11.3.2).

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Fig. 14: Critical rainfall intensity dependent on flow time Minimum mix ratio: If, with commencement of overflow, there is a mix ratio between the rainfall and dry weather component parts below 7 in the critical discharge then the mix ratio MSO is the basis for the stormwater overflow. If the mean COD concentration in the dry weather discharge lies above 600 mg/l then the minimum mix ratio MSO is to be increased in order to achieve a greater dilution MSO = (Qt - Qdw24)/Qddw24 (9.2) MSO · 7 for cdw µ 600 mg/l MSO · (cdw - 180) for cdw > 600 mg/l with Qt in l/s = throttle discharge with commencement of stormwater overflow from Eqn. (6.10) Qdw24 in l/s = daily mean value of the dry weather discharge (6.2.3) cdw in mg/l = mean COD concentration in the dry weather discharge from measurements or from Eqn. (6.16). Daily mean value Qdw24 and concentration cdw are to be determined for the complete upstream catchment area. The minimum throttle discharge derives from Eqn. 9.2 as Qt = (MSO + 1).Qdw24 (9.3) It is relevant if it exceeds the value according to Eqn. (6.10). Wastewater treatment condition: If sewer storage capacity is available upstream of the stormwater overflow which can be activated by a lifted sill with rainfall, then the throttle discharge may be reduced in comparison with the above given dimensioning value only if the minimum mix ratio and the wastewater treatment conditions for the sewer with storage capacity and bottom-end overflow can be observed (Chap. 9.3.2).

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9.2 Stormwater Tanks with Overflow Stormwater tanks with overflow must have at least a volume which meets Chap. 7.4. With stormwater tanks with overflow for settled combined wastewater the treatment condition given below is to be met. For design reasons stormwater tanks with overflow for settled combined wastewater of less than 100 m3 and stormwater tanks retaining the first flush of stormwater of less than 50 m3 are to be recommended. The theoretical emptying time of stormwater tanks with overflow as quotient of the specific storage volume VS and the related ran-off discharge rate qr should not exceed 10 to 15 hours. If this is not possible the design and operation notes from Chap. 10 are to be observed. Minimum mix ratio: It is to be examined for each individual structure whether, in the long-term mean, a minimum mix ratio in accordance with Eqn. (6.17) of MSTO ≥ 7 is maintained. If the mean COD concentration in dry weather lies above 600 mg/l then the minimum mix ratio is to be raised in order to achieve greater dilution. MSTO ≥ 7 for cdw µ 600 mg/l (9.4) MSTO ≥ (cdw - 180)/60 for cdw > 600 mg/l with cdw in mg/l = mean COD concentration in the dry weather discharge from measurements or Eqn. (6.16). In the simplified subdivision procedure the mean mix ratio MSTO is calculated according to Eqn. (6.17). In the verification procedure the mean mix ratio at an overflow structure can be added from the results of a long-term simulation as follows: M = (cdw - ccc)/(ccc - cr) (9.5) with ccc = PLo/VQo cr = PLr/VQr VQo in m3 = combined wastewater quantity overflowed as annual mean and the descriptions in Chap. 7.1.6. An undercutting of the permissible mix ratio can be avoided if, for example, heavily polluted wastewater from commercial and industrial concerns are fed past overflow structures, a reduction of the amount of wastewater is sought or the pre-treatment of heavily polluted wastewater is carried out. Should a significant undercutting be unavoidable advanced measures are to be examined before the discharge of the overflow water into the lake or river. In general, in such cases, a verification procedure in accordance with Chap. 8.2 is necessary. Wastewater treatment condition: In rectangular stormwater tanks with overflow for settled combined wastewater the surface feed rate with a non-reduced critical rainfall intensity of 15 l/(s.ha) should not exceed the value 10/h. In the plug-flow part of a composite tank (Chap. 4.3.2.3) here only the discharge from the non-immediate part of the catchment area is taken into account. With stormwater tanks with overflow for settled combined wastewater one can assume, without particular verification, that a sufficient safety against sludge eddying is provided if the cross-section of the tank is so determined that, with rainfall with non-reduced rainfall intensity of 15 l/(s.ha), the mean horizontal flow velocity in rectangular tanks is not essentially more than 0.05 m/s. The length of a rectangular tank should, in the flow direction, be at least twice the tank width. If a stormwater tank with overflow is divided into individual chambers then this applies for these individual chambers.

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Circular stormwater tanks with overflow for settled combined wastewater with tangential inflow, central discharge and overflow structure for settled combined wastewater at the circumference (Fig. 17) which are designed in accordance with the initial details of Chaps. 10.2.2 and 10.3.2 can be dimensioned with the same surface feed rate of 10 m/h without verification of the flow velocity. In order that the treatment conditions can be observed it is, as a rule, necessary to limit the tank inflow through the tank overflow. However if the discharge can take place over the overflow structure for settled combined wastewater without essentially exceeding the treatment conditions or if the tank overflow comes into action only seldom (less than 10 times annually) then one can dispense with a tank overflow.

9.3 Sewer with Storage Capacity and Overflow

9.3.1 Sewer with Storage Capacity with Top-end Overflow Sewers with storage capacity and top-end overflow, as a rule, are dimensioned as stormwater tanks retaining the first flush of stormwater insofar as the conditions for these stormwater tanks can be met in accordance with Chap. 4.3.2.1. Otherwise they are to be treated as sewers with storage capacity and bottom-end overflow. They are also suitable for storage volumes below 50 m3.

9.3.2 Sewer with Storage Capacity with Bottom-end Overflow Sewers with storage capacity and bottom-end overflow in the simplified distribution procedure receive a supplement due to their poor settling effect. The specific storage volume VS is to be determined as for stormwater tanks with overflow VSSCBO = 1.5.VS

.Ais in m3 (9.6) with VS in m3/ha = specific storage volume (7.2) Ais in ha = impervious surface area of the associated catchment sub-area (6.1.2) In the verification procedure the specialities for sewers with storage capacity and bottom-end overflow are to be observed in accordance with Chap. 8.2.2.3. The theoretical emptying duration of sewers with storage capacity and overflow should not exceed 15 hours. The minimum mix ratio is to be maintained as for stormwater tanks with overflow. Wastewater treatment condition: In sewers with storage capacity and bottom-end overflow the horizontal flow velocity, with an unreduced critical run-off discharge rate of 15 l/(s.ha), should not exceed 0.30 m/s at the start of the structure. A smoothing stretch of sufficient length is to be provided in front of the structure with overflow, eg. through a gradual widening in the ratio 1:10. If this velocity cannot be maintained with existing installations, eg. with the back-up of sewers, then it is to be decided, within the sense of this Standard, whether the existing conditions are still sufficient.

9.4 Stormwater Holding Tanks Stormwater holding tanks are not dimensioned according to this Standard. Their effect on subsequent structures with overflow depends on the run-off discharge rate. Stormwater holding tanks, in the simplified distribution procedure, remain unconsidered in the dimensioning of overflow tanks if their throttle discharge rate is greater than qr > 5 l/(s.ha). Their volume in this case cannot be added to the subsequent storage volume.

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Stormwater holding tanks with throttle discharge rates below 5 l/(s.ha) considerably influence the subsequent structures with overflow. in these cases the volume distribution with the simplified dimensioning procedure is no longer satisfactory; a verification in accordance with Chap. 8.2 must be carried out. In the verification procedure stormwater holding tanks, other than in the preliminary calculation, are taken into account with full volume and actual throttle discharge.

10 Construction and Operation of Structures With Overflow Planning principles for the structures and their arrangement in a sewer network are contained in Chap 4.3. Details of construction methods, maintenance and operation are dealt with below. The structures should be so designed that, with dry weather discharge, no deposits can form. The relief sewer is to be dimensioned for the greatest possible discharge from the upstream overdammed sewer network in order to keep the structure with overflow free from back-up from the relief sewer with an exceeding of the theoretical rainfall. With this, the possible blockage of the discharge sewer, with small diameters of the discharge sewer, is to be taken into account. With larger diameters the throttle discharge of the discharge sewer can be put into the calculation. The discharge sewer to the sewage treatment plant should, as a rule, have a diameter no smaller than 0.30 m. In discharge sewers the installation of a control or exchangeable throttle device should be planned if the further transported discharge is to be matched to the respective actual status of the sewage treatment plant. The efficiency of the throttle is to be verified before being taken into service. A higher selectivity of the discharge can, as a rule, be achieved through controlled slide valves, eddy throttles or similar adjustable throttle devices. Even differing development statuses can be taken into account through this or through a modification of the weir crest using added sills. Longer throttle stretches are not recommended for this. If possible the overflow is to be kept free from floating material through scum boards. For the later monitoring of the effect of structures with overflow the necessary free spaces and empty pipes are to be provided for the installation of measuring equipment.

10.1 Stormwater Overflows

10.1.1 General In order to be able to design stormwater overflows hydraulically correctly the remaining discharge in the sewer should be at least 50 l/s. The connected impervious surface area Ai (6.1.2) should not be less than 2 ha. In addition the flow velocity in the sewers of the inflow and discharge areas, with dry weather, should be at least 0.50 m/s. With smaller flow velocities attention is to be paid to a sufficient flushing effect. With new construction of a stormwater overflow, with a satisfactory gradient immediately below the stormwater overflow, a sufficiently large bottom step should be placed so that a possible, later necessary stormwater tank can be operated with free flow. Lateral junctions in the area of the stormwater tank with overflow are to be avoided in order to maintain hydraulically calculable flow conditions. Attention is to be given to air feed to the stormwater tank with overflow and at the overflow weir. The hydraulic calculation of stormwater tanks with overflow is determined in ATV Standard A 111 (in preparation).

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10.1.2 Method of Construction of Stormwater Overflows with Overflow Weirs A Stormwater overflow with overflow weir is to be provided if the rainfall run-off lies in the laminar regime. With turbulent discharge there is usually a transition point in or before the stormwater overflow so that the overflow conditions cannot be mathematically recorded. Through smoothing stretches with reduced base gradients or suitable braking stretches a laminar discharge at Qcrit can be enforced. These smoothing stretches are not necessary if the overfall crest is raised up at least to the pipe crown of the inflow sewer. Fig. 15 shows a diagram of a stormwater overflow with one-sided, raised weir. The throttle must feed the dry weather discharge Qdwx into the inflow sewer without backing-up. Therefore, a greater base gradient is to be arranged within the stormwater overflow by reduction of the cross-section. In exceptional cases this can also be achieved through a step in the base. The base of the discharge sewer should lie at least 3 cm deeper than the inflow sewer. At Qcrit a back-up due to the throttle may occur up to the height of the overflow crest. The weir crest is horizontal and is to be arranged at least 0.5 m above the crown of the throttle. The overflow crest should be smooth and well rounded. In the interest of a large storage volume (high weir crest) the weir crown should be determined at least the half cross-sectional height of the inflow sewer. With sufficient flushing of the inflow sewer it can be placed as high as the permissible back-up level allows, so that an additional storage space is created. With double-sided overflow the clear space below the throughflow channel should be at least 25 cm high over the whole length. For the most part, due to the throttle effect, the sharp edge formation of the throttle mouth is preferred This should be accessible for cleaning tasks at all times from above (manhole shaft) or from a flooding-free stage.

Fig. 15: Stormwater overflow with one-sided, raised weir

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10.1.3 Method of Construction of Stormwater Overflows with Floor Opening (Spring Overflow) If, due to the large gradient, the rainfall run-off takes place in the stable laminar regime then a stormwater overflow with floor opening is to be provided. The inflow sewer must run in a straight line for a sufficient length.

Fig. 16: Stormwater overflow with floor opening The constructional design emerges from Fig. 16. The base plate may not, for hydraulic reasons, be placed higher than the base of the inflow sewer. The critical combined wastewater discharge Qcrit must flow through the floor opening, without the overflow sewer being acted upon. If an as small as possible extra loading in comparison with Qcrit is required in the discharge sewer to the sewage treatment plant then a flat separation plate is to be selected in the floor opening. With this the flat plate is to be reduced, in a transition stretch, to the cross-section of the overflow channel. Otherwise a curved separation plate matched to the sewer profile can be employed. With very high requirements on the selectivity (extra loading with the greatest possible inflow less than 20 % of Qcrit) an effective throttling is to be planned in the discharge to the sewage treatment plant (see also ATV Standard A 111, currently in draft). The base gradients in the inflow and overflow sewers are to be selected, if possible, with the same size. The length of the floor opening should, with regard to the maintenance, be at least 50 cm. For the fulfilment of these conditions a large base gradient must be available. A growing critical combined wastewater discharge conditioned by the staged expansion of the sewer network requires an extension of the floor opening by the moving back of the drop edge. This can, if required, take place through extension of individual elements (prefabricated components) on the inflow side. In addition the separation plate can be made capable of sliding.

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The drop stream must be ventilated.

10.2 Stormwater Tanks with Overflow

10.2.1 Method of Construction of Separation Structures and Overflows A flow-dividing structure and tank overflow should, if possible, be combined in one structure. With stormwater tanks retaining the first flush of stormwater in the main stream there is only one overflow which is in front of the tank and comes into action only if the tank is filled. It is designated as tank overflow structure (TOS). With stormwater tanks with overflow for settled combined wastewater the overflow, over which the mechanically treated combined wastewater flows into the receiving water, is designated overflow structure for settled combined wastewater (OSC). With the maintenance of certain requirements one can dispense with the tank overflow structure (see Chap. 9.2) Tank overflow structures and flow-dividing structures are basically to be designed as stormwater overflows. However, lateral junctions with sewers into the flow-dividing structure or into the tank flow structure are permissible. A scum board must be sited in front of the overflow structure for settled combined wastewater. If, through a back-up device at the tank overflow, the overflow of the combined wastewater stored in the sewer network and tanks is prevented from overflowing at the tank overflow, then the thus gained storage volume can be added in the determination of the useful volume of the stormwater overflow. The height or type of design of the flow-dividing structure (FDS) do not influence the overflow activity of the tank overflow structure or overflow structure for settled combined wastewater. As long as the stormwater tank with overflow is not filled the combined wastewater inflow will be fed in without the tank overflow to the lake or river activating. The upper edge of the overflow structure for settled combined wastewater determines the theoretical storage volume of the stormwater tank with overflow for settled combined wastewater. It can lie below or above the upper edge of the flow-dividing structure or the base of the inflow. The results of a back-up in the inflow sewer are to be watched. The flow velocity in the inflow should, therefore, with Qdwx, be as far as possible greater than 0.8 m/s in order, again, to remove deposits. This applies for an expansion circumstances from taking into service up to the planning level. The tank overflow structure may, with filled stormwater tank with overflow for settled wastewater, first activate with rainfall whose run-off lies above the required critical combined wastewater discharge (see Chap. 9.2). The upper edge of the tank overflow structure lies at least at the height of the overflow structure for settled combined wastewater hOSC with critical combined wastewater overflow over the upper edge of the overflow structure for settled combined wastewater. In addition, it is recommended so to design the inlet that an even distribution of the inflow takes place and the turbulence in the tank is kept as small as possible. In order to achieve a good treatment effect at the overflow structure for settled combined wastewater, as long as possible overflow edges and a small overflow height should be sought. The overflow edges are, if required, to be arranged in stepped height in order better to be able to measure the overflow event. In opposition to stormwater overflows smoothing stretches are not necessary before the flow-dividing structure and the tank overflow structure, however, for operational reasons and for monitoring with water level measurements, they are recommended.

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10.2.2 Method of Construction of Stormwater Tanks with Overflow With the planning and construction of stormwater tanks with overflow, attention is to be paid to the local and hydraulic conditions as well as to the later operating behaviour, to the maintenance and to the monitoring. Amongst other reasons, the favourable cleaning and maintenance possibilities, the simple control and, possibly, the smaller construction costs favour stormwater tanks with overflow in open construction. Within residential areas it is, inter alia, for hygienic and conservation reasons, often appropriate to favour closed tanks. In addition, efforts should be made that the content of the tank after filling drains in free flow. If this cannot be achieved then at least the continuously flowing dry weather discharge should be further conveyed without it having to be raised. With the design of stormwater tanks with overflow the following is basically to be observed: - with staged tank extension or sub-division of the total volume it is, for operating reasons,

advantageous that the tank is so designed that the individual chambers are filled one after the other, - the rectangular stormwater tanks with overflow with flat floor and without flushing facility should have

a longitudinal gradient of at least 1.0 % (better 2 %) and the lateral gradient is 3 to 5 %. With circular tanks the gradient to the middle of the tank should be at least 2 %. Deposits are to be removed regularly. Therefore cleaning and/or flushing facilities are to be planned. Automatic flushing devices influence the tank design,

- if the dry weather inflow in the main stream is fed right through the tank an individual channel is to be provided for this which is dimensioned for at least 3.Qpx + Qdw24,

- the base drop at the discharge sewer should be at least so large that there continues to be no back-up in the inflow channel with the theoretical tank discharge,

- stormwater tanks with overflow for settled combined wastewater are to be so designed that conditions favourable for laminar flow are achieved. The relatively small retention times require an even distribution of the inflow within the tank. This can be achieved through appropriate inlet and outlet design,

- tangentially fed tanks should have a central throttle discharge and are to be so designed and equipped that deposits are extensively avoided on emptying the tank. With stormwater tanks with the overflow structure for settled combined wastewater at the tank circumference, this is to be arranged in 4 quadrants (in an emergency in 3 quadrants) (Fig. 17),

Fig. 17: Functional diagram of a circular stormwater tank with overflow for settled combined

wastewater

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- tanks with which transverse or short circuit flow degrades the settling effect should not be employed as stormwater tanks with overflow for settled combined wastewater - with open tanks fencing off is necessary. Raised containing walls or side rails are to be planned at

the tanks. The relevant construction and accident prevention regulations are to be observed, - with closed tanks, due to the formation of deposits, the roof should not be covered in, - in order to avoid the accumulation of health damaging and potentially explosive substances closed

tanks are to be provided with ventilation openings. These openings can, at the same time, serve as air exits on filling of these tanks and for light exposure. Within residential areas the air exhaust pipes should be made as high as possible. In special cases forced ventilation is necessary,

- with segmented floors it is recommended that a serpentine channel is preferred to ridged floors. With this the minimum velocity of 0.80 m/s with dry weather discharge Qdwx should be maintained in the individual channels. The hydraulic loss of height should be accounted for with 0.01 to 0.02 m per bend,

- the tanks are to be satisfactorily exposed, - electrical installations in closed, wastewater carrying spaces are to be explosion protected, - the tanks are to be designed with good access. Rescue routes must be capable of easy access.

10.2.3 Method of Construction of Sewers with Storage Volume The weir crest of the tank overflow is placed as high as the back-up with maximum inflow allows. The profile of the inflow sewer is continued with at least the same cross-section. Circular cross-sections with more than 1.5 m diameter or other cross-sections with a similar profile width and heavily angled floor surface are practical. For the reduction of deposits the flow velocity with the dry weather discharge peak should not be below 0.80 m/s and the water depth should be over 0.05 m. The drag tension should be 2 to 3 N/m2, however, not less than 1.3 N/m2. With flow velocities below 0.50 m/s a possibility of flushing must be ensured. As tank overflow at the overflow point a high placed weir crest which must lie above the static sewer volume comes into consideration. Discharge regulating slide valves or other suitable throttling devices are arranged at the storage space end. Through these the discharge to the sewage treatment plant is limited with rainfall.

10.2.4 Method of Construction for Discharges The tank discharge can be limited by suitable throttle installations, eg. eddy throttles, controlled throttle flaps and slide valves or pumps and/or through long throttle stretches. Throttle stretches have the disadvantage that they are no longer adjustable. Therefore they should only be provided in exceptional cases. To avoid sludge deposits an as large as possible floor step should be provided at the outlet. Its depth is based on the selected throttle device. Through this the discharge losses in the discharge sewer are balanced. Throttle stretches are to be so designed that the maximum permissible discharge with safety may not be exceeded. With consideration of the danger of blockage the pipe diameter is not to selected smaller than 0.30 m. In special cases and if, through increased operational monitoring a danger of blockage can be excluded then the diameter may be reduced to 0.20 m. Throttle sliding valves can be arranged as individual sliding valves or as several at different heights. The outlet cross-section in unregulated throttle slide valves should be at least 0.06 m2 and have a minimum opening height of 0.20 m. as throttle slide valves do not serve primarily as closing devices they max be placed under water. An approximately constant discharge can be achieved through controlled throttle installations, eddy throttle or other control devices. With emptying by pumps a regulation of the discharge can be achieved which approximates to that of throttle slide valves. With the selection of pump an as constant as possible discharge is to be sought. In addition the following is to be observed:

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- the emptying pump is to be equipped with sufficiently large opening in order to avoid blockages and,

if necessary, to allow flushing of the tank (opening ≥ 100 mm), - the tank is to be emptied as rapidly as possible, - the pressure pipeline of the pumps should, as a rule, junction with the sewer so that the theoretical

throttle discharge is not exceeded. A flushing of the tank is made possible through circulation flow or other flushing aids.

The areas of application of various throttle devices are very different. With low throttle discharges (eg. below 30 l/s) attention is to be paid to depositing. The throttle installation is to be so designed that an approximately constant discharge is achieved and the maximum permissible discharge with all water levels in the tank is not exceeded. It must be matched to the respective development status of the sewage treatment plant. Verification can be carried out with the aid of a discharge-water level diagram. It is necessary for the maintenance of the objective of this Standard to examine the discharge for acceptance through a simple, local measurement. For reasons of economy as well as for the avoidance of large variations in discharge, in exceptional cases, throttle stretches can also be formed over several sewer sections. With flotation controlled flaps the design of the shafts is to be made accordingly. Fixed slide valves and screens alone are not suitable as throttle installations. In addition to operating installations a further pipeline should be provided through which the tank, with operating defects, can be emptied (base outlet). In emergency it is slid open. Due to the danger of blockage it should be arranged some 0.50 m above the outlet pipeline. Slide valves at outlets are to be so designed that can be operated from outside the tank. Slide valve control rods are to be extended to ground level. The combined wastewater discharge from the stormwater tanks with overflow should be at least 2.Qpx + Qdw24. In order to avoid unnecessary sludge deposits the tank floor may not be used for this outlet. With new planning the drain following each overflow structure should be dimensioned for at least 3.Qpx + Qdw24 in order to cope with unforeseen developments.

10.3 Maintenance and Operation

10.3.1 Maintenance Facilities Maintenance and operation of stormwater overflow installations should be a component part of a total sewer network operational concept. The fault free and, with regard to solid substances, complete discharge with dry weather is for this a decisive precondition for the objective stormwater discharge. With enclosed tanks easily accessible manhole and working openings are to be provided. As manholes must also serve as escape routes it is practical to provide slip-free ladders or steps (see also Accident Prevention Regulations). Ventilation of the tanks should be sufficiently strong so that condensation water inside the tank is extensively prevented. During the filling process the velocity in the ventilation openings my not exceed 10 m/s. An opening is to be placed over the outlet for the removal of blockages there: if no throttle stretch is planned then the submerged part of the throttle device must also be made accessible through a shaft which allows the removal of blockages under water. As, with filled tanks, one may enter these shafts only up to the limit of back-up, fittings for scaffolding are practical.

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10.3.2 Cleaning and Flushing Facilities With the storage of combined wastewater sludge deposits occur, particularly with heavy throttling. This sludge and the other tank contents must reach the sewage treatment plant or be disposed of harmlessly in another manner. With this flushing installations have proved themselves; if required a later installation can be considered. In every case it is to be ensured that the tank can be hosed out manually. Inflow water as well as receiving water or groundwater may be employed for flushing. The hygienic requirements are to be observed. The employment range of flushing installations varies considerably. Experience and information are, for example, contained in Kaul, 1986; Stier, 1986 and 1987; RW-Behandlung in BW, Stuttgart, 1987.

10.3.3 Measurement Facilities In tanks of water management significance, registering water level measurement facilities should be installed within the scope of the self-monitoring. In addition it is recommended that the throttle discharge and the precipitation in the catchment area are measured. Through this the frequency of overflow of the overflow structures and the effects of the stormwater tanks with overflow on the receiving water can be estimated. This is particularly important with intramission considerations with extensive requirements and control of the sewer network. the registration of the overflow frequency can take place on measurement tapes or on other data carrying media. A remote transmission to a central monitoring location (as a rule to the sewage treatment plant) is practical, in particular for the immediate transmission of defect and operational messages as well as for deliberate emptying of tanks. With the other tanks it is sufficient to check, at regular intervals, whether the formulations selected in the calculation (eg. impervious surface areas, run-off discharge rate) still apply.

10.3.4 Other Records It is to be recorded in an operational diary when the following take place: - stormwater tanks with overflow examined, - sludge deposits in what quantity and how removed, - fittings and control devices inspected and serviced, - measurement facilities examined and adjusted, - which particular activities were observed. Normal commercially available forms are available for the operational monitoring of stormwater tanks (Hirthammer, 1989).

11 Dimensioning Example 11.1 Local Situation A drainage area is shown diagrammatically in Fig. 18. The domestic and commercial wastewater amounts occurring were summarised in the Table under "Inhabitants" with a water consumption ws = 180 l/(I.d). Sub-area 1 is, as boundary community, connected via a stormwater holding tank SHT with 2000 m3 storage content to a combined area. The throttle discharge is, in the mean between start of back-up and the water level at activation of the emergency overflow, 100 l/s. Sub-area 2 includes a commercial area in which heavily polluted wastewater occurs. It is to flow over a stormwater overflow SO1. Sub-area 3 contains the inflow of the commercial area stormwater overflow SO1. It is also to be overflowed over a stormwater overflow SO2.

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Sub-area 4 junctions in the stormwater tank retaining the first flush of stormwater STRRF which must be emptied with a pump. The throttle discharge is 2.Qpx + Qdw24. Sub-area 5 is drained using a separate system. The wastewater drain junctions with the main collector sewer in combined area 6. Sub-area 6 receives the inflow from all other overflow installations. Here a stormwater tank with overflow for settled combined wastewater STOSC is planned whose throttle discharge corresponds with the inflow to the sewage treatment plant. The biological treatment stage of the sewage treatment plant can accept a combined wastewater flow of 98 l/s. With dry weather a mean COD concentration of 475 mg/l was measured before the preliminary treatment stage.

Example C = combined area S = separate area hPr = 722 mm ws = 180 l/(I ⋅ d) Ais = impervious surface area tf = flow time SG = terrain slope group Qiw24 = infiltration water flow x = number of hours cw = wastewater concentration

Sub-area 1 2 3 4 5 6 STP

Inhabitants Ais tf SG x

in ha in min - in h

2240 14

10+7 1

550322

420432

13501072

1100 - - -

5600 35 20 1

112600

6637

1.2613.8

Qw24 QrS24 Qiw24 Qdw24 Σ Qdw24

Qnx Qdwx Σ Qdwx Qt,Qcw qr cw cdw Σcdw

in l/s in l/s in l/s in l/s in l/s in l/s in l/s in l/s l/(s.ha)

in mg/l in mg/l in mg/l

4.7 -

1.4 6.1 6.1 9.3

10,7 10.7 100 6.7 600 462 462

1.1 -

0.31.41.42.02.32.350

16.21200951951

0.9-

0.41.32.71.82.24.4

105.514.6600412698

2.8-

1.03.83.85.66.66.6

12.30,85600443443

2.3 2.3 1.0 3.3 3.3 4.6 5.6 5.6

- -

600 418 418

11.7 -

3.5 15.2 31.1 17.7 21.0 48.4

98 0.98 600 462 475

23.52.37.6

31.131.140.848.448.4

980.98629475475

Fig. 18: Schematic plan and area characteristic values for a mathematical example

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11.2 Necessary Total Storage Volumes First, a permissible overflow rate must be established for the total catchment area of the sewage treatment plant. It is carried out as follows (associated Eqn. nos. are in brackets): (6.4) Qw24 = 11260 . 180/86400 = 23.5 l/s, (6.5) Qpx = 23.5 . 24/13.8 = 40.8 l/s, (6.4) Qd24 = 23.5 + 7.6 = 31.1 l/s, (6.5) Qdx = 40.8 + 7.6 = 48.4 l/s, (6.6) QrS24 = 1100 . 180/86400 = 2.3 l/s, (6.16) cdw = 629 . 23.5/31.1 = 475.0 mg/l. From these discharge and concentration data further values can be calculated with the aid of the form (Appx. 3): Rainwater run-off (6.7) Qr24 = 98 - 31.1 - 2.3 = 64.6 l/s, DW discharge rate (6.14)Qdw24 = 31.1/66 = 0,471 l/(s.ha), Run-off discharge rate (6.15) qr = 64.6/66 = 0.979 l/(s.ha), Flow time reduction (6.13) af = 0.5 + 50/(37 + 100) = 0.865, however, minimum value = 0.885, Mean rainwater run-off with overflows (6.12) Qro = 0.885(3.0 . 66 + 3.2 . 64.6) = 358 l/s, Mean mix ratio (6.17) m = (358 + 2.3)/31.1 = 11.6, Coefficient of influence DW concentration (7.2) ap = 475/600 = 0.792, however, minimum value = 1.0, Coefficient of influence annual precipitation (7.3) ah = 722/800 - 1 = - 0.097, Coefficient of influence sewer deposits (Fig. 12 or Appx. 4) xa value for sewer deposits (7.4) xa = 24 . 31.1/48.4 = 15.4 Assuming a mean terrain slope group SGm = 1.26 in the left-hand diagram of Fig. 12, go up to the line for the DW discharge rate qdw24 = 0.47, right to the line for the value xa = 15.4, then vertically down to the abscissa: aa = 0.372, Dimensioning concentration (7.5) cd = 600 . (1.0 - 0.097 + 0.372) = 764 mg/l, Theoretical overflow concentration (7.6) cc = (107 . 11.6 + 765)/(11.6 + 1) = 159 mg/l, Permissible overflow rate (7.7) eo = 3700/(159 - 70) = 41.5 %,

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These results show that, in the annual mean with maximum combined wastewater inflow, 67 l/s of rainwater and 31 l/s dry weather discharge can be fed to the sewage treatment plant. From the connected separate area one must reckon with a rainwater run-off qr of 2.3 l/s so that for the combined wastewater treatment one can assume some 65 l/s of rainwater run-off or a run-off discharge rate qr = 0.98 l/(s.ha). According to Fig. 13 or Appx. 4 there results a necessary specific storage volume and thus a necessary total volume of Vs = 21.6 m3/ha V = 14.26 m3.

11.2.1 Simplified Distribution Procedure First, it is examined whether the scope of application of the simplified distribution procedure is fulfilled (Chap. 8.1.2). The run-off discharge rate of the stormwater tank with overflow is (6.8) Qr24 = 100.0 - 6.1 - 0.0 = 93.9 l/s, (6.15) qr = 93.9/14.0 = 6.7 l/(s.ha). The available run-off discharge rate qr lies above the necessary minimum value of 5 l/(s.ha). As also the permissible number of pre-overflows are not exceeded and the stormwater overflows are dimensioned according to this Standard the simplified distribution procedure can be applied. If now the same calculation according to the Form A 128 is carried out for the stormwater tank retaining the first flush of stormwater there results 18.5 m3/ha for the specific storage volume and 185 m3 for the storage volume. The emptying pump must be designed with this for 12.3 l/s or some 45 m3/h. From this there result the following sizes for the stormwater tank with overflow for settled combined wastewater total volume V = 1426 m3, stormwater tank retaining the first flush of stormwater V = 185 m3, stormwater tank with overflow for settled combined wastewater V = 1241 m3.

11.2.2 Verification Procedure The drainage system upon which this example is based can be represented as an outline network according to the system plan in Fig. 19. In this there are shown sub-division into 18 sub-areas and 31 calculation stretches whereby, a combination of similar sections and the disregarding of subordinate sewers has already been carried out. The degree of detailing in the presentation of the network is thus essentially coarser than is normal in the network calculation. For the application of a verification procedure the following peculiarities are to be pointed out: - the network shows a stormwater holding tank with qr > 5 l/(s.ha) which is thus, in the simplified

distribution procedure in accordance with Chap. 8.1.2, is disregarded in the dimensioning of the stormwater tanks with overflow,

- some sections are hydraulically overloaded for which the network contains a series of interconnections.

This method of approach in the application of a verification procedure is shown in Fig. 20. Following this the system processing in form of an example for a hydrological and a hydrodynamic calculation method is sketched.

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Fig. 19: System plan of the drainage area

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The characteristic values of the calculation stretches and sub-areas are to be taken from Table 1 Table 1: Characteristic values of the drainage network for a hydrodynamic calculation (coarse

network): - Actual Inventory data taking into account structures with overflow - Collector sewer data for the back-up free preliminary calculation with the central tank,

excluding structures with overflow Existing sewer

network Preliminary

calculation central tank

No. Junction L Grad. DN Qv Design. Ais SG DN Qv Above Below m ‰ mm l/s ha - mm l/s

1 .2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

103 102 101 SHT 191 201 SO1 302 301 SO2 403 402 401 STRFF 611 610 609 608 607 606 605 604 608 681 682 683 603 602 601 STOSC 701

102 101 SHT 191 611 SO1 302 301 SO2 611 402 401 STRFF 611 610 609 608 607 606 605 604 603 681*) 682 683 603 602 601 STOSC 701 702

120 120 120 250 250 100 50 50 40 10 140 120 50 20 150 180 100 210 190 100 150 210 100 80 170 100 250 220 110 50 50

5.0 5.0 5.0 8.0 8.0 5.0 5.0 7.0 7.0 20.0 7.0 7.0 7.0 5.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 7.0 7.0 7.0 7.0 3.0 3.0 3.0 4.0 4.0

600 700 800 300 400 400 300 400 500 300 500 500 700 300 600 600 700 700 700 900 900 1000 400 400 500 500 1200 1200 1200 400 400

433 650 924 88 187 148 69 175 316 138 316 316 770 69 335 335 503 503 503 975 975 1287 175 175 316 316 2079 2079 2079 132 132

F1/1 F1/2 F2/1 F3/1 F3/2 F4/1 F4/2 F4/3 F6/1 F6/2 F6/3 F6/4 F6/5 F6/6 F6/7 F6/8 F6/9

8.0 0.0 6.0 0.0 0.0 3.0 0.0 2.0 2.0 0.0 4.0 4.0 2.0 0.0 0.0 0.0 4.0 4.0 0.0 3.0 2.0 8.0 0.0 3.0 2.0 0.0 5.0 0.0 4.0 0.0 0.0

1 - 1 - - 2 - 2 2 - 2 2 2 - - - 1 1 - 1 1 1 - 1 1 - 1 - - - -

800 1000 1000 1000 1000 600 600 700 700 700 600 800 800 900 1400 1400 1600 1600 1600 1600 1800 1800 - 500 600 700 1800 2000 2000 400 400

924 1663 1663 2105 2105 433 433 770 770 1303 512 1094 1094 1260 3116 3116 4424 4424 4424 4424 6025 6025 - 316 512 770 6025 7940 7940 132 132

*) Dispensed with in the preliminary calculation of the central tank

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Fig. 20: Method of approach with the application of a verification procedure

11.2.2.1 Hydrologic Method The network representation for the execution of the preliminary calculation with the central tank for the determination of the theoretical, model-dependent overflow load is shown in the system graph in Fig. 21. The system representation of the application of the verification procedure on the actual available or planned network with inclusion of existing or planned stormwater overflows is shown in Fig. 22. In both cases an outline system presentation with representation of 6 sub-areas, the structures in the system and the interconnection of the individual elements is chosen.

11.2.2.2 Hydrodynamic Method With the application of a detailed pollution load model with hydrodynamic discharge calculation a more detailed system presentation is necessary than with the hydrological method. The outline network structure in Fig. 19 is based on this. For the preliminary calculation for the determination of the theoretical, model-dependent overflow load the existing structures are dispensed with. Therefore an enlargement of the sewer diameter to ensure a back-up free discharge to the fictitious central tank at the sewage treatment plant is necessary. The diameter resulting from a rough measurement with the rainfall intensity r15(1) = 100 l/(s.ha) - the other characteristic data remains unaltered - are included in Table 1. Furthermore it can be advantageous for the simplification of the preliminary calculation to get rid of the existing interconnections, eg. by factoring out the stretch 608 - 681.

EDP PREPARATION COARSE NETWORK SEWER NETWORK CALCULATION FROM NETWORK DATA

VERIFICATION OF THE HYDRAULIC EQUIVALENCE OF THE COARSE NETWORK

POLLUTION LOAD CALCULATION ACTUAL STATUS

DETERMINATION TOTAL STORAGE VOLUME IN ACCORDANCE WITH CHAP. 7.

PREPARATION OF COARSE NETWORK IN ACCORDANCE WITH CHAP. 8. (FICTITIOUS CENTRAL TANK)

PRELIMINARY CALCULATION FOR FICTITIOUS CENTRAL TANK THEORETICAL; MODEL-DEPENDENT OVERFLOW LOAD

DERIVATION OF REHABILITATION REQUIREMENT PLANNING OF REHABILITATION MEASURES

POLLUTION LOAD CALCULATION REHABILITATION ALTERNATIVES

SELECTION OF THE REHABILITATION CONDITION

POLLUTION LOAD CALCULATION REHABILITATED CONDITION VERIFICATION OVERFLOW LOAD < PERMISSIBLE LOAD

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The verification for the actual available or planned network, including all special structures, can take place immediately using the system according to Fig. 19.

11.2.2.3 Presentation of Results The presentation of the most important results of a verification procedure is shown as an example in Table 2. The therein contained values can be calculated with both the hydrologic as well as the hydrodynamic method. Furthermore a pollution load calculation gives further results on the overflow behaviour of the total system and the individual structures, such as, for example, overflow volumes, overflow load, overflow duration and frequency. In addition the inclusion of other pollutant parameters is possible.

Fig. 21: Hydraulically equivalent replacement system for the preliminary calculation of the permitted relief load - hydrological method

Fig. 22: System plan of the roughly subdivided drainage network - hydrological method

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Table 2: Structure and overflow characteristic values as result of verification procedures

Structure characteristic values Overflow characteristic values Structure Volume

m3 Qt l/s

qr l/(s.ha)

te h

ne 1/a

To h

VOo m3

Plo kg

ccc mg/l

mmin1)

SHT 2000 100 6.7 5.9 1 0.2 116 20 130 14 SO1 50 16.2 45 11 2313 290 125 35 SO2 105.5 14.6 45 12 3189 380 120 38 STRFF 185 12.3 0.85 6.0 84 128 23801 3150 132 12 SO5 5.6 Not counted STOSC 1241 98.0 0.98 5.3 56 149 98289 14110 144 9 Sum 1426 98.0 0.98 127708 17950 141 Central tank

1426 98.0 119 130481 18280 140 10

1) Mix ratio m is calculated for SO according to Eqn. (9.2), for SHT according to Eqn. (9.5).

11.3 Dimensioning of Stormwater Overflows

11.3.1 Stormwater Overflow SO1 in Commercial Area 2 The following applies for stormwater overflow SO1: (9.1) rcrit = 15 . 120/(2 + 10) = 14.8 l/(s.ha), (6.9) Qrcrit = 14.8 . 3. = 44.3 l/s, (6.5) Qdw24 = 1.4 l/s, (6.10) ΣQt,i = 0.0 l/s, (6.10) Qt,SO1 = 44.3 + 1.4 + 0.0 = 45.7 l/s For design reasons the throttle discharge of a stormwater overflow should not, however, lie below 50 l/s, so that Qt = 50 l/s is selected (Chap. 10.1.1). Minimum mix ratio (9.2) mSO1 = (951 - 180)/60 = 12.9 An examination of the actual mix ratio gives (9.2) mSO1 = (50 - 1.4)/1.4 = 34.7 > 12.9, so that the requirement according to the mix ratio is maintained.

11.3.2 Stormwater Overflow SO2 in Sub-area 3 The same formulas are used as in the last Chap. (9.1) rcrit = 15 . 120/(3 + 120) = 14.6 l/(s.ha), (6.9) Qrcrit = 14.8 . 4.0 = 58.5 l/s, (6.5) Qdw24 = 1.3 l/s, (6.10) ΣQt,i = 45.7 l/s, (6.10) QtSO2 = 58.5 + 1.3 + 45.7 = 105.5 l/s For the inflow of the above lying stormwater overflows it is not the actual throttle discharge of 50 l/s but rather the theoretically required discharge of 45.7 l/s which is to be applied (Chap. 6.2.7). The minimum mix ratio is (9.2) mSO2 = (698 - 180)/60 = 8.6 Actually available is (9.2) mSO2 = (105 5 - 2.7)/2.7 = 38.1 > 8.6 so that here a sufficient dilution is available when the stormwater overflow comes into action.

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12 Terms Symbol Unit Technical term ACA Ais Ared aa ac af ah ai ap bc bi COD CT c ccc cdc cdwc cr ctp cw DW d e eo FDS hPr hPr,eff I i JT MLQ m mSO mSTO OSSC PLo Plr Pltp Q Qc24 Qcrit Qcw Qd24 Qdw24 Qdwx Qi24 Qiw,Qiw24 QOS Qr24 Qrcrit Qro

ha ha ha - h - - h - d d - -

mg/l mg/l mg/l mg/l mg/l mg/l

- - % % -

mm mm

- - % l/s - - - -

kg kg kg l/s l/s

l/s l/s l/s

l/s l/s l/s

l/s l/s

l/s l/s l/s

Catchment area connected to a sewer network Impervious surface area Hardened surface area Coefficient of influence for sewer deposits Working hours per day in a commercial concern Flow time factor Coefficient of influence for annual precipitation Working hours per day in an industrial concern Coefficient of influence for heavy polluters Production days per year in a commercial concern Production days per year in a industrial concern Chemical oxygen demand Composite tanks Index for "commercial" Theoretical combined concentration in overflow water (COD) Theoretical dimensioning concentration in DW discharge (COD) Mean COD concentration in DW discharge Mean COD concentration in running off rainwater Mean discharge concentration of the sewage treatment plant (COD) Mean COD concentration in domestic and industrial wastewater Dry weather Index for "domestic" Annual overflow rate, overflow discharge rate Permissible annual overflow rate Flow-dividing structure Long-term mean annual precipitation Effective mean annual precipitation Number of inhabitants Index for "industrial" Terrain gradient Mean low water discharge Mean mix ratio in overflow water Minimum mix ratio for stormwater overflows Minimum mix ration for stormwater tanks with overflow Overflow structure for settled combined wastewater Overflowed annual pollution load Annual pollution load flushed from the surface by rainfall Annual pollution load in stormwater of the STP effluent Discharge Daily mean of the commercial wastewater discharge calculated from the annual mean Critical combined wastewater discharge Combined wastewater discharge to the sewage treatment plant Daily mean of the domestic wastewater discharge calculated from the annual meanDaily mean dry weather discharge Daily peak of dry weather discharge Daily mean of the industrial wastewater discharge calculated from the annual meanInfiltration water discharge as annual mean Discharge over the Overflow structure for settled combined wastewater Daily mean of rainwater run-off Critical rainwater run-off Mean stormwater discharge during overflows Daily mean of rainwater run-off from separate areas Throttle discharge Discharge over the tank overflow

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QrS24 Qt QTO Qw24 QwS24 Qwx q qdw24 qr rcrit SG SGm SHT SO SSCBO SSCO SSCTO STO STOSC STP STRFF TO To te tf V VQo VQr VSSCBO Vs Vs,min Vstat vdw vf ws x xa

l/s l/s l/s l/s l/s l/s

l/(s.ha) l/(s.ha) l/(s.ha) l/(s.ha)

- - - - - - - - - - - - h h

min m3 m3 m3 m3

m3/ha m3/ha

m3 m/s m/s

l/(I.d) h -

Wastewater discharge in daily mean Wastewater discharge from separate areas in daily mean Daily peak of wastewater discharge Discharge rate Dry weather discharge rate in daily mean Run-off discharge rate Rainfall intensity at which a stormwater overflow will come into action Terrain slope group according to ATV Standard A 118 Mean terrain slope group Stormwater holding tank Stormwater overflow Sewer with storage capacity and bottom overflow Sewer with storage capacity and overflow Sewer with storage capacity and top overflow Stormwater tank with overflow Stormwater tank with overflow for settled combined wastewater Sewage treatment plant Stormwater tank retaining the first flush of stormwater Tank overflow Overflow duration at a structure summed over one year Theoretical emptying time of a stormwater tank Flow time Storage volume Annual mean overflowed combined wastewater quantity Rainwater run-off sum for one year Usable volume of a sewer with storage capacity and bottom overflow Specific storage volume Specific minimum storage volume Static sewer volume Flow velocity with dry weather discharge Flow velocity with full filling Water consumption per inhabitant and day Hourly formulation according to ATV Standard A 118 Peak coefficient to take into account sewer deposits

13 References ATV (77/1) Arbeitsblatt A 128, Richtlinien für die Bemessung und Gestaltung von Regenent-

lastungen in Mischwasserkanälen, GFA 1977. ATV (77/2) Arbeitsblatt A 118, Richtlinien für die hydraulische Berechnung von Schmutz-,

Regen- und Mischwasser Kanälen, GFA 1977. ATV (77/3) Arbeitsblatt A 117, Richtlinien für die Bemessung, die Gestaltung und den Betrieb

von Regenrückhaltebecken, GFA 1977 ATV (83/1) Arbeitsblatt A 115, Hinweise für das Einleiten von Abwasser in eine öffentliche

Abwasser Anlage, GFA 1983. ATV (83/2) Arbeitsblatt A 105, Hinweise für die Wahl des Entwässerungsverfahrens (Misch-

verfahren/Trennverfahren)), GFA 1983. ATV (84) Arbeitsblatt A 119, Grundsätze für die Berechnung von Entwässerungsnetzen mit

elektronischen Daten-verarbeitungsanlagen, GFA 1984. ATV (88) Arbeitsblatt A 110, Richtlinien für die hydraulische Dimensionierung und den

Leistungsnachweis von Abwasser Kanälen und Leitungen (GFA 1988). ATV AG 1.2.4 (85) Arbeitsbericht "Abflußsteurung in Kanalnetzen", Korrespondenz Abwasser

H. 5/1985.

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ATV AG 1.2.6 (86) Die Berechnung des Oberflächenabflußes in Kanalnetz-modellen, Teil 1 -

Abflußbildung. Arbeitsbericht de ATV-AG 1.2.6 "Hydrologie der Stadtent-wässerung" gemeinsam mit dem DVWK, Korrespondenz Abwasser H. 2/1986.

ATV AG 1.2.6 (87) Die Berechnung des Oberflächenabflußes in Kanalnetz-modellen, Teil 2 - Abfluß-Konzentration. Arbeitsbericht de ATV-AG 1.2.6 "Hydrologie der Stadtentwässerung"

ATV AG 1.9.3 (85) Veranlassung und Anwendungsziele zur Durchführung von Schmutzfracht Berechnungen. 1. Arbeitsbericht der ATV-AG 1.9.3 "Schmutzfrachtberechnung", Korrespondenz Abwasser H. 8/1985.

ATV AG 1.9.3 (86) Der Schmutz-Niederschlag-Transport-Prozeß-Phänomeno-logische Beschreibung und Terminologie. 2. Arbeitsbericht der ATV-AG 1.9.3 "Schmutzfrachtberech-nung", Korrespondenz Abwasser H. 3/1986.

ATV AG 1.9.3 (88) Charackterisierung von Schmutzfrachtberechnungsmethoden - Anwendungsziele, Systemstruktur, Datenbasis, Ergebnisse. 4. Arbeitsbericht der ATV-AG 1.9.3 "Schmutzfrachtberechnung", Korrespondenz Abwasser H. 3/1988.

ATV AG 1.9.3 (89) Ausgewählte Grundlagen für die Anwendungsziele von Schmutzfrachtberechnungensmethoden. 5. Arbeitsbericht der ATV-AG 1.9.3 "Schmutzfrachtberechnung", Korrespondenz Abwasser H. 12/1989.

Brunner, P.G. (75) Die Verschmutzung des Regenwasserabflußes im Trennverfahren, Untersuchun-gen unter besonderer Berücksichtigung der Niederschlags-Verhältnisse im voralpinen Raum. Berichte aus Wassergütewirtschaft und Gesundheitsingenieur- wesen der TU München, Nr. 9, 1975

Durchschlag, A. (89) Bemessung von Mischwasserspeichern im Nachweisverfahren der Berücksichti-gung der Gesamtemission von Mischwasserentlastung und Kläranlagenablauf. Schriftenreihe für Stadtentwässerung und Gewässerschutz, Bd. 3, 1989.

Euler, G., Jacobi, D., Heizelmann, CH. (85)

Die Berechnung des Schmutzfrachtabflußes aus Niederschlägen. Eine vergleichende Darstellung und Wertung der Modellansätze. Techn. Berichte Nr. 33 aus dem Institut für Wasserbau, Fachgebiet Ingenieurhydrologie und Hydraulik der TH Darmstadt, 1985.

Geiger, W.F. (84) Mischwasserabfluß und dessen Beschaffenheit, ein Beitrag zur Kanalnetzplanung. Berichte aus Wassergütewirtschaft und Gesundheitsingenieurwesen der TU München, Nr. 50, 1984

Göttle, A. (79) Ursachen und Mechanismen der Regenwasser verschmutzung, ein Beitrag zur Modellierung der Abflußbeschaffenheit in städtischen Gebieten. Berichte aus Wassergütewirtschaft und Gesundheitsingenieurwesen der TU München, Nr. 23, 1978.

Hailer, W. (86) Einfluß des Einzugsgebietes auf die Rückhaltung von Schmutzfrachten an Regenüberlaufbecken. Korrespondenz Abwasser: Teil 1, H.6/1986, Teil 2 H. 7/1986.

Hirthammer (89) Betriebsaufzeichnungen zur Überwachung von Regenüberlaufbecken. Formblatt der ATV-Landesgruppe Bayern, Hirthammer Verlag, München, 1989.

Jacobi, D. (88) Unterscheidungsmerkmale von Schmutzfrachtberechnungsmethoden. Korrespondenz Abwasser H. 1/1988.

Kaul, G. (86) Mindestabfluß von Drosseleinrichtungen in Mischwasserkanälen. Korrespondenz Abwasser H. 7/1986.

Krauth, Kh. (79) Der Regenabfluß und seine Behandlung beim Mischverfahren. Stuttgarter Berichte zur Siedlungswasserwirtschaft, Bd. 66, München, 1979.

Meißner, E. (88) Vereinfachtes Verfahren zur Abschätzung entlasteter Jahresschmutzfrachten aus Mischkanalisationen. Korrespondenz Abwasser, H. 11/1988.

Paulsen, O. (87) Kontinuierliche Simulation von Abflüssen und Stofffrachten in der Trennent-wässerung. Mitteilungen aus dem Institut für Wasserwirtschaft, Hydrologie und landwirtschaftlichen Wasserbau der Universität Hannover, Nr. 62, 1987.

Pecher, R. (86) Kosten der Regenwasserbehandlung in mischkanalisierten Entwässerungsgebieten und Auswirkungen auf den Gewässerschutz. gwf Wasser Abwasser, H. 8/1986.

Pfeiff, S. (88) Die Entwicklung der Methoden zur Berechnung der Regenentlastungen von Mischwasserkanälen. Schriftenreihe für Städtentwässerung und Gewässerschutz,

ATV-A 128 E

62 December 1992

Heft 2, 1988, S. 55-72

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Schmitt, T.G. (85) .Der instationäre Kanalabfluß in der Schmutzfrachtmodellierung. Schriftenreihe des

Instituts für Siedlungswasserwirtschaft der Universität Karlsruhe, Bd. 42, 1985. Sperling, F. (85) Auswirkung von Regenwassereinleitung aus Mischkanalisationen auf die

Gewässergüte. Vortrag 18, Essener Tagung, Febr. 1985. Stier, E. (86) Untersuchungsprogramm an Regenüberlaufbecken in Bayern, Zwischenbericht.

Korrespondenz Abwasser, H. 1/1986. Stier, E. (87) Planungshilfen für die Gestaltung von Regenüberlaufbecken. Informationsberichte

Bayerisches Landesamt für Wasserwirtschaft, H. 1/1987. - Abschlußbericht des Forschungsvorhabens "Lokale Steuerungseinrichtungen in

Kanalnetzen": RWTH Aachen, Institut für Siedlungswasserwirtschaft, 1990. - Regenwasserbehandlung in Baden-Württemberg. Ministerium für Umwelt, Heft 20,

Stuttgart 1987

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Appendix 1 Notes on Advanced Requirements Presented by the ATV Working Group 2.1.1 "Principles and Decision Aids for Advanced Requirements on Combined Wastewater Discharges Taking into Account the Nature of the Running Waters" with the collaboration of Dipl.-Biol. Borchardt, Kassel Prof. Dr.-Ing. Brunner, Karlsruhe Dipl.-Ing. Krejci, Dübendorf (CH) Dr. rer. nat. Mauch, Augsburg Dipl.-Ing. Sperling, Essen (Spokesman) Dr. habil. rer. nat. Statzner, Essen Dr.-Ing. Stotz, Stuttgart Dr.-Ing. Winter, Bremen Prof. Dr-Ing. Wolf, Kassel ATV Standard A 128 defines normal requirements on stormwater structures with overflow. With the observation of these normal requirements and with the lack of special protection or management needs of the affected section of lake or river it is assumed that no essential impairment of the usage of the lake or river takes place through the combined wastewater overflows. Fundamentally sewage treatment plant discharges and combined wastewater overflows should be considered together. However, it should be pointed out that an automatic tying to advanced requirements for sewage treatment plant effluents is, from the point of view of the Working Group, to be avoided as the reasons are often not transferable to combined wastewater overflows. ATV Working Group 2.1.1 is currently preparing a report for the definition of advanced requirements with overflow into flowing waters. The following information can be given in advance.

Protection or Management Need Type and scope of advanced requirements on combined wastewater overflows are derived from the water management objectives. Subject to detailed considerations the examination of advanced requirements is necessary where a particular protection or management need of the flowing waters exists, in particular if - a management plan with regard to relevant parameters has been published, - requirements which are based on intromission relevant parameters are placed on the sewage

treatment plant effluent over and above the minimum requirements, - appropriate intromission considerations give an essential impairment of the nature of the lake or

river as a result of the combined wastewater overflow considered.

Effects The following summary shows the structure of possible effects of wastewater overflows on flowing waters as well as the respectively relevant substance groups and parameters

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Hydraulic

- near bottom flow velocity - bottom shear stress

Material - oxygen content - suspended substance content (solids) - acute toxicity (in particular NH3) - Single - pathogenic germs event

Material

- oxygen content (org.C, NH4-N, sediment) - toxic substances (in particular NH3, NO2) - sedimentation of solids - pathogenic germs in the sediment

Material

- organic persistent substances - metals - Annual - inorganic and organic sediments load - eutrophying substances

The effects are characterised briefly below. The surge-type increased discharge through combined wastewater overflows can effect a bottom instability and a resuspension of sedimented solids (see suspended and turbid matter). Higher flow velocities rearrange substrates and create stress in invertebrates (fish nourishing animals) and fishes. Rearrangement of substrate causes a drifting or dying off of organisms and results in, as a minimum, critical shear stress following high discharges. This critical shear stress is dependent upon the morphology of the lake or river and on the type of substrate. Rearrangement of substrates are particularly effective even in winter. The oxygen-depleting substances from combined wastewater overflows have a damaging effect on flowing waters organisms, if the oxygen concentration sinks below a boundary level or if the reduction in a time unit climbs above a critical size. A delayed oxygen-depleting effect of combined wastewater overflows can also occur with resuspension of sediment. The better the water quality the more essential is the undisturbed oxygen content for the maintenance of the organism spectrum of the respective water quality class. Turbid and suspended matter influences the biological picture (and thus the saprobic index) in the area of depositing. They are damaging, with short and long range effect, both as direct pollutant parameter (thickening of substrate, removal of light) as well as carrier of contaminants (eg. heavy metals, PCB). With surge-type accrual of the turbid matter (dependent upon the flow velocity) the damaging effects can be observed up to the dying off of organisms at untypical times. Synergism: solids, with high flow velocities, have the effect of a "sand blaster". Difference to naturally high flow velocities (flood water): flood water usually occurs gradually, combined wastewater overflow occurs suddenly. With sufficient available hollow space and interstice system in the flow bed, in the first case many organisms manage to escape without drift; in the second case usually not. If additional pollutants have an effect the protective space is again left and is thus useless. The increased feed of suspended matter can fill the interstice system in the flow bed and thus remove living space. The toxic effect of ammoniac is known and has been seen practically with the dying off of fish following combined wastewater overflows. The persistent toxicity for sensitive organisms (eg. fish breeding) is, on the other hand, in practice hardly documented.

Ecological effects of combined wastewater discharges

Acute effects

Delayed effects

Long-term effects

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Wastewater, particularly raw sewage from combined wastewater overflows also carries pathogenic germs which, over a certain flow time, remain virulent in flowing waters. The increasing damaging effect of mineral oil is reported upon. Here it is mainly the surfaces of lakes and rivers and the contact surfaces of substrates which are affected. Due to numerous recent sewer film investigations the possible significance of heavy metals in together with particular substances in the overflows is pointed out. However, here it can be observed that floating sewer film in retention spaces settles quickly. The same applies for the organic micro-pollutants (eg. CHC, PAK). However, the enrichment by, for example, trichloroethylene, tetrachloroethylene and other solvents, PCB through, for example, adsorption in sewer film, sediment and lipophilic substances exceeds the harmful effect. Sub-lethal effects over the nutrient chain are to be presumed. The nutrients which come from combined wastewater overflows have probably only a small share in the long range effects (eg. nutrient load in the North Sea). At certain times of the year combined wastewater overflows can contribute to the short range effects (eutrophication) in the area of the overflows. Damaging effects of combined wastewater overflows can be considerably greater in times of high biological activity (vegetation period, spawning time) than at other times.

Assessment Criteria Advanced requirements are to be examined in the following cases: 1. The self-cleaning potential of the lake or river is too small for the pollutants potential of the

catchment area. The evaluation of the relationship of overflow discharge of the catchment area to the low water discharge into the lake or river taking into account the material transport and the material conversion in the lake or river is, for example, suitable as criterion.

2. Concentrations and/or loads of relevant parameters place the desired usage of the lake or river in question. This case can, for example, be present with heavy polluters or with particular usages of the lake or river (eg. swimming waters).

3. The maximum overflow discharges set the bottom of the lake or river in motion. Then the channel is not sufficiently capable of dealing with the discharge which in any case bans discharge for reasons of water engineering. The evaluation of the maximum total discharge of the relevant catchment area to the flood water discharge (maximum discharge with design rainfall with the repetition time of one year to the flood water discharge into the lake or river with the same repetition time) is, for example, suitable as criterion.

4. The flowing waters section has no significant refuge space for organisms. This is the case when, for example in the area of greater, effected by the overflow, bottom velocities in the lake or river, neither interstice system nor still water zones are available which make it possible that organisms typical of lakes and rivers remain.

5. Possibilities for migration and remigration for drifted organisms are, in addition to lacking refuge space, disrupted. Flow stretches can no longer be populated after drifting away if migration obstacles (eg. drop) exist or flow stretches or lake or river characteristics (eg. with regard to the temperature), atypical for the drifted organisms, prevent migration or remigration.

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Measures Consequent priority in comparison with the measures for the reduction of effects, is to be given to the measures for reduction of the causes. Fundamentally the following measures come into consideration: 1. Measures in the catchment area, eg. - percolation, unsealing, surface retention, roof grassing, - relevant reduction of toxic substances at the site of occurrence, - deliberate material retention in specially dimensioned and designed spaces (eg. weather

dependent controlled storage of domestic and industrial wastewater with indirect dischargers), - if required, discharge of rainwater run-off into other lakes or rivers or precipitation area, 2. Measures in the sewer system, eg. - deliberate material retention in specially dimensioned and designed flow stretches behind the

overflow of structures with overflow and before the lake or river, - increased retention of rainwater run-off, eg. in storage spaces or by discharge control, - further decentralisation of the retention spaces within a total drainage concept; with this special

observation also with surface intensive indirect dischargers. 3. Measures in and at the lake or river, eg. - improvement of the bottom and bank structure of the lake or river below the overflow structure

through refuge spaces, - creation of accessible migration possibilities for organisms upstream and downstream of the lake

or river, - deliberate discharge delay in specially dimensioned and designed flow stretches below a

structure with overflow before discharge into a lake or river (eg. diversion stretch), - deliberate discharge delay of bottom proximate flow velocity or shear stress, for example, in

specially dimensioned and designed discharge profiles in the lake or river, - improvement of the degradation of consumptive materials. It is recommended, with the assessment of measures in and at the lake or river for the reduction of material and hydraulic effects of overflows, that an experienced flowing waters ecologist is brought in..

Further action The shortly to be expected 1st Report of the ATV Working Group 2.1.1 includes a selection procedure with whose aid situations can be determined which require more detailed considerations with regard to advanced measures. With this material and hydraulic effects are taken into account. In addition the differences between types of lake and river such as, for example, mountain, plain, dammed and tidal waters, are embodied. With the aid of examples it is shown how the procedure is to be applied.

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Appendix 2

Pollution Load Calculation Methods

1. General All pollution load calculation methods simulate the precipitation, discharge and material transport processes. Therefore, fundamentally, both material balances as well as discharge balances can be carried out. Detailed information on which application aims of the described methods are to be achieved, which system structures can be dealt with, which database is available or can be taken into account, which verifications are to be carried out and which lake or river related statements can be made can be found in ATV-AG 1.9.3, 1988. A presentation of the calculation formulations of various named models can be found in Jacobi, 1988.

2. Hydrologic-Empirical Methods The mean annual precipitation events are described in an area dependent rainfall spectrum. The local discharge formation is hydraulically recorded for each of the therefrom derived model rainfalls. The discharge concentration at the surface , i.e. the together flowing of the discharge effective precipitation from the surfaces and the discharge transformation in the sewer are represented globally with simple hydrologic formulations ( eg. flood plan). The pollutant concentration and its temporal process are described for every discharge event with the aid of empirical formulations. "Mean" conditions are represented using "mean" loadings which encounter "mean" initial conditions in the drainage network. The characteristic values of the model rainfall (frequency, duration) contained in the local rainfall spectrum are transferred to the overflow characteristic values. With this the non-linear transfer behaviour of the sub-system and, through this, the modification of the statistic dimensions remain unconsidered. The results which are achieved in accordance with this calculation method represent approximate mean conditions insofar as the rainfall spectrum used is representative of statistic criteria. As the overflow characteristic values are derived from model rainfall and mean pollution concentrations are assumed they can only be checked in long-term measurements.

3. Deterministic Models Deterministic models attempt to represent mathematically and to combine the total precipitation-discharge activity in the individual sub-systems and process phases (see ATV-AG 1.9.3, 1986). Locally measured precipitation series are used in their actual chronological sequence as input data. The therefrom resultant discharge activity on the surface and in the sewer separately simulated with hydrologic and hydraulic formulations. According to the differentiated discharge calculation deterministic pollution load calculation models also represent more or less in detail the processes with material transport, and combine these with the results of precipitation discharge calculations. In addition input data can be derived from locally specific measurements. If such measurements are not available then the parameters must be transferred from experience in similar areas. Different algorithms are applied for the description of material accumulation on the surface and in the sewer. In these various influencing parameters, such as duration of dry periods, time of the year, traffic activity, development structure, area structure, street cleaning and winter service can be taken into account. With the representation of the material erosion various empirical algorithms are also applied. They differ from one another according to the degree of detailing with the sub-division of the area - in part both sub-systems, surface and sewer, are considered together, in part dealt with separately - and within these whether the

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material erosion is simulated dependent or independent of events. Relevant influencing parameters are gradient, roughness, surface structure, network structure, rainfall and/or discharge intensity and thus the transport capacity of the discharge. The formulations for the description of the material accumulation and the material erosion can, however, only be covered by region through the hitherto available measurements. The material transport is either simulated as plug-flow or the transport stretch is dealt with as a completely mixed or partially mixed reactor. After calibration on measurement data the models given in Chaps. 3.1 and 3.2 are basically in a position to simulate, continuously and chronologically, discharge and pollution loads and, taking into account the transfer behaviour of the sub-systems, to represent the actual overflow behaviour of a drainage system chronologically. The pertinent determination of the input data and model parameters here relevantly determine the quality of the calculation results.

3.1 Hydrologic-Deterministic Models With hydrologic-deterministic models the discharge activities on the surface and in the sewer are simulated separately hydrological formulations. The discharge concentration on the surface is represented with hydrologic formulations (transfer functions). With the description of the discharge transformation in the network the discharges, but not the water levels, are worked out. The translation and retention behaviour with discharge in the sewer is described with the aid of transfer functions, i.e. linear, time invariable transfer behaviour of the "sewer" sub-system is assumed. The application of hydrologic methods is limited. If change of flow or back-up occurs at overflow structures then hydrodynamic methods are to be preferred.

3.2 Hydrologic-Hydrodynamic Models The hydrologic-hydrodynamic models and hydrologic-hydrodynamic-deterministic models differ from each other essentially in the simulation of the discharge transformation in the sewer network.(transport module), the with this associated degree of detailing with the division of the area and in the relevant modelling with heavy loading, in particular with flat, back-up prevalent and meshed networks. The discharge concentration on the surface is represented with hydrologic formulations (transfer functions) as described under Chap. 3.1. Hydrodynamic calculation formulations are applied for the description of discharge transformation in the sewer which, along with the discharge, also show the water levels. With this, for example, the situation at special structures can be dealt with, the available sewer volume can be taken into account and overloading in the network can be verified. In the hydrodynamic formulations the St. Vernant Equations (movement and continuity equation) are solved numerically, starting with various assumptions. In addition one differentiates implicit and explicit solution methods (difference procedures). To save calculation time the St. Vernant Equations are, in part, simplified. Here, it should be noted, that a disregarding of the local acceleration or the convective acceleration involves errors with opposing tendency. (ATV Standard A 110).

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4. Characterisation of Pollution Load Calculation Methods Symbols Used A Hydraulic-empirical procedure B Hydrological-deterministic models C Hydrologic-hydrodynamic-deterministic models + Taken into account/possible O Conditionally taken into account/conditionally possible - Not taken into account/not possible

METHODS 1. APPLICATION AIMS Grouping and dimensioning of stormwater overflows Examination and correction of the overall conception Priority of individual rehabilitation measures Quantifying of overflow quantities, durations, frequencies Preparation of management concepts Statements on material accumulation and erosion Advanced and scientific application aims

+ + +

- -

+ + + + + +

+ + + + + +

2. SYSTEM STRUCTURE Area structure, calculable Network structure, calculable - branched - meshed - subject to back-up Advanced hydraulic specialities (pumps, etc) Special operational and constructional characteristics (control/flaps, etc)

+ - - - -

+

+

+

+ + + + +

3. DATABASE 3.1 Discharge related: Statistically derived precipitation data - single event

i = const i = f(t)

- model rainfall spectrum - local rainfall spectrum Measured rainfall data - single event - series - continuum Area data - Qdw (dimensioning discharge) - Qdw (chronologically, locally variable) - tf (full filling or from time coefficient) - Ais - further data (gradient, inhabitants...) Network data - geometric - hydraulic Parameters for: - discharge formation - discharge concentration Receiving waters data - discharge (t) - water level (t) 3.2 Quality related Mean local pollution concentration and chronological concentration process Area dependent and time dependent pollution characteristic values for: - material accumulation - material erosion - material transport Preloading of receiving waters

+

+ + - - - +

+ + + +

+ - - - + - - - -

+ + + + + + + + + - + + + + +

+

+ + + +

+ + + + + +

+ + - + + + + + + + + + + + +

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4. VERIFICATION STANDARD Verification possible - Details of the on average relieved annual discharge sum (%) annual pollutant load % for various substance characteristic values, derived from the calculation results - Details of the relief characteristic values derived from hydrographic curves - Statistical information on frequency, duration, discharge sum, pollutant load derived from the above for various substances Interaction with the water regime and preloading in surface waters for each event and for the annual average Method capable of calibration Calculation results verifiable using measurements - in general - in detail

+ +

+

- -

+ -

+ +

+

+

+ +

+ +

+ +

+

+

+ +

+ +

5. STATEMENTS RELATED TO SURFACE WATERS With regard to: the discharged mean annual load for - BOD5/COD - further substances further loading parameters (discharge sum, duration, frequency, pollution load) the chronological combination of stormwater overflow and lake or river

+ +

-

+ +

+ +

+ +

+ +

The overview should show the different possibilities of pollution load calculation and the associated assumptions. They should ease the selection of the suitable method for the fulfilment of a certain tasking. Here there are two different things to differentiate. On one side one must be clear which statements with regard to the question posed - dependent upon the available initial data - are to be expected; on the other side it is to be examined which initial data must, as a minimum, be taken into account in order to reach concrete results. A specification of the requirements on the pollution load modelling to fulfil the various tasks is given in Jacobi, 1986. The details in the tables characterise the fundamental possibilities of the method groups, not those of the individual offerer who can be assigned to the respective method group. Within the same method group different calculation algorithms can be applied which take into account different initial data and parameters. These can be derived either from correlation relationships or can be physically interpreted directly. Therefore it is to be examined in individual cases to what extent a specially offered pollution load calculation method has all the characteristics which they, as representative of one of the listed method groups, could in principle have.

ATV-A 128 E

72 December 1992

Appendix 3

Ref. No. Date:

Total catchment Area of a Sewage Treatment Plant (STP) Project: ____________________________________________________________________________________ Sewage treatment plant: ________________________ Surface water: ________________________________Mean annual precipitation Impervious surface area Longest flow time in tot. area Mean terrain slope group MW discharge of STP DW discharge, 24 hr daily mean DW discharge, daily peak Rain run-off from separate area COD conc. in DW discharge

German Weather Service significant areas only SGm = Σ (SGi ⋅ ACAi)/(Σ (ACAi) biology with wet weather from combined and separate areas from combined and separate areas 100 % Qw24 from sep. area annual mean incl. Qiw24

hPr = mm Ais = ha tf = min SGm = - Qm = l/s Qdw24 = l/s Qdwx = l/s QrS24 = l/s cdw = mg/l

Mean infiltn. water discharge Utilisation value of STP Rain run-off, 24 hr daily mean Run-off discharge rate DW disch. rate from tot. area

contained in Qdw24 n = (Qcw - Qiw24)/(Qdwx - Qiw24) Qr24 = Qcw-Qdw24-QrS24 qr = Qr24/Ais qdw = Qdw24/Ais

Qdw24 = mg/l n = - Qr24 = l/s qr = l/(s.ha) qdw24 = l/(s.ha)

Flow time reduction Mean rain run-off with overflow Mean mix ratio xa value for sewer deposits Coeff. of influence DW conc. Coeff. of influence annual precip. Coeff. of influence sewer deposits Dimensioning concentration Theoretical overflow concentration Permissible overflow rate

af = 0.5 + 50/(tf + 100);·≥ 0.885 Qro =af

.(3.0 + 3.2qr) ⋅ Ais m = (Qro + QrS24)/Qdw24 xa =24Qdw24/Qdwx ap =cdw/600; ≥ 1.0 ah = hPr/800-1; ≥ 0.25; ≤ 0.25 from A 128, Fig. 12; Appx. 4 cdc = 600(ap + ah + aa) ccc = (107m + cdc)/(m + 1) eo = 3700/(ccc - 70)

af = - Qro = l/s m = - xa = - ap = - ah = - aa = - cdc = mg/l ccc = mg/l eo = %

Specific storage volume Required total volume

from A 128, Fig. 13; Appx 4 V =Vs ⋅ Ais

Vs = m3/ha V = m

Form A 128

ATV-A 128 E

December 1992 73

Appendix 4 Calculation Formulas for Figs. 12 and 13 Calculation formulas for the influence of sewer deposits aa from the mean terrain slope group SGm, the daily mean Qdw24 in l/s, the daily peak Qdwx in l/s and the dry weather discharge rate qdw24 in l/(s.ha): dl = 0.001.[1 + 2(SGm - 1)] xa = 24.Qdw24/Qdwx

τ = 430.

qdw240.45.dl

aa = (24/xa)2.(2 - τ)/10 but: aa ≥ 0 Calculation formulas for the determination of specific tank volume Vs in m3/ha from the rainfall discharge rate qr in l/(s.ha) and the permissible annual overflow rate eo in %: H1 = (4000 + 25qr)/(0.551 + qr) H2 = (36.8 + 13.5qr)/(0.5 + qr) Vs = H1/(eo + 6) - H2 but: Vs,min·≥ 3.60 + 3.84qr with: qr ≤ [(48/xa - 1)Qdw24 - QrS24]/Ais Area of application of last formula: 0.2 ≤ qr ≤ 2.0 l/(s.ha), 25 ≤ eo ≤ 75 %, Vs,min ≤ Vs ≤ 40 m3/ha.

74 December 1992

Appendix 5 Discharge Diagram for the Simplified Dimensioning Procedure (Th. Bettmann)

°for qr < 2 l/(s.ha) ° | Total area for cdw > 600 mg/l ° | m · (cdw-180)/60 ° | Stormwater discharge rate qr=qr24/A cd=600.(ap+ah+aa) cco=(m.cr+cd)/(m+1)

Area specific input data impervious surface area Ais

Area specific input data No. of inhabitants I

Combined wastewater Qcwreceipt of STP

Area specific input data flow time tf

Commercial dischargeQc24=0.2 to 0.8

Industrial discharge Qi24= 0.2 to 0.8 l/(s.ha)°

Domestic dischargeQd24= I.ws/86400

Working hours perday ac,i (1 shift = 8 hrs

Production days per year bc,i (d)

Wastewater discharge Qw24=Qd24+Qc24+Qi24

Domestic and industrial wastewater discharge Qwx=24.Qd24/x+24/ac

.365/bc.Qc24+24/ai

.365/bi.Qi24

Infiltration water yield Qiw24=0.00 to 0.15

Dry weather discharge Qdwx=Qwx+Qiw24

Dry weather flow Qdw=Qw+Qiw24

Dry weather discharge qdw24 = Qw24/Ais

Rainwater run-off QrS24=Qw24 (Sep. area)

Rainwater run-offQr24=Qm-Qdw24-QrS24

Total area

Stormwater dischargeq’ = Qr24/Ais

Flow time reductionaf = 0.50+50/(tf+100) tf ≤ 30 min af = 0.885 tf > 30 min for qr < 2 l/(s.ha)

Mean relief inflow Qro=VQo/(To.3.6)+Qr24) Qro=af.(3.0.Ais+3.2.Qr24

PLo+PLtp ≤ PLr

Mean mix ratio m=(Qro + QrS24)/Qt24 ≥ 7

for cdwc > 600 mg/l m ≥ (cdwc - 180)/60

Theor. overflow conc. ccc = (m.cr + cdc)/(m + 1)

Influence value heavy polluter aw = 1 cdw ≤ 600 mg/l aw = Cdw/600 cdw > 600 mg/l

Influence value for sewer depositsxa = 24.Qdw24/Qdwx dI = 0.001.[1 + 2.(SGm - 1)] τ = = 430.qdw24 aa = (24/xa)2.(2 - τ)10 aa ≥· 0

Influence value for annual precipitationad = hPr/800 - 1 600 · hPr <·1000 ad= - 0.25 hPr < 600 ad = + 0.25 hPr > 1000

Area specific input data dry weather conc. cdw

Area specific input datamean slope group SGm

Area specific input dataannual precipitation hPr

cdw:cr:ctp = 600:107:70

Dimensioning conc. in dryweather cdc = 600.(ap + ah + aa)

H1 = (4000 + 25qr)/(0.551 + qr)

H2 = (36.8 + 13.5qr)/(0.5 + qr)

qrmin = {[(48/xa)-1].Qdw24 - QrS24}/Ais

0.2 ≤ qr ≤ 2.0 l/(s.ha)25 ≤ eo ≤ 75% Vs,min ≤ Vs ≤ 40 m3/ha

Permissible overflow rateeo=100.(cr - ctp)/(cco - ctp)

Specific tank volume Vs,min =3.60+3.84 qrmin Vs=H1/(eo + 6) - H2

Total volume V = Vs

.Ais