GERMAN ATV-DVWK RULES AND STANDARDS

Advisory Leaflet ATV-DVWK-M 206E

Automation of chemical phosphate removal November 2001 ISBN 3-937758-63-1

Publisher/marketing:GFA the publishing company of the German Association for Wa-ter, Wastewater and Waste, Theodor-Heuss-Allee 17 D-53773 Hennef Tel. ++49-22 42 / 8 72-120 Fax:++49 22 42 / 8 72-100 E-Mail: vertrieb@gfa-verlag.de Internet: www.gfa-verlag.de

ATV-DVWK-M 206E

2 November 2001

The main fields of activity of the ATV-DVWK are technical-scientific subjects and the economic as well as the legal concerns of environmental protection. The politically and economically independent association works nationally and internationally in the fields of pollution control, wastewater, water-hazardous substances, waste, hydraulic engineering, hydraulic power, hydrology, soil protection and contaminated sites. The ca. 16,000 members are active in municipalities, engineer offices, authorities, firms and associations and also in universities. Of these there are 10,000 specialists with personal membership; these are engineers, scientists, lawyers, business persons, operating personnel and technicians. Via the corporate membership in the ATV-DVWK there is access to ca. 160,000 specialists.

All rights, in particular those of translation into other languages, are reserved. No part of this Advisory Leaflet may be reproduced in any form – by photocopy, microfilm or any other process – or transferred into a language usable in machines, in particular data processing machines, without the written approval of the publisher.

Publisher: ATV-DVWK Deutsche Vereinigung für Wasserwirtschaft, Abwasser und Abfall e.V., Theodor-Heuss-Allee 17, 53773 Hennef

Marketing: GFA Gesellschaft zur Förderung der Abwassertechnik e.V., Hennef

Setting and printing of the German original: DCM, Meckenheim

© GFA Gesellschaft zur Förderung der Abwassertechnik e. V., Hennef 2001

Die Deutsche Bibliothek [The German Library] – CIP-Einheitsaufnahme

ATV-DVWK, German Association for Water, Wastewater and Waste: ATV-DVWK Rules and Standards (Medium combination) / ATV-DVWK, Wasserwirtschaft, Abwasser, Abfall. - Hennef : GFA, Publishing Company of the ATV-DVWK Formerly under the [German] title: Abwassertechnische Vereinigung: ATV-Regelwerk

Advisory Leaflet M 206E. Automation of chemical phosphate removal

ISBN 3-937758-63-1

ATV-DVWK-M 206E

November 2001 3

Foreword

Since the publication of Advisory Leaflet ATV-M 206 “Automation of the Chemical Removal of Phosphate” in July 1994 a continuous technical further development has taken place in this field. As a result of the increas-ing expansion of process analysis technology in wastewater treatment plants, the further development of regulation and control strategies as well as new knowledge on the combination of a deliberate biological with a chemical removal of phosphorus a revision and amendment of the 1994 edition has become necessary.

Authors

The original German Advisory Leaflet ATV-DVWK-M 206 was elaborated by the ATV-DVWK Specialist Committee KA-13 “Automation of wastewater treatment plants”. The following have collaborated in the preparation: Dr. rer. nat. J.-U. Arnold, Bergisch-Gladbach Dr.-Ing. P. Baumann, Stuttgart Dipl.-Ing. U. Blöhm, Berlin Dr.-Ing. P. Hartwig, Hannover Dr.-Ing. U. Jumar, Magdeburg Dipl.-Ing. E. Michel, Waldbronn Dr.-Ing. J. Reichert, Viersen Dr.-Ing. S. Schlegel, Essen (Chairman) Dr.-Ing. H.-H. Schneider, Berlin Dipl.-Phys. Ing. W. Worringen, Düsseldorf

ATV-DVWK-M 206E

4 November 2001

Contents

Foreword ........................................................................................................................................................3

Authors ..........................................................................................................................................................3

User Notes......................................................................................................................................................5

1 Area of Application ........................................................................................................................5

2 Abbreviations .................................................................................................................................5

3 Introduction ....................................................................................................................................5

4 Basic Elements and Process Description ...................................................................................6

5 Continuous Measurement of the Phosphate or Phosphorus Concentration ..........................7 5.1 General.............................................................................................................................................7 5.2 Measurement of Orthophosphate (SPO4)..........................................................................................7 5.2.1 Molybdenum Blue Process ..............................................................................................................7 5.2.2 Vanadate Molybdate Process ..........................................................................................................8 5.3 Measurement of Total Phosphorus (CP) ..........................................................................................8 5.4 Operation and Maintenance.............................................................................................................8

6 Automation Concept involving Metal Salts and Sodium Aluminates ........................................8 6.1 Preamble ..........................................................................................................................................8 6.2 Measuring Sites and Dosing Points .................................................................................................8 6.3 Control and Regulation Concept for Phosphate Removal ...............................................................10 6.3.1 Control according to Timeplan .........................................................................................................10 6.3.2 Control according to P-Load ............................................................................................................11 6.3.3 Control according to Wastewater Flow ............................................................................................11 6.3.4 Regulation of SPO4 ............................................................................................................................12 6.3.5 Other Control Concepts....................................................................................................................14 6.3.6 Substitutional Value Strategies ........................................................................................................14

7 Storage and Dosing Technology ..................................................................................................15 7.1 General.............................................................................................................................................15 7.2 Dosing Facilities ...............................................................................................................................16 7.3 Storage and Dosing..........................................................................................................................17 7.3.1 Liquid Precipitants ............................................................................................................................17 7.3.2 Non-Pourable Precipitants ...............................................................................................................18 7.3.3 Pourable Precipitants .......................................................................................................................19 7.4 Measurement of the Precipitation Concentration.............................................................................21 7.5 P-Removal by Raising the pH-Value................................................................................................21 8 Economic Efficiency ......................................................................................................................21

9 Ordinances, Standard Specifications and Standards ................................................................22

ATV-DVWK-M 206E

November 2001 5

User Notes

This Advisory Leaflet is the result of honorary, technical-scientific/economic collaboration which has been achieved in accordance with the princi-ples applicable therefor (statutes, rules of proce-dure of the ATV-DVWK and the Standard ATV-DVWK-A 400). For this, according to precedents, there exists an actual presumption that it is textually and technically correct and also generally recog-nised. The application of this Advisory Leaflet is open to everyone. However, an obligation for application can arise from legal or administrative regulations, a contract or other legal reason. This Advisory Leaflet is an important, however, not the sole source of information for correct solutions. With its application no one avoids responsibility for his own action or for the correct application in spe-cific cases; this applies in particular for the correct handling of the margins described in the Advisory leaflet.

1 Area of Application

This Advisory leaflet applies for activated sludge and fixed bed plants for the treatment of wastewa-ter which essentially originates from households or from facilities which serve commercial or agricul-tural purposes provided that the harmfulness of this wastewater can be reduced using biological proc-esses with the same result as with wastewater from households.

2 Abbreviations

AbwV Abwasserverordnung [German Wastewa-ter Ordinance

XSS Concentration of suspended solids (0.45 µm)

SPO4,des PO4-P design value at the dosing point e.g. in mg/l

CP,MV Monitoring value for the Ptot concentration in the effluent e.g. in mg/l

CP,Part P-concentration in the effluent of the plant due to residual suspensions e.g. in mg/l

CP Measured concentration of phosphorus, e.g. in mg/l

CCOD,InB COD in the inflow to the biological reactor I Inhabitant PT Total number of inhabitants and popula-

tion equivalents F Safety factor as empirical value, e.g. in

mg/l

f Safety factor FM Precipitant k Proportionality factor mMe Effective metal content of a precipitant

solution, e.g. in mol/l mP,SS Phosphorus content of the suspended

solids, e.g. in mg/kg SPO4 Orthophosphate-P Q Wastewater flow at the point of the P-

concentration measurement, e.g. in m3/h QPF Precipitant flow, e.g. in m3/h UV Ultraviolet VAwS Verordnung über Anlagen zum Umgang

mit wassergefährdenden Stoffen und über Fachbetriebe [German Ordinance on plants for the handling of water-hazardous substances and on technical operations]

WGK Wassergefährdungsklasse [German Wa-ter Hazard Class]

WHG Wasserhaushaltsgesetz [German Water Resources Management Law]

ß-value Ratio of mol metal to mol phosphorus re-lated to the P-content in the influent to the precipitation reactor. In order to be able to assess the effec-tiveness of the precipitation the ß-value must actually be related to the P-content at the dosing point. In these cases higher values result than this normal operational definition.

ρ Specific weight of the precipitant solution, e.g. in kg/m3

3 Introduction

Through legal regulations (§ 7a WHG; AbwV) the permitted phosphorus content in the effluent of mu-nicipal wastewater treatment plants is limited. With plants ≥ 10,000 PT up to 100,000 PT a monitoring value of 2 mg/l, with plants > 100,000 PT of 1 mg/l is to be maintained. More extensive requirements are possible in individual cases. Phosphates are removed through biological proc-esses and through chemical precipitation. Aim of an automatic dosing of precipitant/flocculation agent is to achieve an extensive removal of phosphates from the wastewater with as small as possible em-ployment of chemicals in order, in addition to the costs of precipitant, in particular to minimise the expenses for a disposal of the addition precipitation sludge produced. This should also be sought in order to keep the salting of the water low.

ATV-DVWK-M 206E

6 November 2001

4 Basic Elements and Process Description

Phosphorus is a wastewater content substance which can lead to eutrophication with discharge into a slowly moving or static body of water. In most water bodies the phosphorus content determines the degree of algae growth. In wastewater it usually originates from domestic discharges. Although a reduction of the specific total phosphorus produc-tion from ca. 5 to approximately 1.8 g/(I d) through the very extensive reduction of the phosphate com-ponent in detergents, the normal concentrations in the influent of municipal wastewater treatment plants with 5 - 10 mg/l, however, lie as a rule still so high that an extensive removal is required. In indi-vidual cases, however, lower concentrations can also occur which can be traced back to either the dilution of the wastewater with infiltration water or to a higher component of industrial discharges. The latter usually only shows small phosphorus concen-trations. A part of the phosphates contained in the influent is carried out both with the primary sludge (ca. 10 - 15 %) and also incorporated in the biomass and re-moved with the surplus sludge. Integration into the surplus sludge can, as a rule, can be estimated as 1 % of the added BOD5 or 0,005 CCOD,InB (comp. ATV-DVWK A-131E). A small P/BOD5-ratio there-fore requires less expense for extensive removal measures. A deliberately executed, increased biological P-removal can be achieved in the aeration stage through a special process technology (suitable switching in of anaerobic zones). The biological removal of P, however, does not always suffice in order to maintain safely the required monitoring value in the effluent. Therefore, as a rule, the pos-sibility of dosing precipitant/flocculation chemicals is also planned for these cases. In general acidic iron salts as iron sulphate, iron chloride, iron chloride-sulphate, aluminium salts as aluminium chloride, the alkaline reacting sodium aluminate or, in special cases, also lime hydrate are applied as precipitants. For further basic infor-mation attention is drawn to the appropriate litera-ture (ATV-DVWK-A 202E and the ATV Manual “Biological and advanced treatment of wastewater” [Not available in English]). The dosing of precipitant must be capable of being matched to the fluctuating influent and/or effluent values. With this, due to competing reactions, dos-ing has to be more or less hyperstoichiometric. Every overdosing, however, leads to a formation of

hydroxides or carbonates which, due to increased sludge formation, are unwanted. For the adjustment of the dosing with plants with exclusively chemical phosphate removal a ß-value of 1.5 is set. As, however, usually phosphate is increasingly re-moved biologically, in practice the ß-value, elated to the P-content in the influent to the precipitation reactor (as a rule the aeration stage) very fre-quently lies under 1.0. With the addition of lime hydrate the precipitation of phosphates takes place through the raising of the pH value. With this, insoluble calcium phosphates are formed using the calcium ions contained in the wastewater. With the exception of very soft water there are sufficient calcium ions available. The cor-rect selection of the pH value at the dosing point is dependent on the local conditions. With pre-precipitation pH values up to 9.3 are permitted. With a dosing of lime hydrate into the aeration stage a pH value of 9.0 may normally not be ex-ceeded in the effluent of the plant, as ammoniac, toxic to fish, can result due to the high pH value in the bodies of water. The chemical P-removal takes place in two steps, the rapid chemical reaction (precipitation) and the subsequent agglomeration of the small flocs into larger, easily separable floc formations (floccula-tion). Here, for the chemical reaction, a rapid, en-ergy-intensive admixture of the precipitant into the wastewater is required. The flocculation itself on the other hand requires only a very small input of energy. Frequently points can be found in a waste-water treatment plant at which the admixture is en-sured without additional input of energy. For exam-ple, overflows can be used for this. If necessary the required turbulence can also be created through the incorporation of chicanes or input of additional energy (e.g. employment of pumps). An alternative is also to improve the utilisation of precipitants by dosing the precipitant at several points of the aera-tion tank or over the complete width of the channel. In larger plants the production of a separate precipi-tation and flocculation reactor can be sensible with new construction measures. Depending on the place of dosing one differentiates the dosing into pre-precipitation, simultaneous pre-cipitation and post-precipitation. With pre-precipitation the precipitant dosing takes place in the grit chamber or in the inlet to the pri-mary settling with separation of the flocs in the pri-mary settling tank. Due to the negative effects on the denitrification (reduction of the BOD5 load) it is, however, rarely employed. With a high component of industrial wastewater with high BOD5 and phos-phorus concentrations or a downstream fixed bed plant, this process can, however, be employed

ATV-DVWK-M 206E

November 2001 7

thoroughly practically for the relief of the biological stage. In any case attention is to be paid that for the subsequent biological process still sufficient phosphorus remains in the wastewater. Simultaneous precipitation is currently the most widely employed process with which the addition of precipitant takes place directly into the activated sludge stage; the separation of the flocs takes place in the secondary settling tank. In addition to a good utilisation of the precipitant an improvement of the sludge index often presents itself as a positive side effect of simultaneous precipitation. The low P-concentrations normal today in the wastewater usually allow the maintenance of the monitoring values of 2 mg/l and 1 mg/l respectively using this simple to operate method with sufficient efficiency of the secondary settling stage. If, with regard to an especially weak performance receiving water an even lower P-concentration is required usually additional measures have to be taken. Such measures are: – filtration, sieving or similar for separation of

suspensa; polishing ponds can also support this.

– post-precipitation in the form of flocculation filtration: in order not to load the filter with too high a quantity of precipitation sludge, flocculation fil-tration, depending on the required effluent value, assumes a previous partial removal to 1 to 2 mg/l P.

– conventional post-precipitation: this takes place in a separate stage following the secondary settling tank. It consists of pre-cipitation and flocculation tanks as well as the separation stage with a further secondary set-tling tank or a flotation plant.

5 Continuous Measure- ment of the Phos- phate or Phosphorus Concentration

5.1 General

The continuous measuring facilities with process analysis equipment today employed for monitoring, control and regulation of the removal process oper-ate according to expensive physical-chemical measuring processes. For this reason and due to the content substances in the wastewater, they re-

quire a certain maintenance expense and the em-ployment of chemicals. As comprehensive informa-tion for the employment and operation of process analysis equipment including the systems for the pre-treatment of samples are contained in ATV-DVWK Advisory Leaflet M 269 [Not available in English], the essential analysis processes for the determination of phosphorus compounds are only gone into briefly in the following sections. Using process analysis equipment both orthophos-phate (SPO4) and also Ptot (CP) can be determined. With measuring equipment for the determination of Ptot at most a coarse filtration may be placed up-stream for the protection of the equipment, as an extensive separation of suspended solids which contain phosphorus leads to considerably reduced findings. The determination of the precipitable orthophos-phate compounds, on the other hand, takes place as a rule following a sample pre-treatment, in order to be able to analyse reliably the then extensively solid matter-free wastewater. The sample pre-treatment systems which come into question are described in detail in ATV-DVWK-M 269.

5.2 Measurement of Orthophosphate (SPO4)

In the continuous analytics of orthophosphate up to now two photometric processes have gained in sig-nificance. With both processes polyphosphate and organic phosphorus compounds are not recorded. Depending on the application purpose there are, at the forefront, requirements for higher accuracy, DIN conformity or economic efficiency. The measuring process is to be selected accordingly.

5.2.1 Molybdenum Blue Process With the molybdenum blue process (EN 1189) or-thophosphate with ammonium molybdate in an acid medium converts into complex phosphorus molybic acid. This is subsequently converted using reduc-tion agent into phosphorus molybdenum blue. The light attenuation brought about by the colouring is determined photometrically and is a measure for the orthophosphate concentration. The process covers the range from 0.01 to 5 mg/l SPO4 and is therefore particularly suitable for precise measure-ment with low concentrations. With higher concen-trations the process has to be appropriately ad-justed (e.g. through dilution of the wastewater sam-ple). The chemicals used are, however, relatively expensive and are of only limited use.

ATV-DVWK-M 206E

8 November 2001

5.2.2 Vanadate Molybdate Process With the vanadate molybdate process (yellow proc-ess), orthophosphates react in the acid medium with ammonium molybdate and ammonium vanadate into yellow ammonium phosphorus vanado-molybdate. The light attenuation brought about by the colouring is determined photometrically and is a measure for the orthophosphate concentration. Yellow wastewater content substances influence the measured value. This effect, as a rule, can be compensated through special automatic calibration procedures. The process covers a range from 0.1 to 20 mg/l SPO4. In compari-son to the molybdenum blue process the chemicals are inexpensive and last longer.

5.3 Measurement of Total Phosphorus (CP)

Total phosphorus measuring equipment, as a rule, functions in accordance with the molybdenum blue process, but following previous digestion. Some equipment also allows the separate determination of the orthophosphate content. Digestion, as a rule, takes place through the heat-ing with peroxodisulphate and sulphuric acid (mod-elled on EN 1189), partially under pressure, in or-der to shorten the degradation times. A degradation though UV radiation can only be applied for sam-ples free of solid matter. The measurement of the total phosphorus content requires the inclusion of all solid matter in the di-gestion, as the greatest part of the phosphates are bonded to solid matter. Therefore it must be guar-anteed that an unfiltered sample is analysed. In addition care is to be taken with sampling and preparation that a homogenous sample is pro-duced. The lower limit of the measurement range of total phosphorus measuring equipment with the molyb-denum blue process lies between 0.01 mg/l and 0.1 mg/l P. The upper limit of the measurement range varies between 5 mg/l and 15 mg/l P.

5.4 Operation and Maintenance

Process analysis equipment for the determination of phosphorus compounds requires a significantly higher maintenance expense than normal process measurement equipment such as, for example, for temperature or pH-value. Information on the gen-

eral equipment requirements, for measures of qual-ity assurance, for maintenance and servicing as well as the necessary training measures are to be found in Advisory Leaflet ATV-DVWK-M 269.

6 Automation Concept involving Metal Salts and Sodium Alumi-nates

6.1 Preamble

Although only the dissolved phosphates can be determined with precipitation the actual objective is the observation of the monitoring value for the total phosphorus concentration in the plant effluent with the lowest possible costs. With the choice of whether to integrate in a regulation/control of the Ptot (CP) or only to record the dissolved ortho-phosphate compounds (SPO4), the selected measur-ing site and dosing point as well as the intended control or regulation strategy are by all means to be taken into account. More detailed information is given by Table 1 in the following section. As, with precipitation, as far as possible phosphate loads should be taken into account, an integration of the wastewater flow into the automated concept is fun-damentally to be recommended.

6.2 Measuring Sites and Dosing Points

In principle, for regulation and control tasks, various procedures come into consideration. Fig. 1 reflects schematically the possible measurement sites and dosing points in wastewater treatment plants as well as their suitability for this. The actual arrange-ment of measurement sites and dosing points is, inter alia, dependent on: – the local conditions (arrangement of tanks and

pipeline layout), – the selected automation concept (comp. Chap.

6.3), – the process technology (chemical or combined

biological-chemical phosphate removal, precipi-tation process) and

– the precipitant used.

ATV-DVWK-M 206E

November 2001 9

Fig. 1: Measurement sites and dosing points

With pre-precipitation using acidic metal salts or sodium aluminate, the phosphate or Ptot concentra-tion is measured conveniently in the influent to the primary settling tank (B). Using this measurement a regulation of the input of precipitant into the grit chamber (A) or a control with input into the influent of the primary settling tank (B) is possible. The problem of a continuous measurement at this measurement site corresponds with that in the in-fluent to the biological reactor (comp. Table 1). With simultaneous precipitation the dosing of precipitant can take place at various points in the system. A regulated input into a possibly available anaerobic stage or into its effluent (E) is basically not practical as the process of biological phosphate removal is not finished here and the take up of re-dissolved phosphate takes place first in the aerated stage. Frequently the precipitant is dosed into the return sludge circuit (C) and the phosphate content meas-ured in the effluent of the aeration tank (F or G). This process has disadvantages due to the delay and dead times existing here. In addition there is only a little phosphate in the return sludge available for the reaction with the precipitant. The precipitant is used for the formation of metal hydroxide so that it has only limited availability for P-removal in the activated sludge stage. Through this, as a rule, an overdosing must take place. With input into the in-

fluent of the aeration tank (D) a certain overdosing must also take place, because the removal through incorporation of phosphorus into the surplus sludge has to be estimated. With a measurement in the effluent to the biological reactor the influence of the time delay on the regulation behaviour is to be taken into account. Furthermore attention is to be paid that the measured value in the effluent of the biological reactor does not correspond with the ac-tual value in the plant effluent as subsequent influ-ences such as, for example, balancing of concen-trations or redissolving events in the downstream treatment stages (in particular with systems with biological phosphate removal) are not recorded. The best control reaction is to be expected if the precipitant is dosed into the effluent of the aeration tank or possibly into the last tank of a cascade (F). Measurement takes place conveniently in the influ-ent to the secondary settling stage (G). With this variant a particularly careful mixing of the precipi-tant in the wastewater is to be ensured so that at the not far removed measuring site the precipitation reaction is extensively completed. Dosing in the influent to the secondary settling stage G) has also shown itself to be favourable. With unfavourable conditions here, however, only a control can be realised, measurement then takes place before the dosing point.

ATV-DVWK-M 206E

10 November 2001

The dosing into the effluent of the aeration tank (F) with a measurement in the effluent of the secon-dary settling stage (H), due to the long retention time of the wastewater in the secondary settling tank leads to large dead times. Therefore a rapid reaction of changes of the P-load, which occurs particularly with combined wastewater influents, is impossible. With a post-precipitation the precipitant dosing takes place before the downstream stage (H). For

regulation measurement is carried out in the efflu-ent of this stage (I), with control in (H). The floccula-tion filtration with both concepts can be realised. In addition to the measuring site the measurement parameter for the control and regulation tasks is of particular significance. As an example Table 1 shows an overview of possible combinations of measurement sites and measurement parameters with the mainly applied simultaneous precipitation.

Table 1: Combinations of measurement sites and measurement parameters with simultaneous precipitation

Measurement parameter

Measurement site

CP SPO4

Influent biological reac-tor

Possible The incorporation of P in the surplus sludge is to be taken into account by estimation. A continuous sam-pling must take place so that the sample is not influ-enced by a pre-treatment which, possibly, can lead to a separation of particularly bonded phosphorus. This can, however, be guaranteed with difficulty at this measurement site.

Possible At this measurement site only 60-75 % of the phosphorus compounds are already available as orthophosphate. The phosphate component which can be removed chemically has to be estimated. In this case the incorporation of P in the surplus sludge is also to be taken into account. Preparation of the sample at this measurement site can be expensive.

Effluent biological reac-tor

Impractical The phosphate bonded in the sludge flocs but which is not precipitable is also recorded therefore is not suitable for inclusion in the regulation of the precipi-tation.

Practical Direct recording of the precipitable P-component, through short delay times suitable for inclusion in the regulation (with suitable dosing site). Disadvantage: no complete recording of the moni-toring parameters.

Effluent secondary settling stage

Impractical for an inclusion in the regulation*

The monitoring parameter is recorded completely. The delay time is, however, too large for an inclusion in the regulation.

Impractical As for the effluent of the biological reactor but, due to long delay time not suitable for inclusion in the regulation.

* Although an input (proportional regulation) of the precipitant depending on the phosphorus concentration in the effluent of the secondary settling stage (both SPO4 as well as CP) is not to be recommended, such a measurement can, however, provide information on the effectiveness of the precipitant in-put and, under certain circumstances, enables an iterative adjustment of the precipitant input. Taking into account the real wastewater flow the regula-tion of the Ptot concentration in the effluent of the wastewater treatment plant in combination with a subordinate regulation of the PO4-P concentration in the effluent of the biological reactor (cascade regulation within the sense of control engineering) can increase the certainty of maintaining the monitor-ing values.

6.3 Control and Regulation Concept for Phosphate Removal

6.3.1 Control according to Timeplan With constant precipitant dosing in general a con-siderable overdosing of precipitant has to be under-taken as otherwise load peaks cannot be covered with certainty. With regard to the costs of precipitant and with the higher sludge yield associated with the input of precipitant, this strategy is not to be rec-ommended for large plants. The quantity of precipi-tant can already be reduced effectively through the specification of different day and night dosing quan-tities. A further improvement is also possible through the dosing according to a specified load hydrographic curve. With hydrograph control one is concerned in principle with the replacement of a measured quan-tity with an empirical value. Representative daily

hydrographs of the phosphate load are determined through measurements and placed in automation systems for control. Here, it has shown itself to be sensible to record hydrographs working days as well as for both weekends and holidays separately. Industrial discharges must, under certain circum-stances, also be recorded separately. For practical purposes the hydrographs are determined in the effluent of the activated sludge stage as, through this, the influence of the biological phosphorus re-moval is taken into account. In principle control according to a hydrograph is, however, not in a position to react to unforeseen variations of the phosphate load. As, for example, the efficiency of the biological phosphorus removal can also vary, a safety reserve must always be cre-ated with this concept through overdosing. The hy-drograph should be capable of being amended simply by operating personnel and thus matched to the changing requirements.

ATV-DVWK-M 206E

November 2001 11

Fig. 2: Control according to P-load

6.3.2 Control according to P-Load With this process the product from the wastewater flow at the site of the P measurement and the phosphorus concentration for the control of the dos-ing facility is used (comp. Fig. 2). The precipitant flow QFM here results as

QPF = k •. Q •. CP (Eqn. 1) with: QPF = precipitant flow, e.g. in m3/h k = proportionality factor, e.g. in l/mg Q = wastewater flow at the site of the measurement of the P-concentration,

e.g. m3/h CP = measured phosphorus concentration, z. B. mg/l The proportionality factor k (here for example for precipitant containing iron) is

k = f •. ß •. 55.8/30.9•. 1/ρ •. 1/mMe (Eqn. 2) Here the following is taken into account: f = safety factor ß = ß-value, e.g. 1.1 mol Fe/mol P ρ = specific weight of the precipitant

solution, e.g. 1200 kg/m3 mMe = effective metal content, e.g. 87 kg

Fe/1000 kg precipitant solution 55.8/30.9 = ratio of the mol masses of iron and phosphorus The safety factor f is to be set according to operat-ing experience and normally lies between 1.0 and 1.5. This concept can be realised with simultaneous precipitation basically in two process engineering variants (comp. Fig. 1):

Variant a: Measurement site for CP and dosing

point in the influent to the aeration tank (D)

Variant b: Measurement site for SPO4 and dosing point in the effluent of the aeration tank (F or G)

With Variant a the proportionality factor k must also take into account the estimated incorporation of phosphorus in the surplus sludge. The concept therefore contains a large uncertainty which has to be balanced through higher precipitant dosing. With Variant b the biological P-removal - both planned and unplanned – is completed and there-fore no longer requires to be taken into account in the proportionality factor. Using this concept changes of the phosphorus concentration can be reacted to very rapidly and accurately. The load-controlled dosing can be applied particu-larly where other concepts, for example due to large delay and dead times or control engineering unfavourable arrangement of the reactors, cannot be applied. With this process, however, no direct control of the effectiveness of the precipitation is possible. 6.3.3 Control according to Wastewater

Flow The control of the precipitant dosing according to the wastewater flow is a simplified variant in com-parison with control of the P load. This concept is suitable when the P-concentrations in the influent vary only slightly. With combined wastewater flows, which cause a reduction of the P-concentration, such a procedure, however, leads to a significant overdosing.

ATV-DVWK-M 206E

November 2001 12

Fig. 3: Control according to wastewater flow

Fig. 3 shows a typical characteristic curve for the control of the precipitant dosing according to the wastewater flow. With the undercutting of the value b) for the quantity of wastewater the proportionality factor k between wastewater flow and precipitant no longer applies, rather the dosing remains at a con-stant level. With a long-term undercutting of a “limiting quantity of water” c), which is to be determined em-pirically, a dosing of precipitant can possibly be dis-pensed with. If the wastewater flow exceeds the restart limit d), the dosing of precipitant is restarted. If the “limiting quantity of water” c), with the lower-ing of the water flow, is not achieved or is undercut, the dosing of precipitant remains up to the achievement of the value b) at a constant level. The selection of the limiting value must be in such a way that stable dosing conditions are set, that means no too frequent changing between switching in and out. In particular it must be checked in nor-mal plant operation whether the restart limit d) can be set smaller or larger than b).

In smaller and more medium sized plants this strat-egy, an effective and economic application of pre-cipitant can be realised without additional technical measuring expense.

6.3.4 Regulation of SPO4 The most favourable solution for technical regula-tion is an addition of precipitant into the effluent or the outlet area of the aeration tank, whereby the dosing of the precipitant is undertaken dependent on the orthophosphate concentration. Through a locking in of the wastewater flow (or the load) this regulation can be improved further. In both cases a continuous measuring of the SPO4 is necessary which, with thorough mixing, can take place several metres, otherwise up to 20 m or more behind the dosing point.

ATV-DVWK-M 206E

November 2001 13

Fig. 4: Control of the orthophosphate concentration

In order to guarantee the maintenance of the moni-toring value for the phosphate concentration (CP) in the effluent of the wastewater treatment plant the design value for SPO4 at the measuring site must be smaller than the monitoring value. That is neces-sary in order to take into account the P-load which is contained in the residual suspended matter which is contained in the effluent. The P-concentration, which is contained in the solid mat-ter, results from the percentage by mass of the phosphorus of filterable solids and their concentra-tion in the effluent of the plant: CP,Part = mP,SS • CXSS (Eqn. 3) with: CP,Part = P-concentration due to residual sus-

pended (in mg/l) mP,SS = P-contents of dry matter in the sludge (in

mg P/g SS). This value is normally 25 – 35 mg P/g SS

CXSS = concentration of the filterable solids (in g/l)

Furthermore an “increased factor of safety” F to take into account the resolution effects in the downstream treatment stages of non-precipitable phosphate compounds as well as uncertainties in measurement of the process analysis equipment is required which, on the basis of operational experi-ence, is to be set at about 0.2 mg/l SPO4. Thus there results as PO4-P design value for the regulation of the P-dosing a concentration SPO4,Spec of:

SPO4,des = CP,MV – CP,Part – F (Eqn. 4) with: SPO4,des = orthophosphate design value in mg/l CP,MV = monitoring value for the Ptot concen-

tration in the effluent in mg/l CP,Part = P-concentration through negative lift

in mg/l F = increased factor of safety as empirical

value in mg/l Fig. 4 shows the regulation concept for simple regulation of the phosphate concentration. The quality of regulation can be improved still fur-ther through the locking in of the wastewater flow as influence quantity (comp. Fig. 5). This is particu-larly interesting with plants with combined biologi-cal–chemical phosphate removal. Here, with hy-draulic peaks, resolved phosphate is often dis-placed in surges from the anaerobic zone into the aerobic zone. With too short retention times for an extensive take up of phosphate or with short-circuit flow, the phosphate concentration in the effluent of the activated sludge stage can increase very rap-idly. This effect can be countered through a distur-bance variable compensation depending on the quantity of water as this affects a timely increase of the dosing quantity. Attention is to be paid that the locking in of influencing quantities is so arranged that, following the start up of a combined wastewa-ter inflow, it remains effective for a certain time only.

ATV-DVWK-M 206E

November 2001 14

Fig. 5: Control of the PO4-P concentration with locking in of the influencing quantity of the

wastewater flow

6.3.5 Other Control Concepts The control concepts described can be expanded or combined taking into account the existing process technical conditions in the practical case. For exam-ple the regulation of the PO4-P concentration with a locking in of the wastewater flow shown as an ex-ample in Fig.5 can be so expanded that, in accor-dance with the model of Fig. 2, not the wastewater flow but rather the P-load is locked in as influencing quantity. Furthermore it can be practical or neces-sary to carry out the locking in not as a static locking in but rather that this is done via dynamic elements, for example as “moderating locking in” (comp. Sec-tion 6.3.4). In individual cases so-called “knowledge based regulation systems” are also employed of which fuzzy controls have achieved the highest degree of familiarity. With these approaches, control is defined in the form of verbally formulated, blurred rules. As-suming a clear number of these rules, control solu-tions result which are characterised in general through transparency and good reproducibility. The necessary process knowledge for the design of such control systems corresponds with that for the design of the simple conventional control and regulation described in Sections 6.3.1 to 6.3.4. The dynamic systems behaviour of a fuzzy control system can be interpreted as non-linear characteristic diagram. Due to the few plants managed by this type of regulation a detailed handling of this area is not included in the scope of this Advisory Leaflet.

6.3.6 Substitutional Value Strategies The above presented automated concepts as-sume that the actual phosphorus concentra-tions present are available reliably and as far as possible continuously as measured values. In order that a failure of the measurement signal, for example during a calibration or with an equipment fault is not associated with negative consequences for the process control, substitu-tional values must therefore be secured for the automation which, in place of the real process values, ensure emergency operation. Further-more, it is to be defined when the measured value is to be considered as faulty and, instead of this, a substitute value is to be applied. Within the scope of the substitute value strat-egy it is determined which of the following processes are to be applied for the creation of substitute values: – declaration of a fixed value as default

value, – specification of characteristic curves, – adoption of measured values from a train

operated in parallel, – employment of the last undisturbed value

with or without extrapolation in time, – employment of auxiliary parameters. The following can be viewed as criteria for a “measured value fault” with the activation of a substitute value strategy:

ATV-DVWK-M 206E

November 2001 15

– so called “live zero” – monitoring of the 4 – 20 mA signal of the measuring transducer,

– status report (self-monitoring) of the analysis equipment,

– deviation from the plausible range of measured values through monitoring of the upper and lower boundary values,

– operation of a “maintenance switch” with calibra-tion procedures.

7 Storage and Dosing Technology

7.1 General

Common precipitants are summarised with their im-portant physical and chemical data in Table 2. Solu-bility, density and viscosity depend strongly on the temperature and the substance contents and there-

fore place particular requirements on the dosing facilities. The precipitants containing iron and aluminium as well as milk of lime are classified as weak water hazarding substances (WHC 1). The fa-cilities for the storage and dosing of chemicals are to be established and operated in accor-dance with the requirements of the respective [German] Federal State (e.g. Ordnance on the Handling of Water Hazarding Substances and on Specialist Operation [In Germany = VAwS]). The aqueous solution of acidic precipitants based on iron and aluminium salts is, as a re-sult of the low pH-value and the high salt con-centration, extremely corrosive. All parts in con-tact with the solution must therefore be made resistant to acid, for example covered in plastic or with a suitable coating. With the employment of GRP, attention should be paid that the plastic coating is acid- and alkali-resistant, in order that acidic or alkaline precipitants can be stored as desired.

Table 2: Physical and chemical data on the most common precipitants

Product supplied Examples for normal solution Percentage by Prec Active

subst.

Density or bulk density

Viscosity Operating temperature

Precipitant (main component)) Typical form of de-livery

WHC

% % g/cm3 mPa s °C Iron(III) chloride FeCl3 Solution (32-42 %) 1 40 13.8 1.43 (20 °C) 10 (20 °C) > - 12 Iron(II) chloride FeCl2 Solution (20 – 30 %) 1 20 8.7 1.36 3 (20 °C) > - 15 Iron(III) chloride-sulphate

FeClSO4 Solution (ca. 40 %) 1 41 12.3 1.52 42 (15 °C) > - 10

Iron(II) sulphate FeSO4 . 7H2O Crystalline bulk ma-terial

1 - 17.8 – 19.6

1.2 (20 °C) 3 (20 °C) > - 2

Aluminium chlori-de

AlCl3 Solution (30-40 %) 1 30 6 1.3 (20 °C) 10 (20 °C) > - 20

Polyaluminium chloride

Al(OH)3-xClx Solution (5-10 %) 1 - 5.9 – 7.5 1.3 10 (20 °C) > -15

Aluminium sul-phate

Al2(SO4)3 Solution 1 24 4 1.27 10 (20 °C) > -15

Sodium aluminate NaAl(OH)4 Solution (5-12 %) 1 - 7.3 – 11 1.3 (20 °C) 20 (20 °C) * to 200 ** (20 °C)

> 200 (20 °C) **

> - 20

White fine lime CaO Powder 1 - - 0.8 –1.0 - - White lime hy-drate

Ca(OH)2 Powder 1 - - 0.4 - -

Milk of lime Ca(OH)2 Suspension (20-40 %)

1 20 - 1.1 (20 °C) 100-150 *** 800-1200 ****

> 0

* 5 % Al content ** 11 % Al content ***, **** 10 % solution; different values through different grain size distribution

Table 3 contains information on the resistance of various material compared with aqueous solutions of the precipitant.

ATV-DVWK-M 206E

16 November 2001

Tab. 3: Resistance of various materials to aqueous solutions of precipitants at 20 °C (1 = good resistance, 2 = limited resistance, 3 = non-resistant)

Material:

Aqueous solution of:

St 35, St 37

1.4301 (V2A)

1.4571 (V4A)

GCI*, NGI**

Titanium PVC-U

HD PE

GRP

Iron(II) chloride 3 3 3 3 1 1 1 2 Iron(III) chloride 3 3 3 3 1 1 1 2 Iron(III) chloride-sulphate 3 3 2 3 1 1 1 2 Iron(II) sulphate 3 2 1 3 1 1 1 1 Sodium aluminate 1 1 1 1 1 1 1 1 Aluminium chloride 3 3 2 3 1 1 1 1 Aluminium sulphate 3 2 1 3 1 1 1 1 Aluminium hydroxide chloride 3 2 2 3 1 1 1 1 Milk of lime 1 1 1 1 1 1 1 1

* GCI = grey cast iron ** NGI = nodular graphite iron

Precipitants based on iron and aluminium salts and sodium aluminate are yielded as by-product in the metal and chemical industries. They can contain residues and foreign material which complicate the employment in wastewater treatment. Type and share of these substances as well as the observa-tion of the permitted limiting values are to be given or guaranteed by the producer. Maximum values are, inter alia, contained in ATV-A 202E. Pour able precipitants tend, with longer storage to material compacting and hardening (bridge build-ing). Therefore special measures for storage are necessary in particular with hygroscopic products. (see Chap. 7.3.3). Sodium aluminates are as a rule also not stable for longer than six months. Here precipitation and hardening can take place.

7.2 Dosing Facilities

The dosing facilities and the pipelines associated with these must satisfy the requirements of the VAwS. Also with regard to the material used the same requirements are to be applied as for facilities for disposal (so-called produce – treat – use plants [in German: HBV plants: Herstellen Behandeln Ver-wenden]) and for warehousing (so-called store – fill – tranship plants [in German: LAU plants: Lagern Abfüllen Umschlagen] ). As the dosing facility functions as final control ele-ment its delivery range is to be agreed with the re-quired delivery performance in the actual and de-sign condition (expansion status). A too high dosing performance is frequently applied, whereby the dosing accuracy in the lower operation range is affected adversely or too high quantities of precipi-tant are always applied. The highest dosing performance per hour should therefore be no more

than 15 % of the daily performance. In case the pump is dimensioned too large an intermittent dos-ing can in emergency further reduce the consump-tion of precipitant. Exceptions here are formed by plants with deliberate biological phosphate removal. Here, if required, even a separate pump to cover peak loads can be practical. The precipitants in general are dosed through dos-ing pumps or under gravity, e.g. from a levelling bulb, through a fitting. With plants with several dos-ing points it is recommended to carry out dosing from a ring circuit in order to ensure an even distri-bution and to avoid incrustation. Ring circuits should also be employed with suspensions which tend towards depositing. As dosing pumps mainly reciprocating and rotary pumps are used, such as diaphragm pumps, piston diaphragm pumps and eccentric screw pumps as well as, less often, hose pumps. Due to their form reciprocating and rotary pumps maintain with higher accuracy the dosing quantities (design values) specified by the control or regulation facility. For circulation mainly centrifu-gal and eccentric screw pumps are employed. Connections for flushing, air removal and emptying are to be supplied in sufficient numbers for running up and running down. They should be installed at points where depositing and precipitation takes place. Externally laid pipelines are to be insulated and equipped with concomitant heating. With frost-free laying in the ground this can be dispensed with. They are basically to be produced as pipe-in-pipe systems which have a gradient to a visible outlet or manhole. They are to be checked before commis-sioning and subsequently according to the provi-sions of the applicable VawS. The output with reciprocating and rotary pumps, depending on the system, is influenced by the

ATV-DVWK-M 206E

November 2001 17

number of revolutions per minute, piston stroke or number strokes per minute, with eccentric screw pumps by the revolutions per minute, with shut-off devices by the duration of the opening time and with regulator devices by the degree of opening. The dosing and circulation pumps should as far as possible be set up in the vicinity of the storage tanks, in order to keep the suction lines short. Fun-damentally the suction line should be a nominal width larger than the pressure line. With piston and piston diaphragm pumps operating behaviour is improved by pulsation dampers. With dosing using pumps care is to be taken to provide a sufficient back pressure on the delivery side. With several dosing points and different deliv-ery heads the pressure drop is to be taken into ac-count. Therefore appropriate measures, such as pressuriser valves or separate dosing pumps, are to be planned. For the layout it is to be noted that the performance data of the dosing facilities are

related to clean water, but precipitants have a higher viscosity and density. In particular, the dif-ferent viscosity of the precipitant depending on temperature and metal content is often not taken into account. The high - in comparison with iron salts – viscosity of sodium aluminate with higher Al contents (ca. 10 %) and low precipitant tempera-tures is to be noted particularly in this case.

7.3 Storage and Dosing

7.3.1 Liquid Precipitants The necessary equipping of a plant for the storage and dosing of liquid precipitant is shown in Fig. 6. Here the storage tank (1) is represented with dou-ble walls. Single-wall tanks are to be placed in col-lecting troughs without outlet which have the same capacity.

Fig. 6: Storage and dosing station for liquid precipitants

ATV-DVWK-M 206E

18 November 2001

The following details are pointed out: – start-up facilities (10,14), which can be re-

placed by a filling connection for process water as an alternative (17).

– flap trap with strainer basket (16). – overflow valve (22) for the security of the pump

membrane and the pressure side pipeline sys-tem.

– the delivery flow can be determined using sus-pended solid particle flowmeters or MIF (24). with larger plants the continuous registration of the precipitant flow using MIF is sensible for operating reasons. At least one container should be firmly installed for the gauging of ca-pacity in litres in order thus to be able to check the dosing simply at certain time intervals.

– leakage monitoring (29), which indicates leaks in the dosing station and the pipeline system and which possibly automatically activates a

ventilation valve on the suction line in order to avoid a siphoning of the tank.

– expansion fittings are to be provided for the maintenance intensive systems.

– in order to avoid overfilling with certainty, a second, independent measuring systems (digi-tal limit selector) should be installed.

– with a change of precipitant the system – in particular the tank – is to be washed out care-fully as precipitant residues can react with the new product (precipitation). Furthermore the plant must be approved for the new precipitant.

7.3.2 Non-Pourable Precipitants Fig. 7 shows the system setup of a facility for the dosing of iron(II) sulphate, in which the installations for storage, dissolving of the salts and the storage of the solution (1) are combined in one structural unit.

Fig. 7: Storage and dosing station for non-pourable precipitants

ATV-DVWK-M 206E

November 2001 19

Attention is drawn particularly to the following: – it is recommended that the cover of the filling

opening (6) is equipped with load-relieving weights or springs for easy handling. As faulty filling cannot be avoided the tank should be provided with openings, safeguarded by sieves (5) for the introduction of flushing water. With the planning of the structure and the access routes attention is to be paid that vehicles can approach and dump their cargo without prob-lem.

– in order to avoid a spilling of crystalline salt dur-ing filling, the dissolving bunker must be exten-sively emptied before charging (19).

– the consumption of precipitant solution is com-pleted automatically through the addition of so-lution water (7).

– dosing takes place in Fig. 7 from the levelling tank (11). Attention is to be paid to the careful retention of undissolved salt in the dissolving chamber.

– for improved dissolving it is expedient to pump over the solution (17, 18).

In addition the last five bullet points in Section 7.3.1 are to be taken into account.

7.3.3 Pourable Precipitants Fig. 8 shows the necessary equipping for the stor-age, preparation and dosing of milk of lime as lime hydrate. In general lime hydrate is delivered in silo vehicles and transferred into the storage silo (6). The delivery of precipitant is supported pneumati-cally via air cushions (5). The blower (2) should, as far as possible, be mounted directly on the silo. An automatic monitoring of the cleaning intervals and a monitoring of the pressure difference of the exhaust air filters (12) is recommended. The filling level is monitored using mechanical detectors (7, 8), for example rotating blades, pivoting forks, or moni-tored via the silo weight through continuous meas-urement equipment (15), for example: pressure pickup. Cellular wheel sluices prevent the “shoot-ing” of the silo content material and serves as initial distributors for the downstream spiral conveyer (14) or container scales. The material is transported via the spiral conveyor (14) by charge into the batching and storage tanks (16). The addition of dilution wa-ter (13) is controlled via the filling level (19). Nor-mally the silo material, as no high demands have for accuracy have to be placed on the mass flow, is added via metering screws proportional to volume. By means of special structural design, for example as hollow screw, opposed double-lead screws or as metering screw with superimposed rotational and axial movement, a self-cleaning effect is achieved and encrustation prevented.

ATV-DVWK-M 206E

November 2001 20

Fig. 8: Storage and dosing station for pourable precipitants

The milk of lime is dosed, from the storage tank, from a ring circuit (23), for example via rubber pinch valves (24). The dosing of milk of lime, in addition, sets special requirements: – concentration of the suspension: 5-10 %,

maximum 15 %. – with several dosing points as well as with long

transport paths it is practical to dose from a ring circuit. The suspension is pumped around this in order to avoid sedimentation.

– ring circuits and dosing pipelines are for practi-cal purposes made from fabric reinforced plas-tic hose (∅ > 1″, radius of curvature > 5 times ∅). Baffles and changes of cross-section are to be avoided; flow rate > 1.5 m/s.

– dosing pipeline outlets from the ring circuit may branch upwards only in order to avoid sedimen-tation of the milk of lime.

– rubber pinch valves have proved to be successful as dosing fixtures.

– dosing lines, as far as possible, should dis-charge below the water level.

– ring circuits should always be filled, when idle with water. Longer pipelines should be flushed with water on completion of dosing.

– connections for the acidulation of the pipeline system are to be provided.

In addition the requirements from the last three bul-let points in Section 7.3.1 are to be considered analogously. Below some peculiarities with pourable precipitants are additionally pointed out: – with the storage of pourable iron(II) sulphate

and iron(III) chloride it is necessary to protect the silos from the sun as the products upwards from ca. 40 °C tend to form lumps.

– with hygroscopic precipitants the exit opening of the dry material screw conveyor (14), the screw mouth, is particularly sensitive to back-ing-up, as the moist air rising from the solution tank below condenses here. In this case an as-

ATV-DVWK-M 206E

November 2001 21

sociated heating and an automatic gate valve below the screw outlet opening should be pro-vided. However, basically, the screw mouth should be easily accessible in order to be able to carry out possible necessary cleaning tasks.

7.4 Measurement of the Precipitation Concentration

The density and concentration of the precipitant are linearly dependent on each other. Thus the density can be used in order to determine the salt content of the precipitant solution. This can take place manually discontinuously by means of spindle measurement according to Beaumé, continuously using a differential pressure measurement or a us-ing a mass flowmeter in accordance with the Corio-lis principle. The concentration of the working sub-stance is to be taken from the data sheet of the re-spective supplier.

7.5 P-Removal by Raising the pH-Value

With a precipitation using hydrated lime, the dosing system must be so designed that, despite varia-tions of the flow of wastewater, the desired pH value can be maintained at +/- 0.1 pH precisely, as the pH value to be maintained in the effluent lies very close to the operating point of simultaneous precipitation (pH = 8.6 – 9.0). Gel electrodes with short reaction times have shown themselves to be suitable for pH measure-ment. The manufacturers offer electrodes which are especially suitable for the dosing of milk of lime (e.g. electrodes with Teflon diaphragms). The elec-trode brackets should have large flow openings so that they cannot clog. In addition they must be mounted in such a way that the electrodes are eas-ily accessible for maintenance and can be wetted at every water level. In addition, it is an advantage for regular mechanical cleaning to employ an auto-matic electrode cleaner with which the electrode(s) can be washed or acidated at short intervals. The acid flushing facility must be designed in such a way that no tangles can form on the spray ele-ments. Systems which clean the electrodes in a separate tank can be an advantage here. During cleaning the control facility (automatic) must be in-terrupted and the last dosed quantity of lime kept constant until the pH electrode is again ready for operation. It is recommended that, in addition to the control electrode an additional further electrode is applied for the monitoring of the system.

8 Economic Efficiency

The economic efficiency of the measures for the automation of chemical phosphate removal is to be checked carefully in every case. As a rule, savings with the required quantity of precipitant as well as with sludge treatment and disposal counter the ex-penditure for measurement and control technology. Advantageous, but barely appraisable, is the stable maintenance of the monitoring value. Already the installation of simple controls (input of precipitant dependent on specified characteristic curves or dependent on flow of wastewater) is worthwhile even for smaller plants. Upwards from a certain design capacity the employment of process analysis equipment in combination with the previ-ously described regulation and control concepts is, however, more economic. The total system of the measurement system is always to be considered for the assessment of the costs. In addition to investment costs for the ana-lyst, a possible necessary space for the accommo-dation of the system, the pre-treatment of samples and the continuous transfer of samples, in particu-lar also the running costs for reagents, replacement parts and expendable items, servicing and mainte-nance and other incidentals are to be included (comp. ATV-DVWK M-269). To be added to these are the costs for automation. Cost considerations according to the Cost Com-parison Calculation (KVR) Directive of the LAWA [German Federal State Working Group Water] show that the employment of process analysis equipment, depending on local conditions, as a rule is economical upwards from a connection capacity of some 40,000 PT. Frequently such equipment is already present for other reasons (e.g. monitoring of documentation and operation). Then it is possi-ble to combine these also into the regulation and control concept, if necessary the equipment for this can be transferred to suitable positions (comp. Chapter 6). The economic efficiency of process control with solely chemical P-removal is substantially depend-ent on the variations of the P-concentration at the dosing point. Thus, with relatively constant P-concentration, only a limited amount of precipitant can be saved and the precipitation sludge yield re-duced. If the chemical P-removal is employed to supplement biological P-removal, although the P-concentrations at the dosing point in the outflow of the biological stage overall are significantly smaller than without biological P-removal, the variations in

ATV-DVWK-M 206E

22 November 2001

concentration can, in certain cases, be large due the varying process behaviour of the biological P-removal. Here, through process control, there is the possibility of stabilising the biological P-removal through prevention of a large input of precipitant and thus indirectly reduction of the requirement for precipitant and the sludge yield. Furthermore, it is pointed out that an adaption of the storage and dosing facilities also contributes to the economical efficiency of phosphate removal. In particular with very small plants it is not practical to install very large storage tanks as here, with com-plete filling, the precipitant has to be stored too long. This is to be noted particularly with sodium aluminate and pourable precipitants. Although higher costs result with smaller storage tanks and smaller delivery quantities, the investment costs of smaller tanks are lower and the operation simpler.

9 Ordinances, Standard Specifications and Standards

[Translator’s note: References available in English are shown as such. For those references with no known of-ficial translation a courtesy translation is provided in square brackets]

EN 879 Aluminium sulphate, iron-free, for

treatment of water intended for human consumption: 1992

EN 881 Aluminium chloride, aluminium hydroxide chloride and aluminium hydroxide chloride sulphate (monomer), for treatment of water intended for human consumption: 1997

EN 882 Sodium aluminate, for treatment of water intended for human con-sumption: 1997

EN 883 Polyaluminium chloride hydroxide and –chloride hydroxide sulphate, for treatment of water intended for human consumption: 1997

EN 887 Aluminium iron sulphate, for treatment of water intended for human consumption: 1992

EN 888 Iron(III) chloride, for treatment of water intended for human con-sumption: 1998

EN 889 Iron(II) sulphate, for treatment of water intended for human con-sumption: 1998

EN 890 Iron(III) sulphate, for treatment of water intended for human con-sumption: 1998

EN 891 Iron(III) chloride sulphate, for treatment of water intended for human consumption: 1998

EN 935 Aluminium iron chloride and alu-minium iron hydroxide chloride (monomer), for treatment of water intended for human consumption: 1992

EN 1189 Water quality – determination of phosphorus – by the ammonium molybdate spectrometric method: 1996

EN 12255-13 Wastewater treatment plants, Part 13: Wastewater treatment through addition of chemicals

DIN 19 611 Weißkalk zur Wasseraufbereitung – Technische Lieferbedingungen [White lime for the processing of water – Technical delivery condi-tions], Issue 1983-04

DVGW W 622 Dosieranlagen für Flockungs-mittel und Flockungshilfsmittel [Dosing facilities for flocculants agents and flocculation aids.

ATV-DVWK-A 131E Dimensioning of Single-Stage Activated Sludge Plants, (2000) ATV-A 202 Verfahren zur Elimination von Phosphor aus Ab-wasser [Processes for the Removal of Phosphorus from Wastewater] (1992) ATV-DVWK-M 269 Prozessanalysengeräte zur Bestimmung von N, P und C in Abwasseranlagen [Process Analysis Equipment for the Determination of N, P and C in Wastewater Systems] (2000) KVR-Richtlinie [CCC Directive] Leitlinie zur Durchführung dynamischer Kostenver-gleichsrechnungen [Guideline for the carrying out of dynamic cost comparison calculations], Publ.: LAWA, Kultur-buchverlag Berlin GmbH, ISBN 3-88961-228-8 NN Verordnung über Anlagen zum Umgang mit wassergefährdenden Stoffen und über Fachbe-triebe (VAwS), länderspezifisch [German Ordinance for plants on the handling of water-hazardous substances and on specialist op-eration (VAwS), specific for each German Federal State] NN Herstellung, Lagerung und Dosierung von Kalkpro-dukten [Production, storage and dosing of lime products];

ATV-DVWK-M 206E

November 2001 23

Bundesverband der Deutschen Kalkindustrie e.V., Köln (1992), pH-gesteuerte Dosierung von Kalk-milch zur simultanen Phosphorelimination auf Kläranlagen [pH controlled dosing of milk of lime for simultane-ous phosphorus removal in wastewater treatment plants]

NN Herstellung und Dosierung von Kalkmilch [Production and dosing of milk of lime]; Bundesver-band der Deutschen Kalkindustrie e.V., Köln (1986)