BS6297 1983 Code of Practice for Design and Installation of Small Sewage Treatment Works and...

8/11/2019 BS6297 1983 Code of Practice for Design and Installation of Small Sewage Treatment Works and Cesspools

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BRITISH STANDARD BS 6297:1983Incorporating

Amendment No. 1

Code of practice for

Design and installationof small sewagetreatment works and

cesspools (Formerly CP 302 and CP 302.200)

UDC 628.314.2-181.4+696.138

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BS 6297:1983

This British Standard, havingbeen prepared under thedirection of the Building ServicesStandards Committee, waspublished under the authorityof the Board of BSI andcomes into effect on29 April 1983

BSI 01-1999

The following BSI referencesrelate to the work on thisstandard:Committee reference SEB/19Draft for comment 80/15003 DC

ISBN 0 580 13123 8

Cooperating organizations

The Building Services Standards Committee, under whose direction thisBritish Standard was prepared, consists of representatives from the following:

Association of District Councils* Greater London CouncilBath Manufacturers Co-ordinating Committee Heating and Ventilating ContractorsBritish Gas Corporation AssociationBritish Ironfounders Association Incorporated Association of Architects andBritish Plastics Federation Surveyors*British Plumbing Employers Council Institute of PlumbingBritish Precast Concrete Federation Ltd. Institution of Environmental Health OfficersBuilders Merchants Federation Institution of Gas EngineersBuilding Services Research and Information Institution of Municipal Engineers*

Association Institution of Public Health Engineers*Chartered Institution of Building Services Institution of Water Engineers and Scientists*Clay Pipe Development Association Limited Local Authorities OrganizationConsumer Standards Advisory Committee of Metal Sink Manufacturers Association

BSI National Brassfoundry AssociationConvention of Scott ish Local Authorit ies* National Coal BoardCouncil of British Ceramic Sanitaryware National Federation of Building Trades

Manufacturers EmployersDepartment of Health and Social Security National Water CouncilDepartment of the Environment (PSA)* Royal Institute of British Architects*Department of the Environment (Building Royal Institution of Chartered Surveyors

Research Establishment) Royal Society of Health*Department of the Environment (Water Scottish Development Department*

Directorate) Trades Union CongressDepartment of the Environment (Housing Water Companies Association

and Construction)*Domestic Solid Fuel Appliances Approval

Scheme

The organizations marked with an asterisk in the above list, together with thefollowing, were directly represented on the Technical Committee entrustedwith the preparation of this British Standard:

British Water and Effluent Treatment Plant Institute of Clerks of Works of Great Britain Association Inc.

Cement and Concrete Association Institute of Water Pollution ControlInstitute of Building Control Officers Water Research Centre

Amendments issued since publication

Amd. No. Date of issue Comments

6150 December 1990 Indicated by a sideline in the margin

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Contents

PageCooperating organizations Inside front cover

Foreword iiiSection 1. General1 Scope 12 References 13 Definitions 14 Collection of information 3

Section 2. Materials5 Notes on materials 3

Section 3. Design6 Design: general 47 General requirements for tanks 6

8 Cesspools 69 Septic tanks 710 Preliminary treatment 1011 Primary and secondary settlement tanks 1112 Biological filters including rotating biological contactors,

and secondary settlement tanks 1313 Activated sludge units 1614 Tertiary treatment (polishing) processes 1915 Disposal of final effluent 2016 Pumping 2217 Automatic monitoring and alarm systems 23

Section 4. Installation18 Installation 23

Appendix A References to standards not included in the text 25 Appendix B Relevant sections of statutes and regulations currentlyapplicable to small treatment works and cesspools 26

Index 41

Figure 1 Sewage treatment: broad options for small communities 8Figure 2 Typical septic tanks, two in series, separate, forpopulations of up to 30 27Figure 3 Typical septic tanks, two in series, separate, forpopulations of over 30 28

Figure 4 Typical septic tanks, two in series, combined,for populations of up to 30 29Figure 5 Typical septic tanks, two in series, combinedfor populations of over 30 30Figure 6 Twin inlets for tanks in excess of 1 200 mm wide 31Figure 7 Typical tank inlet (crested weir) 32Figure 8 Typical upward flow settlement tank 33Figure 9 Typical horizontal flow settlement tank 34Figure 10 Typical rectangular biological filter 35Figure 11 Typical circular biological filter 36Figure 12 Treatment on grass plots 37

Figure 13 Typical upward flow clarifier 38Figure 14 Cross section of typical underdrain 39Figure 15 Typical sludge drying bed 40d

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PageTable 1 Filter medium capacity 14Table 2 Grading limits for 50 mm filter medium 15Table 3 Air supply 17

Publications referred to Inside back cover

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Foreword

This code of practice, prepared under the direction of the Building ServicesStandards Committee, encompasses the subject matter previously covered by

codes of recommended practice for small sewage treatment works, CP 302:1972,and for cesspools, CP 302.200:1949. CP 302 and CP 302.200 are withdrawn.The use of cesspools or of septic tanks without further treatment of their effluentis generally considered not to be good practice but it is recognized that in somesituations such installations are the only practicable means of dealing withsewage.The increasing and wider application of synthetic materials and the production oflarger prefabricated (package) units has been recognized and the scope of the codehas been extended to include treatment units to deal with sewage frompopulations of up to 1 000 persons.New processes developed since the publication of the previous code, such as therotary biological contactor, are now included and it is intended that the codeshould not inhibit the development and application of other suitable newprocesses.The code gives guidance for those experienced in the design of small sewagetreatment works. However, it is recognized that not all works are regularlydesigned by such persons and it is strongly recommended that specialist adviceshould be sought where appropriate, including where ground conditions aredifficult, where there are likely to be abnormal flow or pollution loads, and for thedesign of tanks, and biological or other processes.It is not within the scope of this code of practice to set out in detail the fulloperation and maintenance requirements of small sewage treatment works.

Adequate maintenance is essential for even the smallest works to ensure that itproduces the standard of effluent required, and instructions for proper operationand maintenance should be provided by the designer. For general guidancesee National Water Council Technical Paper No. 4 The Operation andMaintenance of Small Sewage Works.

A British Standard does not purport to include all the necessary provisions of acontract. Users of British Standards are responsible for their correct application.

This code of practice represents a standard of good practice and takesthe form of recommendations. Compliance with it does not of itselfconfer immunity from relevant legal obligations.

Summary of pagesThis document comprises a front cover, an inside front cover, pages i to iv,pages 1 to 42, an inside back cover and a back cover.This standard has been updated (see copyright date) and may have hadamendments incorporated. This will be indicated in the amendment table onthe inside front cover.

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Section 1. General

1 ScopeThis code of practice deals with the design andinstallation of sewage treatment works suitable forthe domestic discharge from domestic andindustrial communities ranging from singlehouseholds up to about 1 000 population equivalentand with the storage of sewage by means of acesspool, the contents of which are periodicallyremoved for disposal or treatment.Domestic discharges are taken to include those fromschools, hotels, restaurants, etc. but the code doesnot deal with the treatment of trade effluents, or the

effluent from chemical closets.General guidance only is given on good design andinstallation practice. Particular requirements willbe determined by local conditions. The codesrecommendations should be supplemented asrequired by skilled engineering advice based on aknowledge of sewage works practice and of localconditions.Materials for tanks and other structures are notindicated on the diagrams, which are included forguidance on general proportions and details ofinlets, outlets and other features, and should not bescaled.

2 ReferencesThe titles of the publications referred to in thisBritish Standard are listed on the inside back cover.

A list of other standards of interest in this field isgiven in Appendix A.

3 DefinitionsFor the purposes of this code of practice thefollowing definitions apply.

3.1

activated sludgea flocculent microbial mass, produced when sewageis continuously aerated

3.2aerobic action

a biological process promoted by action of bacteria inthe presence of dissolved oxygen

3.3anaerobic action

a biological process promoted by the action ofbacteria in the absence of dissolved oxygen

3.4baffle

a device used in a tank to check eddies and promotea more uniform flow through the tank

3.5biochemical oxygen demand (BOD)

the amount of dissolved oxygen consumed bymicrobiological action when a sample is incubated,usually for 5 days at 20 C

3.6biological filter

a bed of relatively inert material (such as slag,moulded plastics, clinker, etc.) to promote or assist

natural aerobic degradation of sewage3.7bottom water level (BWL)

the minimum operating water level in a pump wellor dosing chamber

3.8cesspool

a covered watertight tank used for receiving andstoring sewage from premises which cannot beconnected to a public sewer and where groundconditions prevent the use of a small sewagetreatment works including a septic tank

3.9combined system

a drainage system in which both foul and surfacewaters are conveyed in the same pipe

3.10distributor

a device for spreading settled sewage over thesurface of a biological filter

3.11dosing chamber

a small tank which receives settled sewage until thedesired quantity has accumulated, when it isdischarged automatically to the distributor of abiological filter

3.12dry weather flow (DWF)

when the sewage flow is mainly domestic incharacter, the average daily flow to the treatmentworks during seven consecutive days without rain(excluding a period which includes public or localholidays) following seven days during which therainfall did not exceed 0.25 mm on any one dayNOTE With domestic sewage from industrial premises the dry

weather flow should be based on the flows during five workingdays if production is limited to that period. Preferably, the flowsduring two periods in the year, one in the summer and one in thewinter, should be averaged to obtain the average dry weatherflow.

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3.13effluent polishing (tertiary treatment)

a further stage of treating sewage by removingsuspended solids. Consequential removal of residualBOD may occur

3.14filter medium

the material of which the biological filter is formedand on which a biological film containing bacteriaand fungi develops

3.15final effluent

the effluent discharged from a sewage treatment

plant3.16humus tank

see secondary settlement tank

3.17mixed liquor

a mixture of sewage and activated sludgeundergoing circulation and aeration in the aerationtank or channel of an activated sludge plant

3.18mixed liquor suspended solids (MLSS)

the concentration of dry solids in milligrams perlitre of mixed liquor in the aeration tank or channelof an activated sludge plant

3.19package plant

a prefabricated factory-built sewage treatmentinstallation

3.20population equivalent

the equivalent, in terms of a fixed population, of avarying or transient population, e.g. of a hospital orrestaurant, based upon a figure of 0.060 kg BOD perhead per day or 120 L per head per day3.21primary settlement tank

a tank in which the majority of settlable solids areremoved from the crude sewage flowing through it

3.22rotary biological contactor

a unit consisting of a series of closely spaced,parallel discs, mounted on a rotating shaft which issupported just above the surface of the waste waterto be treated

3.23scumboard

a device used at the outlet end of a tank to retainscum and other floating material

3.24secondary settlement tank

a tank in which settlable solids or humus isseparated from the effluent flowing through it frombiological filters or an activated sludge plant

3.25separate system

a drainage system in which foul and surface waterare conveyed by separate pipes

3.26septic tank

a type of settlement tank in which the sludge isretained for sufficient time for the organic matter toundergo anaerobic decomposition

3.27sewage

the water-borne wastes of a community

3.28storm sewage

sewage flowing to a treatment works in wet weather

or discharged from storm overflows, when thesewage is diluted with rainwater

3.29sludge

a mixture of solids and water produced during thetreatment of waste water

3.30sludge loading

the mass of BOD applied daily per unit mass ofactivated sludge MLSS

3.31specific surface

a property of biological filter media expressed assurface area per unit volume (m 2/m 3)

3.32supernatant liquor

the liquor in a settlement tank, lying between thedeposited solids and any floating scum

3.33suspended solids (SS)

solids in suspension in sewage liquors as measuredby filtration either through a glass fibre filter paperfollowed by washing and drying at 105 C, or bycentrifuging followed by washing and removal of thesupernatant liquidd

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3.34top water level (TWL)

the maximum water level in a settlement tank, anaeration tank, or a sludge storage tank

3.35water table

the level below which the ground is saturated withwater

4 Collection of informationThe following main items of basic informationshould be obtained before designing small sewagetreatment works:

a) requirements of the local building control andplanning authority;b) requirements of the appropriate water/riverauthority or its agent;c) minimum and maximum number of persons(resident and non-resident) to be served;d) average 24 h water consumption, and anyspecial conditions affecting the composition ofsewage and peak rates of flow; data areobtainable from the local water undertaking inmany instances;e) existence of infiltration water;

f) particulars of site;1) distance from nearest habitable building2) prevailing winds3) levels4) information as to the nature of the groundincluding the level and variations of the watertable5) access for vehicles and plant

g) particulars of outfall, e.g. tidal or inlandwaters, rivers, streams, ditches or soakage; alsothe proximity, highest known flood level and

minimum flow of any stream or otherwatercourse to which discharge of the effluent ispossible;h) conditions under which the works willnormally operate and be maintained;i) possibility of the need for future extensions ofthe works or of their elimination by acomprehensive scheme;

j) availability of electric power and mains water;k) facilities for eventual disposal of sludge andscreenings.

Section 2. Materials

5 Notes on materials5.1 General. All materials used in the constructionof any of the works described in this code shouldcomply with the relevant British Standards.Where no British Standard exists, materials shouldbe suitable and adequate for the purpose for whichthey are used.5.2 Aggregates for concrete. Aggregates shouldcomply with the requirements of BS 882, BS 1201or BS 1047. The nominal maximum size of coarseaggregate should be as large as possible within thelimits specified in the appropriate British Standard,

provided that the concrete can be satisfactorilyplaced and compacted. Where tests are requiredthey should be carried out in accordancewith BS 812.5.3 Aggregates for mortar. The fine aggregatesfor mortar should consist of sand complying with therequirements of BS 1198, BS 1199 or BS 1200, orconcreting sands in zones 3 and 4 from which theexcess coarse materials in grading zones 1 and 2of BS 882, BS 1201 have been removed.5.4 Cement. Cement used for works included inthis code should comply with the requirementsof BS 12, BS 146, BS 915, BS 4027 or BS 4248 1) .5.5 Cement mortar. Selection of the correctcement and aggregate for use in mortars shouldfollow the recommendations of 5.3 and 5.4 . A mortarmix having a 1 : 3 cement sand ratio is suitable forthe following purposes:

brickwork; jointing clay or concrete pipes where flexible joints cannot be used;rendering of inverts and benchings;bedding and haunching manhole covers andframes.

Calcium chloride should not be added to mortars.5.6 Concrete5.6.1 General. Concrete work should be inaccordance with CP 110-1 and BS 5328, as well aswith Building Research Establishment DigestNo. 174 and other publications issued by theBuilding Research Establishment and by theCement and Concrete Association. Reference shouldalso be made to the National Water Council CivilEngineering Specification for the Water Industry.

1) It should be noted that supersulphated cement made to BS 4248 is no longer manufactured in the United Kingdom although itcan be imported if it is required in relatively large quantities.

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5.6.2 Admixtures. Admixtures for promotingworkability, for improving strength, for entraining

air or for any other purpose should be used only withthe prior approval of the client or his representative.Calcium chloride as an admixture should not beused in reinforced concrete, prestressed concrete orany concrete made from sulphate-resisting Portlandcement. For guidance, reference should be made toCP 110.5.6.3 Workmanship. Concrete should be mixed in amechanical mixer until there is a uniformdistribution of the materials and the mix is uniformin colour. It should be transported to the point ofplacing as rapidly as practicable by methods thatwill prevent segregation or the loss of any of theingredients, placed as soon as possible andthoroughly compacted by rodding, tamping orvibration so as to form a void-free mass around anyreinforcement and into the corners of the formworkor excavation. Exposed concrete should be cured bykeeping it in a damp condition for at least four days.Concreting should not be carried out when theambient temperature is below freezing point orwhen a falling temperature of 4 C is indicated.If concreting has to be carried out at or near freezingpoint, precautions should be taken to ensure thatthe concrete when placed has a temperature of at

least 5 C and is maintained above 5 C until it hasthoroughly hardened. When necessary, the finishedconcrete should be insulated and protected afterplacing. Frozen materials, or materials containingice should not be used.5.7 Glass fibre reinforced cement (GRC). Tanksof GRC for use for small septic tanks and cesspoolsare under development; reference should be made toBuilding Research Establishment Digest No. 216 onthe suitability of this material where its use isproposed.5.8 Glass fibre reinforced plastics (GRP). Thestructural performance and durability of tanksconstructed of GRP is dependent upon the quality ofthe resin, the glass fibre reinforcement and theconditions and workmanship involved in thelaying-up and curing of the finished product. GRPhas a lower modulus of elasticity relative to itstensile or compressive strength in comparison tosteel, so it is essential that care be taken in thedesign to limit strain to acceptable levels; liaisonwith the manufacturer and close supervision on siteare therefore strongly recommended, and therequirements of BS 4994 should be followed.

5.9 Steel tanks. Pressed steel tanks complyingwith BS 1564 may be used, but should be

adequately protected, both on the inside and theoutside, against corrosion.5.10 Clay and concrete pipes and fittings. Allclay and concrete pipes and fittings should complywith the relevant British Standards, and whereverpracticable should have flexible joints.

Section 3. Design

6 Design: general6.1 Climatic considerations. Design factors inthis code are for average UK temperate conditionsonly and the performance of treatment units willvary with changes in temperature, exposure andaltitude. Design factors recommended should nottherefore be adopted for use in non-temperateclimates or in temperate climates with extremes ofconditions without special consideration. Theoperation of works is also affected by the prevailingweather. For example, the desludging of tanks mayhave to be carried out more frequently in hotconditions, and during periods of frost, filters andmechanical plant may be affected by freezing.6.2 General design considerations6.2.1 Installation. Sewage treatment works providefor the settlement and retention of solids andusually include biological treatment carried out bythe use of biological filters or activated sludge, or forthe biological treatment of raw sewage followed byseparation of solids. Before any process orcombination of processes is used, the requiredstandard of effluent 2) should be ascertained fromthe water/river authority.Cesspools are provided to receive and retain crudesewage and form no part of sewage treatment(see clause 8).

A septic tank installation provides only partial

treatment of sewage but is permissible withoutundue risk of pollution in some locations. Biologicaltreatment to follow primary settlement may benecessary and, in some cases, a further polishingstage is required (see clause 14 ).The design criteria given in this code relatespecifically to foul drainage flows and surface waterand subsoil water should be excluded from worksdesigned in accordance with the code. Where this isnot possible, i.e. with partially separate or combinedsystems, specialist design advice should be sought.

2) For example, normal requirements are 30 mg/L max. suspended solids and 20 mg/L max. BOD (described as 30 : 20).

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It is emphasized that a multiplicity of small sewagetreatment works in a limited area, particularly for

single houses, is undesirable. Greater efficiency ofoperation as well as economy of construction can beachieved by collective drainage and treatmentarrangements. Consideration should first be givento the possibility of providing such a scheme as apreferable alternative to several individual works.In choosing the type of treatment, the designershould compare the costs of maintenance andoperation as well as the initial capital cost of theworks. For example, the availability and type oflabour should be investigated and the cost ofelectricity and fuel in operating treatment units,pumps or other plant should be considered. Withregard to capital costs, the cost of different types offilter media and the probable life of the variousmaterials of construction are relevant.The designer should make adequate provision,where appropriate, for unusual pollution loads.These may arise from the use of waste disposal unitsand from the specialized occupancy of premises(e.g. public houses, industrial premises), or fromhigh flows from establishments such as hospitals,institutions and hotels. Domestic use of detergentsand disinfectants is not detrimental but excessiveuse may have a harmful effect on the performance ofthe works.Rags and floating debris are always a problem insewage treatment works, causing blockages andfouling mechanical plant. Prior removal from theincoming flow is advisable.Excessive quantities of grease and oil may causemalfunction of a small sewage works. In such cases,arrangements should be made where practicable forgrease and oil to be removed at source or for them tobe excluded from the sewerage system.Sludge is continuously produced in the settlementtanks and needs to be removed at frequentintervals. The disposal of the liquid sludge usuallyaccounts for 40 % of the works operating costs.Drying beds on site are an option but can give rise toodour and insect nuisance and pose problemsregarding clearance. The cost of removal by tanker,ideally to a larger works for treatment, or toagricultural land, is dependent upon distancetravelled, but is likely to be comparable in overallcost to drying beds and less problematical.

Measurement of flows on small works is difficult.Nevertheless, where practicable this should be

done. To minimize blockages it is more satisfactoryto measure flow of the final effluent. Wherecontinuous measurement is not installed a facilitysuch as a V-notch weir should be provided to permitthe use of portable measuring equipment whenrequired. Portable tipping troughs with recordingmeters attached have proved to be satisfactory.6.2.2 Location and safety6.2.2.1 Siting. Sewage treatment works should be asfar from habitable buildings as is economicallypracticable. The direction of the prevailing windshould be considered in relation to any propertieswhen siting the works. A small treatment worksserving more than one premises incorporatingconventional biological treatment should be aminimum of 25 m from any dwelling and this shouldbe progressively increased for larger treatmentworks.For works where noise is a factor, e.g. extendedaeration installations, it is difficult to be specific onthe distance from dwellings that will avoid nuisancefrom noise, as different circumstances, including therelative sizes of plant, merit individualconsideration. Compressors mounted directly overtanks produce much more noise, because of

reflection and vibration, than they would if locatedon the ground. Special provisions to reduce noiseinclude placing compressors under cover andsurrounding installations with earth banks orclose-boarded fences.Good road access should be provided to enable thetank-emptying vehicle to operate within itssuction-lift capability. Wherever possible, pumpingshould be avoided by locating the plant lower thanthe premises to be served. If pumping is inevitableit is preferable to use settled sewage rather thancrude sewage. Treatment units should not belocated in an area subject to flooding or where the

water table can rise to such levels as to cause flowinto the treatment units.6.2.2.2 Safety. Safety should be given fullconsideration in the design of sewage treatmentworks. They should be adequately fenced againstunauthorized interference to prevent potentialaccidents.

Attention is drawn to the provisions of the Healthand Safety at Work etc. Act 1974, and to the advicegiven in Health and Safety Guideline No. 2 Safeworking in sewers and at sewage works and otherpublications of the National Joint Health and Safety

Committee for the Water Service.3)

3) Available from Information Services Division, National Water Council, 1 Queen Annes Gate, London SW1H 9BT.

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6.3 Types of installation (see Figure 1). Thefollowing types of installation, or combinations of

them, are covered in the clauses referred to below.Cesspools (see clause 8).Septic tanks (see clause 9 and Figure 2,Figure 3, Figure 4, Figure 5, Figure 6 andFigure 7).

Preliminary treatment: removal of rags anddebris (see clause 10 ).Settlement tanks (see clause 11 ) and Figure 8and Figure 9).

Biological filters, including contactors (see clause 12 and Figure 10 and Figure 11).

Activated sludge units and secondary settlementtanks (see clause 13 ).Tertiary treatment (polishing) processes(see clause 14 )

grass plots (Figure 12)clarifiers (Figure 13)lagoons.

7 General requirements for tanksIt is essential that tanks constructed to hold or treatsewage, e.g. cesspools, septic tanks, primary andsecondary settlement tanks and chambers, should

be of watertight construction so that they permitneither ingress of ground water nor egress of sewageto the ground.Engineering bricks, concrete bricks, in situ concreteand large precast concrete pipes are all used for theconstruction of tanks, also units prefabricated fromsteel and plastics materials, including glassreinforced plastics. Brickwork should normally be incement mortar and of not less than 229 mm nominalthickness. In situ concrete for walls, floors andsurrounds should be not less than 150 mm thick andof C/25/P mix (see clause 4 of BS 5328). Plasticsmaterials should be to an appropriate specification.Where construction in waterlogged ground isunavoidable, provision should be made for theprevention of tank flotation during construction,emptying and maintenance.

A roof should always be provided to a cesspool forsafety purposes, to prevent nuisance and to preclude

entry of surface water and rainwater. The tank roofshould have structural strength adequate for thelocation of the tank. Access, with cover, should beprovided in the roof for emptying, cleansing andmaintenance. Where entry is likely to be required,at least two access openings should be provided.Similarly, a roof should be provided to a septic tank,in which case it may be wholly or partiallyremovable and be of concrete or timber. If f ixed, theroof should have adequate access openings, withcovers, including those necessary for inspection andcleansing of the inlet and outlet arrangements.Where it is not roofed a septic tank should be

provided with a protective fence to preventunauthorized access. Materials should beadequately protected against corrosion andelectrolytic attack where appropriate.Cesspools and septic tanks should be adequatelyventilated and access to rod the horizontal inlet pipeshould be provided. An integral inspection chambercan be provided.

8 Cesspools8.1 General

8.1.1 It is essential that cesspools are, and remain,

impervious to ingress of ground- or surface-waterand to leakage.8.1.2 Before deciding to provide a cesspool, theavailable local facilities for continual emptyingshould be carefully ascertained and whether such aservice will be provided by public authority orprivate contractor. The cost of emptying by tankervehicles may be high, and it should be noted that anaverage household of three persons willproduce 7 m 3(the capacity of a typical tanker) inabout 3 weeks, necessitating some 17 journeys perannum. Each journey may involve the haulage of 7 tof material a distance of several miles.8.1.3 The relevant sections of current legislation areset out in Appendix B. The Building Standards(Scotland) Regulations do not recognize theprovision of cesspools as a means of dealing withfoul drainage.8.1.4 The responsibility for ensuring that new orexisting buildings have a proper means of drainagerests with the local authority whose approval is tobe obtained.

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8.2 Locations

8.2.1 The site selected for a cesspool should not be sonear to any inhabited building as to be liable tobecome a source of nuisance or a danger to health(a minimum of 15 m is desirable) and it is essentialthat no well, stream, river, spring or aquifer likelyto be used for drinking or domestic or amenitypurposes is liable to be polluted.8.2.2 The site of the cesspool should preferably be onground sloping away from and sited lower than anyexisting building in the immediate vicinity.8.2.3 Consideration should also be given to thedirection of the prevailing wind.8.2.4 Adequate means of vehicular access should be

provided to within 30 m of the cesspool.8.2.5 The possibility of connection to a public sewerin the future should be borne in mind as a factor inthe siting of a cesspool.

8.3 Capacity

8.3.1 Normally the capacity required will limit thechoice of a cesspool as a means of disposal to singlehouses or buildings within the same curtilage, thetotal population of which does not exceed abouteight people.8.3.2 Constructional considerations will probablylimit the economic capacity of a single tank cesspoolto a maximum of about 50 m 3 . It should be notedthat the Building Regulations prescribe a minimumof 18 m 3 .8.3.3 As a general rule a capacity of not less than 45days storage should be allowed 4).8.3.4 The drainage should be on the totally separatesystem, and every precaution should be taken toensure that there is no entry of surface or subsoilwater into the foul drains.8.4 Arrangement. The most satisfactory shape fora cesspool is cylindrical with the diameter equal tothe length/depth, but it may be square orrectangular in plan to suit the conditions of the site.The depth from the cover of the access opening tothe floor of the tank should not normally exceed 4 mon a flat site and may need to be further restrictedon a sloping site to limit the suction lift whenemptying.8.5 Drain connection. The inlet drain should beprovided with access appropriate for the drainagesystem and should terminate with the pipeprojecting about 75 mm clear of the inside of thewall of the cesspool.

8.6 Ventilation. Stored sewage in a cesspoolbecomes extremely foul, and particular attention

should therefore be paid to ventilation, which isnormally through the ventilation pipe on the housedrainage system.

A separate fresh air inlet of not less than 100 mmdiameter incorporating a suitable non-return flapshould be provided, its point of entry into thecesspool being as high as possible beneath the cover.This should have a suitable head or gratingabout 800 mm above ground level and well clear ofthe cesspool cover. Attention should be paid to themaintenance of the flap.8.7 Entry into confined spaces. The accessshould not be less than 600 mm clear opening toenable inspection, maintenance or removal ofconsolidated sludge to be carried out. Entry into thecesspool chamber may be necessary. The chambershould be made as safe a place of work as possibleand safe methods of working should be adopted(see 6.2.2.2 ).8.8 Abandonment. When a cesspool is to beabandoned, it is essential that it be left in acondition that is neither dangerous nor prejudicialto health. This normally entails removal of theremaining contents and backfilling of the chamberwith hardcore or similar stable non-compressible

material, demolition of the structure within 500 mmof ground level, and reinstatement of the groundsurface.

9 Septic tanks9.1 Capacity. Calculation of the total capacity ofseptic tanks for the populations covered by this codeshould be made on the basis of the number ofpersons to be served, and the following formula isrecommended for general use, where desludging iscarried out at not more than 12-monthly intervals:

C = (180 P + 2000)

where

This formula allows for proportionately largerretention at the lower populations in order to coverthe surges in flow which are experienced in smallsystems.

4) Effective storage time will depend on the population served, the water consumption and whether there is any infiltration intothe foul drains, but in general a minimum of 150 L per head per day should be a reasonable provision for average circumstances.

C is the capacity of the tank (in L) with aminimum value of 2 720 L; and

P is the design population (see clause 4) with aminimum value of 4.

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For schools, similar premises and hotels, capacityrequirements can be evaluated separately or

included in the general formula using populationequivalent (see 3.20 ) figures for P after taking intoaccount factors such as part-time occupancy andshared cooking facilities; for example, inappropriate circumstances allowance might bemade in the factor P on the basis of two part-timeoccupants being equivalent to one full-timeoccupant. Specialist advice is necessary for plantstreating abnormal flows or non-domestic sewage.Where waste disposal units are installed, additionalsludge solids are discharged with the sewage andthe capacity of septic tanks should be increasedby 70 L for each person served.Where multi-compartment tanks are used, the inlet(settlement) zone should have a capacity of not lessthan 2/3 C and the subsequent zones should have acombined capacity of not less than 1/3 C .The calculated capacity C is recommended as aminimum for all types of septic tanks and the figureof 180 in the formula may be regarded as made upas follows.

NOTE 120 L per head per day is quoted in NWC publicationThe Water Industry in Figures October 1980.

Capacities may, however, be increased to takeaccount of particular circumstances (use of highconsumption fittings, projected growth in waterusage, reliable information on infiltration, etc.).

9.2 Arrangement. The design of septic tanksshould be such that the discharge of solids in thetank effluent is kept to a minimum. This is bestachieved by the use of tanks in series.For rectangular tanks two in series should be used,either by constructing two separate tanks or bydividing a single tank into two by a partition. Ineither case the compartments should be not lessthan 1 200 mm deep below TWL for up to 10 personsand not less than 1 500 mm deep below TWL forlarger populations. The first compartment shouldhave a length of not less than twice its width. In thelarger installations serving over 30 persons, a baffle

should be provided at the inlet and a scumboard atthe outlet. In order to facilitate desludgingoperations, the floor of the first compartment shouldhave a fall of 1 : 4 towards the inlet end.

Typical arrangements for installations using twoseparate tanks are shown diagrammatically in

Figure 2 (for populations of up to 30) and Figure 3(for populations of over 30). Where the twocompartments are separated by a dividing partitionas in Figure 4 (up to 30 persons) and Figure 5(over 30 persons), connection between thecompartments should be made either as orificesarranged horizontally or as vertical slits at each sideof the partition (Figure 4) and not by the use of a dipand communicating pipe (see 9.3 ).For populations of over 60, duplicate tanks, each ofhalf the total calculated capacity required, should beprovided and operated in parallel; this arrangementpermits all the flow to be passed through one unitwhile the other is being desludged. To enable the topwater to be decanted when desludging, a decantingvalve should be provided in the wall dividing the twotanks; the invert of this valve should be 625 mmbelow TWL. Each tank should comprise twocompartments but, where the installation is formore than about 100 persons and surge flows causeless disturbance, consideration should be given tothe use of two single-compartment tanks in parallel.Except where emptying and desludging will becarried out only by a tank emptying vehicle, tanksshould be provided with a valve-controlled sludgepipe not less than 100 mm in diameter at their lowerend, arranged so as to discharge to a sludge dryingbed or beds. Positioning the sludge pipe slightlyabove the floor level of the tank will facilitate theretention of a proportion of the sludge for reseedingpurposes.This code does not preclude the use of prefabricatedmaterials, e.g. concrete and glass fibre reinforcedcement or plastics, and the guidance givenpreviously on the proportioning ofmulti-compartment tanks may be difficult to applydirectly to non-rectangular tanks made from suchmaterials. The inherent principles are nonetheless

commended and should be followed wherereasonably practicable. Some adjustment to volumemay be necessary to achieve this.9.3 Inlets and outlets. The design of septic tankinlets and outlets should be such as to introduce thecrude sewage and to remove the clarified liquid withthe least possible disturbance of the settled sludgeor the surface scum.

A satisfactory form of inlet for rectangular tanks notmore than 1 200 mm wide is a T-shaped dip pipe ofcast iron or other suitable material not less than thenominal bore of the incoming drain, fixed inside thetank, with the top limb rising above scum level andthe bottom limb extending about 450 mm belowTWL.

LSludge storage capacity 90Balance to cover

a) 12 h storage of average domesticwater usage of 120 L per head perday assumed as passing to drains 60

3090b) higher consumptions and/or

infiltration etc.180

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For tanks in excess of 1 200 mm in width, twosubmerged inlets having inverts at the same level

are preferable. One method of overcoming thedifficulties of dividing small flows is by the use ofsubmerged bends of the same nominal bore (not lessthan 100 mm) set as closely together as practicablein a shallow sump formed within a small benchedchamber (see Figure 6). It is important that theinvert of the benched channel of this chamberbe 50 mm above TWL, and the inlet ends of thesubmerged bends should be set flush with the floorof the sump, which should not be less than 75 mmbelow TWL. The sump may be the full width of thechamber, but should not exceed 1.5 times thenominal bore of the inlet bends in the other

direction.The inverts of the outlet end of these bendsshould be between 300 mm and 525 mm below TWLin the tank. A baffle should be provided 150 mmfrom the inlet end of the tank, extending 150 mmbelow the invert of the inlet pipes and 150 mmabove TWL.Where duplicate tanks are required, each of which isin excess of 1 200 mm in width, the flow may bedivided equally by forming a crested weir of suitablelength (see Figure 7) on the centre line of thedividing wall between the tanks at the inlet end sothat the top of the weir is 75 mm below the standingwater level in the tanks and 100 mm below theinvert of the inlet pipe. This should be so arrangedthat the flow from each side of the crested weirpasses through a handstop frame to enable eithertank to be shut off for cleaning purposes by theinsertion of a suitable handstop.It is important that the incoming drain or sewershould be precisely in line with the centre of the twotanks for a distance of at least 6 m.Where the incoming drain has a steep gradient, atleast the last 12 m should be laid at a gradient notsteeper than 1 : 50 in order to minimize turbulence.

The final outlet for tanks which are lessthan 1 200 mm wide should be by a 100 mm

nominal bore dip pipe of cast iron or other suitablematerial fixed inside the tank in a similar manner tothe inlet dip pipe and 25 mm below it. For widertanks it is necessary to use a weir outlet extendingthe full width of the tank and protected by ascumboard, e.g. of suitable protected timber,plastics or asbestos cement fixed 150 mm from theweir and extending 150 mm above and 450 mmbelow TWL. It is important that the top edge of theweir be true and set level 50 mm below the inletdrain. A deflector should be formed either in thestructure of the end (outlet) wall or by apurpose-made deflector to prevent rising particles

from reaching the outlet weir.This deflector shouldbe located 150 mm below the base of the scumboardand protrude 150 mm into the tank (see Figure 3and Figure 5). Consideration should be given to theprovision of access to the outlet pipe for rodding.Where two or more tanks are served by a commonincoming drain or sewer it is important that theinvert of the outlet dip pipes serving tanks upto 1 200 mm wide and of the weirs serving tanksmore than 1 200 mm wide are set at precisely thesame level.9.4 Further treatment of septic tank effluent. When required, further treatment of septic tankeffluent should be carried out by the use of abiological filter or disc. Where this is not practicable,the tank effluent may be given treatment on land.This latter method is unlikely to produce an effluentsatisfying a 30 : 20 standard (see 6.2 ). There are twoways by which this can be carried out, as describedin clause 12 , but the dangers arising from pollutionof local water supplies, from airborne and fly-bornecontamination of food and from rat infestation,should be carefully considered.

10 Preliminary treatment

Rags and floating debris will inevitably form part ofthe flow reaching the works and to reduce blockagesand fouling of plant, particularly with largerinstallations, one of the following methods may beadopted.

a) The placing of a small metal screenwith 30 mm to 75 mm clear spacing between thevertical bars in the inlet channel. Provisionshould be made for overflow or by-pass of thescreen in the event of blockage. Provision shouldalso be made for the regular and safe disposal ofscreenings.b) The provision of a macerator in the inletchannel or pipe to chop up all the debris before itenters the plant.d

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c) If the sewage has to be pumped at any stagebefore treatment, a pump incorporating a cutting

edge or a separate macerator unit.Specialist design advice on the need for grit removalfacilities may be necessary.

11 Primary and secondary settlementtanks11.1 General. It is particularly important thatspecialist engineering advice be obtained whenconsidering the installation of settlement tanks.The efficiency of a settlement tank is dependent onthe velocity of the flow, which is determined by thetank dimensions. In small sewage treatment works

in particular, the considerable variations in flowwhich occur can reduce settlement efficiency.Settlement tanks may be of the horizontal flow orupward flow type. Although generally moreexpensive to construct than a horizontal flow tank,an upward flow tank has two distinctiveadvantages. Since the tank is desludgedhydrostatically, the need for two tanks in parallelfor draining down, as in the case of a horizontaltank, is eliminated, and workmen are not requiredto enter it to remove sludge, thus eliminating anunpleasant and potentially hazardous task.Facilities should be provided for the regular removalof sludge, which is crucial to the performance of allsettlement tanks, and for this to be carried out atintervals such as will prevent the onset ofsepticity.Failure to do so will result in a seriousreduction in the efficiency. In normal operation,tanks should be desludged at least once each week.Unless otherwise specified, scum retention boardsand removal facilities should be provided forsettlement tanks, since small sewage treatmentworks are more likely to receive relatively highproportions of oils, fats and grease than are largeworks.

11.2 Primary settlement tanks. Primarysettlement tanks are used to settle out solids priorto biological treatment and thus reduce the BODload on following units. They should not normally beused for populations of fewer than about 100.

An upward flow tank for the range of populationsconsidered in this code is normally square in planwith a hopper bottom having steeply sloping sides toprovide sludge storage. Sewage enters the tank viaa feed pipe and is initially deflected downwards by astilling box. As the sewage is dispersed into the bodyof the tank it rises steadily towards a peripheralweir and suspended material falls into the hopper.

In designing hopper bottomed tanks an angle ofslope of 60 (giving 51 valley slope) will usually be

satisfactory. In order to reduce possible sludgeaccumulation in the valley angle, a tank of steeperangle of slope of 68 (giving 60 valley slope) may beconsidered. This will be approximately 1.4 times thedepth of the shallower tank, and consequently moreexpensive.

A typical arrangement is shown in Figure 8.Prefabricated units are available in steel or plasticsbased materials.

A horizontal flow settlement tank is normallyrectangular in plan and should have a length ofapproximately three times its width and a depth

below TWL of about 1 500 mm. The floor shouldhave a fall towards the inlet end of the tank with agradient of 1 : 10. To facilitate desludging, twintanks should be provided in parallel, and adecanting valve, having an invert level 300 mmabove the floor level of the higher end of the tanks,should be located in the wall dividing the two tanks.

A desludging valve should be provided from thedeep end of each tank and be connected to sludgedrying beds or a sludge pumping chamber. Thesearrangements will vary according to site conditionsand may be unnecessary when emptying is to becarried out only by a tank emptying vehicle.

Dividing very small flows of crude sewage betweenthe tanks is difficult and the use of submerged bendsor a crested weir as described in 9.3 isrecommended. The final outlet for horizontal flowsettlement tanks should be of the weir type inaccordance with the recommendations for a septictank as described in 9.3 .

A typical arrangement is shown in Figure 7.11.3 Capacities of primary settlement tanks11.3.1 Upward flow tanks. The arrangement of anupward flow settlement tank should be such thatthe nominal upward flow velocity through it is less

than the settling velocity of the material to beremoved. A figure of 0.9 m/h at maximum flow rateis recommended. Where the maximum flow rate isunknown, the surface area of the tank may becalculated from the formula:

where A is the minimum area (in m 2) of the tank at

the top of the hopper; and

P is the design population (see item c) of

clause 4 and 9.1 ).

A 1 10-------- p 0.85=

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This formula allows for increased variability of flowrates which occurs as populations decrease. It is

based on a dry weather flow of 180 L per head perday but should be adjusted pro rata for other valuesof the dry weather flow. The dimensions andcapacity of the hopper can be determined from aknowledge of its volume and surface area.Sludge may accumulate at the rate of 10 L per headper week and should be accommodated in the lowertwo-thirds of the depth of the hopper. At dryweather flows of less than 180 L per head per daythis criterion may be critical in determining thecapacity of the hopper. The layout should be suchthat the inlet arrangement never becomessubmerged in sludge.

Additional capacity should be provided above thehopper in a vertical side-wall section between thetop of the hopper and TWL, as shown in Figure 8.The side-wall height to be adopted should be not lessthan 400 mm and the gross capacity of the tankshould be such as to provide a detention period ofnot more than 12 h at dry weather flow. It is alsorecommended that the gross capacity should be notless than that determined by the formula forcapacity given in 11.3.2 .11.3.2 Primary horizontal flow tanks. Thecalculation of the capacity of a horizontal flow tank

should be based on the number of persons to beserved and the dry weather flow. The detentionperiod should not exceed 12 h at dry weather flowand the following formula is recommended:

where

This formula allows for the increased variability inflow rates which occurs as populations decrease, Itis based on a dry weather flow of 180 L per head perday but should be adjusted pro rata for other valuesof the dry weather flow. Use of the formula will givegross detention periods of less than 12 h at dryweather flow for all values of dry weather flow andfor a population in excess of 100 (i.e. rangingfrom 12.0 h at population of 100 to 8.5 h at 1 000).It is also recommended that the surface area of thetanks should be not less than that determined bymeans of the formula given in 11.3.1 .

11.4 Secondary settlement tanks. Secondarysettlement tanks, usually known as humus tanks

when used in conjunction with biological filters, areessential components of secondary sewagetreatment where a 30:20 or better quality effluent isrequired. They are installed immediately followingbiological treatment, either as independent units oras integral parts of packaged systems. It may beadvantageous to arrange for recirculation of some ofthe final effluent through the biological filters.Secondary sludge may also be transferred to theprimary compartments of septic tanks for storageand final disposal with the septic sludge. In the caseof activated sludge units sludge requires to becontinuously withdrawn from the settlement tank

for return to the aeration tank.The design principles for secondary settlementtanks are similar to those for primary tanks butwhere recirculation of final effluent is adopted aspart of the biological filtration process (see 12.2.4 ) itwill be necessary, with specialist advice, to increasethe surface area and capacity of secondarysettlement tanks relative to the amount of effluentrecycled. Guidance for the design of secondarysettlement tanks to be used with activated sludgeunits is given in 13.5 .For design, constructional and operationalconvenience, it may be desirable to make secondarysettlement tanks of equal size to primary tanks.Otherwise, the formulae in 11.5 for determiningcapacities are recommended.11.5 Capacities of secondary settlement tanks11.5.1 Upward flow tanks. The surface area shouldbe not less than:

where

This formula is based on a dry weather flow of 180 Lper head per day and allows for increased variabilityof flow rates at small populations. It may beadjusted pro rata for other values of dry weatherflow.Sludge should be accommodated in the bottomtwo-thirds of the depth of the hopper and thiscriterion may be critical in determining the capacityof the hopper to be adopted.The layout should besuch that the inlet arrangement never becomessubmerged in sludge.

C is the gross capacity of the tank (in L); and

P is the design population (see item c) ofclause 4 and 9.1 ).

C 180 P 0.85 =

A is the minimum area (in m 2) of the tank at

the top of the hopper; and P is the design population (see item c) of

clause 4 and 9.1 ).

A 3 40-------- P 0.85=

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Additional capacity should be provided above thehopper in a vertical side-wall section between the

top of the hopper and TWL, as shown in Figure 8.The side-wall height should be not less than 400 mmand the gross capacity of the tank should be not lessthan that determined by use of the formula forcapacity given in 11.5.2 .11.5.2 Secondary horizontal flow tanks .(see Figure 9). The calculation of the capacity of ahorizontal flow tank should be based on the numberof persons to be served and the dry weather flow.Thefollowing formula is recommended:

where

This formula is based on a dry weather flow of 180 Lper head per day and allows for increased variabilityof flow rates at small populations. It may beadjusted pro rata for other values of dry weatherflow. Use of the formula will give gross detentionperiods of less than 9 h at dry weather flow for allvalues of dry weather flow and a population inexcess of 100.It is also recommended that the surface area of thetanks be not less than that determined by means ofthe formula given in 11.5.1 .

12 Biological filters, includingrotating biological contactors, andsecondary settlement tanks12.1 General. In a conventional biological filter, theeffluent from a septic tank or a primary settlementtank is brought into contact with a suitable medium,the surface of which becomes coated with abiological film. The film assimilates and oxidizesmuch of the polluting matter through the agency ofmicro-organisms. The biological filter requiresample ventilation and an efficient system ofunderdrains leading to an outlet.

A form of rotary biological contactor has now beenintroduced employing a medium in the form of discsor random elements packed in a perforated drum.

12.2 Conventional biological filters12.2.1 Distribution. The effluent should be

distributed evenly over the surface of the biologicalfilter, through which it percolates to the floor.Biological filters are usually either rectangular(see Figure 10) or circular (see Figure 11) in plan,and various methods of distribution may be used,the most suitable for use in small installations beinga series of fixed channels or a rotating-armdistributor.12.2.2 Fixed channels for rectangular filters. Aseries of fixed channels of suitable material shouldbe provided and so adjusted in level that the effluentflows uniformly through notches in their sides.These channels should be dosed intermittently bymeans of a tipping trough or other mechanism. Thismethod of distribution should not normally be usedfor populations of over 50.12.2.3 Rotating-arm distributor for circular filters.

A rotating-arm distributor, consisting of one or morearms extending from the central axis towards theouter edge of the filter, should be provided. Theeffluent should be fed into the rotating arm or armsso that on discharge it is sprinkled evenly over thewhole surface of the filter medium. This type ofdistributor requires a greater hydraulic head tooperate it than does the fixed channel type.

A head of liquid is necessary to effect rotation of thedistributor arms by hydraulic means. If themechanism is such that the separate dosingchamber with siphon has to be constructed outsidethe bed, the capacity of the chamber should befrom 3 L to 4 L per m 2 of filter. The dose, andtherefore the capacity of the chamber, should besuch as to ensure efficient distribution.12.2.4 Volume of filter. It is essential that thevolume of filter medium provided is sufficient toallow for surge flows which occur with smallinstallations, such variations being morepronounced the smaller the number of personsserved.The volume of mineral medium required can becalculated by the formula

where

C is the gross capacity of the tank (in L); and

P is the design population (see item c) ofclause 4 and 9.1 ).

C 135 P 0.85=

V is the volume of medium (in m 3); and

P is the design population.

V 1.5 P 0.83=

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In Table 1 shown below, the volumes of mediumrequired for representative numbers of users are

given; intermediate values may be interpolated on alinear basis. The volume of medium per user is alsogiven and it can be seen that surge flows are allowedfor. When waste disposal units are installed, thevolume of medium obtained from the formula orfrom the tables should be increased by 30 %, prorata for that part of the population equipped withwaste disposal units.Where

It may be possible, with specialist advice, to reducethe volume of the filter by introducing recirculationof part of the final effluent in order to dilute theinfluent to the filter. This also eliminates the dangerof the filter medium drying out during periods of lowflow. Where this is done the arrangements providedfor secondary settlement in humus tank(s) shouldbe increased relative to the amount of effluent whichis recycled.12.2.5 Mineral filter media. Mineral filter mediashould comply with the requirements of BS 1438and be chosen with regard to the following

considerations.a) It should be strong enough to resist crushingunder its own weight or when walked on.b) It should be obtained washed and dust-free.c) It should not contain any toxic substances orother undesirable matter likely to be dissolvedinto the sewage flow.d) It should be capable of resisting breakdowndue to the flow of the sewage or under frostaction.e) The general shape of the individual piecesshould be roughly cubical rather than veryelongated or flat.

f) The surface of the pieces should preferably berough and pitted.

g) Local availability, having regard to suitability.Several mineral materials are suitable for thispurpose, the most usual being hardburnt clinker,blastfurnace slag, hard broken stones and hardcrushed gravel.Efficiency is dependent on careful grading; asuitable grading for mineral media is 100 mmto 150 mm at the bottom for a depth ofabout 150 mm, the remainder being 50 mm nominalmaximum size which requires, in accordance withBS 1438, the grading limits given in Table 2.12.2.6 Plastics filter media. Media fabricated from

plastics materials are now available and may beused in biological filters in place of mineral media.Three main types are available in the form ofpressed laminar sheets supplied in modules,tubular form media and random fill media. Thesehave high void capacity, extended specific surface,and low bulk density enabling high hydraulic flow tobe accommodated in smaller working volumes andlightweight structures in comparison with mineralmedia. Costs of plastics media are howeverrelatively higher per unit volume than mineralmedia, and the risks of freezing or drying in periodsof low hydraulic flow are greater in plastics media.

The specific surface and wetting properties ofplastics media vary widely from one type to another,and no general simple formula can be given to relatethe performance of plastics media to mineral media.It is essential to follow the manufacturersrecommendations with care and if possible toconsider the performance of existing plastics mediain similar conditions. Manufacturers should beconsulted in all cases on the installation andorientation of plastics media to avoid possibledamage to the filter and to maximize the efficientutilization of the special design of the media.

Table 1 Filter medium capacity

function V = 1.5 P 0.83

V is the volume of medium (in m 3)

P is the design population.

P 4 6 8 10 15 20 25 30 40 50V 4.7 6.6 8.4 10.1 14.2 18.0 21.7 25.2 32.0 38.6V/P 1.18 1.11 1.05 1.01 0.95 0.90 0.87 0.84 0.80 0.77

P 100 200 300 400 500 600 700 800 900 1 000V 69 122 171 217 261 303 345 385 425 464V/P 0.69 0.61 0.57 0.54 0.52 0.51 0.49 0.48 0.47 0.46

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Table 2 Grading limits for 50 mm filtermedium

Where plastics media are used they need to becontained against loss by wind action. A perforatedwalkway should be provided to give access to thedistributor where necessary.

12.2.7 Design. The depth of mineral medium should,where practicable, be 1 800 mm, corresponding withconventional practice. Where insufficient hydraulichead is available the depth may be reducedbut 1 200 mm is the minimum depth recommended.Where adequate head is available the depth may beincreased up to 2 500 mm. Deeper filters enableeconomies to be made in foundations, underdrainsand in distribution. The higher hydraulic flow rateon the surface of the filter reduces problems ofdistribution and drying at low flow but may notnecessarily eliminate them. Where small increasesin loading rate occur filter loading per unit volumemay be reduced by adding medium to the surface ofan existing filter provided hydraulic conditionspermit.12.2.8 Ventilation. Adequate ventilation of abiological filter is essential; air ventscommunicating with the floor level of the filtershould be provided. Where the filter is belowground, the ventilating pipes from the ends of theunderdrains should be carried to 150 mm aboveground level outside the filter (see Figure 11).Normally the filter should not be covered, but wirenetting may be used to prevent falling leaves fouling

the surface of the filter or blocking the ventilatingpipes.Where the filter is above ground, ventilation holesor porous construction can be provided at the base ofthe walls of the filter.

A minimum of four 100 mm diameter ventilatorsshould be applied to underground filters belowground and these should be at a maximum of 2 mcentres for larger units. Gratings should be keptclear.

12.2.9 Grassland. Treatment of filter effluents ongrassland is a method of removing humus as an

alternative to secondary settlement. In this methodthe filter effluent should be evenly distributed overthe grass from a system of channels and, afterflowing over the surface, collected in a secondsystem of channels. The land should be well gradedand to avoid scouring should have a gentle slope ofabout 1 : 60 to 1 : 100. Special seeding of the land isnot necessary and the grass and other vegetationneeds only occasional cutting to keep the growthfrom becoming too rank. The cuttings should beremoved from the irrigation area and theaccumulated solids should be removed periodically.The area of grassland required per head ofpopulation is about 3 m 2 and the total area shouldbe divided into three approximately square plots,used in rotation for a period of 2 months to 3 monthsat a time.This method should not be confused with finaldisposal of effluent over grass plots describedin 14.2 .12.3 Rotary biological contactors12.3.1 General. Rotary biological contactors arenormally package units that incorporate facilitiesfor primary and secondary settlement. Therecommendations of the manufacturer concerning

selection of plant, maintenance and operationshould be followed with care.The biological organisms which oxidize theimpurities in the sewage are supported on a rotatingstructure which exposes them to absorb,alternately, air and sewage. Rotation also assistsaeration of the sewage in the immersion tank.Some units include novel or patented features thatcannot be covered in this code of practice. Usersshould seek to establish that any claims made canbe justified.12.3.2 Input arrangements and capacity. Wherever

possible installations using rotary biologicalcontactors should be supplied by gravity and meansprovided to minimize surges in flow, especiallywhere package units are used. Where crude sewageis admitted by pumping, it is important that theaverage frequency of pumping should not be lessthan four times per hour throughout most of theday.Septic tanks built integrally with rotary biologicalcontactors should be able to hold at least the totalvolume of sludge deposited in 1 month to 3 monthsuse, dependent on the size of the plant, at the fulldesign loading. They should provide convenientaccess for desludging and should be sufficientlyrigid to withstand pressure from adjoiningcompartments during desludging.

BS 410 test sieves Proportion by mass passing mm %

63 100

50 85 to 100

37.5 0 to 30

28 0 to 5

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In integral plants it is desirable for the inlet zone tobe baffled or for a weir providing a headloss

of 10 mm to 20 mm to be installed to minimize theeffect of surges in flow. Treatment is more efficientwhen longitudinal mixing is minimized in thetreatment zone by installation of a number oftransverse baffles each providing a headloss ofabout 10 mm.The design should facilitate the transfer of excessfilm, shed from the rotating surfaces, from thetreatment zone to a secondary settlement unit,either by positive mechanical means or by ensuringthat sufficient turbulence is induced to carry itforward in the effluent stream. Some systemsincorporate novel or patented features of design.12.3.3 Rotor units and drive mechanisms. Therotational speed (usually 1 r/min to 3 r/min) anddiameter of the rotating structure govern theperipheral velocity, which should notexceed 0.35 m/s to avoid stripping of the biomass.Random media, where employed, should be tightlypacked for the same reason. Biological filmaccumulates more thickly on the surfaces nearestthe inlet to the treatment zone, and the spacingbetween adjacent surfaces of discs in this regionshould be designed to prevent the bridging of gapsbetween surfaces.

12.3.4 Construction. The design and alignment ofthe drive shaft should provide adequate strength toassure long trouble-free life. Failure of power orother interruption of rotation may, if continuingmore than 24 h, allow the biomass on the rotor tobecome unbalanced due to drainage and drying ofthe exposed areas. If rotation recommences withoutthe proper maintenance and cleaning of the discs,severe strain will be placed on the shaft and drive.It is therefore essential that proper provision foroverload protection of the motor is made and thatautomatic restart for the motor is provided after anelectrical failure.

Structures supporting the rotor bearings and driveshould have adequate long term rigidity to maintainalignment. Bearings, drive chains and sprocketsshould be protected from moisture and providedwith easy access for lubrication and adjustment.

Discs can be made from a variety of durablematerials including expanded metal, plastics mesh,

GRP, unplasticized polyvinyl chloride or similarmaterials, or high density polystyrene foam. Thepacking used in rotating cylinders may be similar torandom fill media used in high rate biological filters.Rotors are also used with a variety of surfacesdisposed in a spiral or honeycomb form.12.3.5 Secondary settlement tanks for biologicalcontactor units. Secondary settlement tanks can beeither integral parts of package systems or separatestructures and should conform in principle to theprovisions of clauses 10 and 11 . The capacityprovided should be not less than conventionalsecondary settlement tanks and should allow for theaccumulation of about 3 months discharge ofhumus sludge. Provision may also be made forhumus sludge to be transferred to the primarysettlement tank or septic tank.12.3.6 Loading and performance of the biologicalstage. Where full treatment of domestic sewageto 30 : 20 standard (see 6.2.1 ) is required, theloading of the rotating surfaces in the biologicalzone should not exceed 5 g BOD per m 2 per day ofsettled sewage or 7.5 g BOD per m 2 per day as crudesewage entering an integrated package plant 5) . Theloading should be based on the maximumpopulation to be served especially in camping orholiday areas serving varying numbers. Wherequality standards are critical, additional tertiarytreatment (polishing) should be provided(see clause 14 ).

13 Activated sludge units 6)

13.1 General. For the purposes of this code,installations operating on activated sludgeprinciples are those providing for the aeration ofcrude unsettled sewage with activated sludge. Animportant feature of these installations is that along period of aeration should be provided at some

stage in the process in order to bring about oxidationof sludge, thus reducing the rate of production ofsurplus sludge and the frequency with which thissludge should be removed. In all activated sludgesystems there is a need regularly to removequantities of surplus sludge. To ensure that aneffluent of 30 : 20 standard is achieved it may benecessary to provide a polishing stage oftreatment (see clause 14 ).

5) Higher loadings may be used provided that adequate technical support data has been supplied.6) See Technical memorandum on activated-sludge sewage-treatment installations providing for a long period of aeration,HMSO, London, 1969.

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13.2 Location. Attention is drawn to therecommendation given in 6.2.2 that the site should

be sufficiently far from habitable buildings to avoidthe risk of noise nuisance. An electricity supply isrequired.13.3 General requirements. The installationshould incorporate the following features:

a) adequate protection against corrosion;b) standby electrical equipment incorporatingautomatic changeover, where practicable;c) automatic restarting in the event of powerfailure;d) arrangements for the removal and disposal ofsurplus sludge;e) adequate control of flow to minimize risk ofwashout of activated sludge;f) if below ground level, adequate protectionagainst flotation.

13.4 Types of installation13.4.1 General. There are three types of installation:

a) extended-aeration;b) contact stabilization;c) oxidation ditches.

Types a) and b) are normally prefabricatedfactory-built units, often referred to as packageplants. The minimum capacity varies with differentmanufacturers; as a guide, type a) is suitable forpopulations of not less than 25 and types b) and c)for populations of not less than 70.Specialist advice is necessary when considering theuse of any of the three types. Where waste disposalunits are installed, due allowance should be maderegarding air supply requirements and tankcapacities.13.4.2 Extended-aeration installations13.4.2.1 General. The extended-aeration processinvolves treatment in two compartments, anaeration or mixed liquor compartment and asettlement compartment. Sewage, which willusually be screened or macerated, flows to theaeration compartment where it is aerated inadmixture with activated sludge. The sludge isseparated from the mixed liquor in the settlementcompartment which is usually integral with the firstcompartment but separated from it by a partition.The sludge is recycled to the aeration compartmenteither by gravity pump or by air-lift. Thesupernatant liquor (treated effluent) leaves theplant over a weir.

13.4.2.2 Capacities. The capacity of the aerationcompartment should be not less than 230 L 7) per

head of resident population. Retention time shouldbe at least 24 h and up to 48 h may be provideddepending on the strength of sewage and thestandard of effluent required. Maximum daily BODloading should be between 0.05 kg/(kgd)and 0.15 kg/(kgd) MLSS and a concentrationmaintained of 2 000 mg/L to 5 000 mg/L MLSS. Thedesign of the settlement compartment should besuch that the maximum surface loading (flow perunit area) does not exceed 22 m 3/m 2 per 24 h [0.9 m 3/(m 2h)].13.4.2.3 Air supply. The duty air compressor(s)should be capable of producing up to 17 m 3 per dayof air at 2 m water depth per head of population, thevolume required being dependent upon the bubblesize and depth of immersion as shown in Table 3.

Table 3 Air supply

With mechanically aerated systems the aeratorcapacity potential should be not less than 2 g oxygenper g BOD applied.

7) As in the Technical Memorandum, this figure is equivalent to a BOD loading of 240 mg/L of aeration capacity per day on thebasis of 55 g BOD per head per day.

Bubblesize

Depth ofaerator

Air supply perday per head of

population

mm m m 3

Coarse bubbles 8 2 16.8

3 12.0

3.5 9.5

Fine bubbles 2 to 4 2 8.43 6.0

3.5 4.8

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13.4.3 Contact stabilization installations13.4.3.1 General. The contact stabilization process

involves treatment in four distinct compartments.In the first compartment, sewage, which willusually be screened or macerated, is aerated incontact with activated sludge for a period ofbetween h and 2 h, the mixed liquor then passingto the settlement compartment. After settlement,the supernatant liquor (the treated effluent) isdischarged, and the sludge is transferred to a third(re-aeration) compartment where it is aerated for aperiod of several hours during which time oxidationof absorbed organic material occurs. A largeproportion of the activated sludge is then recycled tothe first (contact) compartment. There may be afourth (aerobic digester) compartment wheresurplus sludge is further aerated to oxidize it ascompletely as possible before being removed fordisposal.13.4.3.2 Capacities. The combined capacities of thefirst (contact) and the third (re-aeration)compartments should not be less than 114 L perhead of population served. The design of the second(settlement) compartment should be the same as forextended aeration installations.The capacity of thefourth (aerobic digester) compartment should be notless than 90 L per head of population. TheBOD/sludge loading for the combined stages shouldlie between 0.05 kg/(kgd) and 0.15 kg/(kgd) MLSS.13.4.3.3 Air supply. The duty air compressor(s)should be capable of producing a volume of airconsistent with Table 3. Air input should beallocated to the three stages, contact, re-aerationand digestion, approximately in proportions 2 : 4 : 3respectively.

13.4.4 Oxidation ditch installations13.4.4.1 General . The oxidation ditch consists

essentially of a continuous shallow channel 1 mto 3 m in depth usually forming an oval circuit inplan. The same depth below TWL and preferably ofthe same cross-sectional area should be maintainedfor the complete circuit. The ditch should beequipped with one or more mechanical aeratorsarranged to maintain a velocity of flow in the ditchsufficient to keep the activated sludge insuspension. The construction should be in concreteor in earthwork. Where the latter method is used,some form of lining may be required according toground conditions and type of construction. A rigidlining should always be provided in the vicinity ofthe rotor, extending to at least 4.5 m downstream.Provision should be made for separate settlement ofsludge before discharge of final effluent if the ditchis designed for continuous operation.13.4.4.2 Capacities. The capacity of the ditch shouldbe not less than 260 L per head of population. In thecase of continuous operation, where separatesettlement is required, the design of the settlementcompartment should be the same as forextended-aeration installations. The BOD/sludgeloading should lie between 0.05 kg/(kgd)and 0.15 kg/(kgd). Mixed liquor suspended solidsshould be maintained between 2 000 mg/Land 5 000 mg/L.13.4.4.3 Aeration. The mechanical aerator shouldprovide not less than 2 g of oxygen per g BOD 8) .13.5 Settlement of activated sludge13.5.1 General. Prefabricated factory-builttreatment units generally include settlementfacilities. However, for the larger units it may benecessary to construct settlement facilities in situ orin vessels separate from the biological section.Designs of settlement tanks vary with themanufacturer, but the following basic elementsshould be included.13.5.2 Loading rates. Surface loading should notexceed the rate of 0.9 m 3/(m 2 h) at peak flow, andthe capacity of the tank should be sufficient toprovide a minimum retention of 2 h at peak flow.The maximum solids load per unit surface areashould not exceed the rate of 5 kg/(m 2 h).

8) Biological treatment plants are constantly under development and new methods may become available in the future whichmay lead to different design parameters.

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13.5.3 Inlet arrangements. The inlet pipe shouldbe 100 mm minimum diameter in suitable material

such as low carbon steel, unplasticized polyvinylchloride or cast iron. On entering the centre feedwell or stilling box, the pipe should turn through 90to discharge vertically as close as possible to thetank centre at approximately liquid level. Thestilling box should be square or circular and shouldhave a side length or diameter of approximately 1/6of the tank side length or diameter. Its upper edgeshould be not less than 75 mm above water level andits lower edge should extend to the bottom of thevertical side wall level in the case of an upflow tank,and 800 mm to 1 000 mm below water level in thecase of a scraped circular tank.

13.5.4 Overflow outlets. The effluent should beremoved by overflow at adjustable notched weirs atthe periphery of the tank, discharging into acollection channel of sufficient depth to prevent itflooding at sustained peak flow rate. Notched weirsare essential when weir overflow rates fallbelow 150 m 3/(mh).13.5.5 Sludge withdrawal. Sludge should becontinuously withdrawn from the settlement tankfor return to the aeration tank at a controlled rate.The rate of return should be adjustablebetween 0.5 DWF and 1.5 DWF. The returnsludge pump should be adequately dimensioned toavoid it becoming blocked.

14 Tertiary treatment (polishing)processes14.1 General. Conventional biological treatmentcan produce an effluent of 30 : 20 standard(SS : BOD), or better, after separation of solids, butfor reliable production of higher quality effluents atertiary or polishing stage of treatment isnecessary before final disposal. Polishing processesrely mainly on flocculation, sedimentation orfiltration of residual suspended solids. The BODassociated with the solids is removed and somemethods also provide further biological purification.Polishing is suitable only for dealing with goodquality secondary effluents and, in general, willoperate efficiently only at works where biologicaltreatment is adequate. If a suitably chosenpolishing process is applied to a good qualitysecondary effluent it should normally be possible toachieve at least a 10 : 10 standard.

Several methods are now available. These includeslow sand filtration, rapid sand filtration,

microstraining and retention in lagoons. In smallsewage treatment works the following methods aremore common:

a) treatment over grass plots;b) upward-flow clarifiers (not normally used withactivated sludge plants).

14.2 Treatment over grass plots (see Figure 12).This method is inexpensive and can removeabout 70 % of residual suspended solids and 50 % ofBOD. The rate of treatment, calculated on the areain use at any one time, should notexceed 0.85 m 3/(m 2d) at maximum flow of

about 0.3 m2

per head of population. (The use ofgrassland for irrigation of filter effluent is describedin 12.2.9 .)14.3 Upward flow clarifiers. In the gravel-bedclarifier effluent is passed upwards througha 150 mm layer of 5 mm to 7 mm gravel 9) supportedon a perforated floor in a suitable tank. The floorshould be made of metal suitably protected againstcorrosion, stainless steel, concrete or other suitablematerial. The perforated area of the floor should besuch that the rate of flow does notexceed 1.0 m 3/(m 2h) under peak flow conditions.The method is effective in removing about 50 % of

the suspended solids and 30 % of the BOD providedthat the solids which accumulate in, above andbelow the gravel are regularly removed.It is preferable that duplicate tanks should beprovided if practicable to permit proper cleaning.Each tank should be designed on similar lines to asettlement tank with a surface area sufficient tocontain the gravel bed and allow access at the inletend for cleaning (see Figure 13). As an alternative,the gravel bed may be installed in the humus tankprovided that the size and design of the tank complywith the requirements of this section, as well aswith those in Figure 7 and Figure 8. Solids whichaccumulate in, above, and below the gravel shouldbe removed by back-washing, that is, lowering thewater level by draining off effluent from below thegravel bed and washing the surface of the drainedbed with a jet of water or effluent where necessary.

9) Reference: Tertiary Treatment and Advanced Waste Water Treatment. Manuals of British Practice in Water PollutionControl . The Institute of Water Pollution Control, 1974.

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The top of the perforated floor supporting the gravelshould be 450 mm below the surface of the liquid in

the tank as controlled by the level of the outlet,which should be in the form of a weir 300 mm abovethe surface of the gravel. The perforations in thefloor should be able to retain the medium but be ofsufficient size to allow a free flow of liquid throughit. The floor and its support should be designed toaccommodate both the weight of the medium andthe superimposed weight of a man when cleaning.The edges of the floor adjoining the walls should besealed for a distance of 100 mm to obviate the risk ofthe flow short-circuiting between the gravel and thewall.

A vertical inlet baffle should be provided to achieveeven distribution of flow beneath the gravel bed, andshould project not less than 300 mm below theunderside of the perforated floor. It should projectnot less than 75 mm and not more than 225 mmabove the level of the outlet weir to prevent themedium blowing and thereby discharging solids.The ends of the baffle should be sealed to the tankwalls.There are also a number of methods of clarifyingeffluent by upward flow through fabricatedmaterials. Metal and plastics mesh are those mostcommonly used. These clarifier

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