PJ4 the Basis of Sewage Treatment Works Design

8/13/2019 PJ4 the Basis of Sewage Treatment Works Design

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THE BASIS OF SEWAGE TREATMENT WORKS DESIGN

The Nature of Sewage and Industria Effuents

Genera

Sewages and industrial effluents are complex liquors mainly comprised of water with varyingamounts of a wide range of other substances dispersed throughout their bulk. Theseimpurities vary greatly in both chemical and physical properties. No two sewages andindustrial effluents are exactly alike, and a single sewage or effluent varies in compositioncontinually. The impurities also vary greatly in their polluting effect should they be allowedto enter a water course.

So it is important that we understand the nature of sewages and industrial effluents toenable assessments to be made of such things as polluting substances present and degreeof treatment necessary.

Treatment processes for sewages and industrial effluents have been developed specificallyto deal with the various types of polluting substances present. It is therefore very necessaryfor reasons of plant design, efficient plant operation and treated effluent quality control, tounderstand the nature of raw sewages and industrial effluents.

The Nature of Sewages

Genera Des!ri"tion#

. !olour " fresh domestic sewage is grey or greyish#brown in colour. In industrialsewages the colour may be modified by the presence of trade wastes, e.g. paintpigments and dyestuffs. Stale sewages darken, eventually becoming almost black.

$. %articulate &atter " there is a great variation in particle si'e and much is settleable,(about )*+ of filterable solids will settle in $ hours.

-. Turbidity " crude sewages are invariably highly turbid due to the presence ofsuspended and colloidal solids.

. /dour " a fresh domestic sewage has a musty, earthy, soapy smell. 0s the sewagebecomes stale a sour, 1bad eggs2 smell is apparent. The smell of an industrialsewage may be modified by components of trade effluents.

3. p4 " 5omestic sewage p4 is normally between ).* and 6.*.

The $h%si!a Nature of Sewage#

Sewage is a heterogeneous mixture of solids dispersed through a liquid medium. 0 usefulclassification of the forms which these solids take can be based on an examination ofparticle si'es and densities as follows7

&' Gross Soids

8arger identifiable solid material, either more or less dense than water, and which istransported by the force of the sewage flowing through the sewers. /ften considered to begreater than )mm in at least one dimension. The quantity of gross solids in a given sewagewill vary according to local conditions e.g.


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Nature of sewerage system.9elocities of flow.:se of pumping.;eather conditions.Nature of the community.

(' Minera )i'e' inorgani!* Sus"ended Soids

Small, relatively dense, often abrasive, and chemically inert particles.


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Inorgani! -o0"onents#

The following may be present in the approximate quantities stated in an 1average2 domesticsewage.

-ations Anions

Sodium ** mg=l !hloride )* mg=l%otassium $* mg=l Sulphate 3* mg=l0mmonium -3 mg=l %hosphate $* mg=l!alcium ** mg=l Dicarbonate&agnesium * mg=l !arbonateIron mg=lEinc *.) mg=lTrace &etals F mg=l

5issolved gases may also be present e.g. oxygen up to $ mg=l in fresh sewage, nitrogen,

carbon dioxide, hydrogen sulphide if sewage is stale.

Signifi!an!e#

0mmonium " 0t high p4 ammonia may be toxic to aquatic life, it can cause an oxygendemand, and has a water treatment significance.

!alcium, magnesium and iron " may be sludge forming but has little pollution significance.

Sodium, potassium " little significance.

!hloride " little pollution significance, an indicator of sewage strength and comparability

between sewage samples.

Sulphate " attack on concrete at concentrations over ,*** mg=l.

%hosphate " traces accelerate growth of aquatic flora.

Dicarbonate=!arbonate " no pollution significance, gives buffer capacity to the sewage.

Organi! !o0"onents#

The ability of carbon to form molecular chains allows for there being a vast number oforganic chemicals. &any can be classified into groups with similar chemical properties.

The most important groups in sewage, which account for about )3+ of the organic carbon,are the following.

. !arbohydrates " -*+ of total carbon. 8inear carbon chains with hydrogen atomsand hydroxyl groups attached. They include the sugars.

e.g. cellulose and starch (!@4*/3n

cane sugar and lactose !$4$$/glucose, hexose sugars !@4$/@

pentose sugars !34*/3

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$. Gats " $@+ of total carbon. &uch fatty material is present as insoluable matter(about * mg=l. Gats are compounds of glycerol (!4$(/4 !4 (/4 !4$(/4and various organic acids. !hemically they are glyceryl esters or glycerides. Thecommon organic acids involved are lauric, myristic, palmitic and stearic which aresaturated, and oleic, linoleic and linolenic which are unsaturated. The commonformula is given below7

!4$" / > where >, >H and >2 are acidic groupings. !4 " / >H !4$" / >2

eg. !4$" / " !/ " !)4-3 !4" / " !/ " !)4-3Tristearin

!4$" / " !/ " !)4-3

-. %rotein based material " + of total carbon. %roteins are complex nitrogenousorganic substances formed from long repetitive chains of the following grouping7

# N4 " !4J " !/ "

J will be an organic grouping possible containing sulphur or phosphorus.The chainmay repeat ,*** or more times with differing x groupings.

%roteins degrade into amino#acids, e.g. glycine !4$=(N4$!//4 and cysteine !4$(S4=!4 (N4$=!//4. They are present in sewage along with other breakdown products.

e.g. urea N4$!/ N4$indoleskatole (methyl indole

. 5etergents " + of total carbon. Synthetic substances. The most common arethe alkyl ben'ene sulphonates which are used for almost all domestic detergents.

e.g. sodium alkyl ben'ene sulphonate.

> " !@4S/-Na. > B alkyl group.

In addition to these four groupings there are other trace quantities of organic chemicalswhich have been isolated.

e.g. :ric acid mg=l%henols *.$ mg=l!holesterol trace!reatine and creatinine @ mg=l9itamins

The total organic carbon content in solution in sewage amounts to around ** mg=l with afurther ** mg=l in suspension.


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Sewage i0"urit% as a !e nutrient

8iving cells require certain types of organic matter as nutrients. They are principally7

!arbohydrate Gats energy sources%rotein and amino acids " for synthesis of protoplasm.

9itamins " trace quantities needed for cell processes.

0ll these are present in sewage, which is therefore a favourable substrate to support largebacterial populations i.e. the maKor organic impurities are biodegradable.

Industria Effuents

Trade wastes are many and varied. The trade waste content of a particular sewage willreflect the nature of the local industry.

0 simple classification of industrial effluents according to their effect on the sewage to whichthey are discharged is given below7#

. Strongly organic effluents " high D./.5.e.g. food industries, brewing, slaughtering, dairying.

$. &ineral effluents " low D./.5.e.g. metal finishing, mine drainage, pickling liquors, chemical industries.

-. Affluents containing high suspended solids contente.g. quarrying, ceramics, vegetable washing, papermaking. (nb. Solids may beorganic and inorganic.

. 0cidic effluentse.g. gas scrubbing, rayon production.

3. 0lkaline effluentse.g. textile processing, papermaking.

@. Toxic effluents i.e. those toxic to biological treatment processes.e.g. metal finishing # toxic metals, cyanide

case hardening # cyanideagrochemical production # organochlorine and

organophosphorus compounds

drycleaning # chlorocarbons.

). !oloured effluents " containing quantities of intractable colour matter.e.g. dyeing, printing, papermaking, paint manufacture.

6. 4ot effluentse.g. cooling waters, hot process liquors.

Effe!t on sewage

0ny particular trade effluent may fall into a number of the eight categories listed, and willthus give properties of each to the sewage. In addition, discharges of trade effluents tosewers may have a marked effect on the flow regime within the sewerage system.

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Site -onsiderations

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acquisition may be protracted. It may be more economical to find a site more readilyobtainable but requiring a more expensive process, extra sewerage or access costs.

The land elevation must be such that the sewage can be brought to the plant economicallyand can be readily disposed of. Sites will therefore be low#lying so as to drain as much ofthe ultimate development as possible by gravity sewerage and hence avoid expensivepumping.

$. Goundation conditions and soil stability

!osts are increased when dealing with difficult or unpredictable ground conditions,excavating rock and supporting instable earthworks. 0s previously noted, sites aregenerally low#lying and the problems associated with such sites are high water tables, peatdeposits and low bearing capacity. /ccasionally overburden is thin, and the depth of tanksmay need to be restricted to avoid extensive rock excavation, possibly affecting the process.

%reliminary site investigations reveal enough useful information to be able to compare sites,

but the importance of thorough investigation into a chosen site must not be under#estimated.Inadequate site investigation leads to uneconomical design, and unforeseen problemscausing further expense.

The engineer must assess whether a site which may be ideally situated with regard tosewerage, access and cost is worth the expensive foundation engineering that may benecessary to construct a plant that will be free from long term faults due to differentialsettlement, or may need expensive temporary works during construction.

-. Glood levels

0s a sewage treatment works is likely to be situated on low#lying ground in close proximity to

a river or coastline the occurrence of flooding must be investigated.


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Ideally the site should slope gently and uniformly so that the process can be arranged forthe sewage to flow through the works without pumping, whilst maintaining the civilengineering works at a reasonable height above ground level. The slope of the site also hasa bearing on the type of process proposed as head losses through biological filters aremuch higher than those through aeration tanks.

3. !ompatibility with existing sewerage

New sewage treatment works are often required to treat sewage from mixed existing andnew development, and often to replace one or more existing works. ;ith greater emphasison a cleaner environment, works have now been proposed for towns and cities whichpreviously discharged to the sea. In each case, there is an existing sewerage systemdesigned to gravitate to particular locations. In a new scheme, the amount of modification tothe existing system should be minimised. To achieve operational economics, a new plantmay replace several small works, to which existing sewerage systems drain. These sewersare connected by an interceptor sewer, which will preferably drain by gravity to the newworks, thus avoiding pumping.

Decause of the nature of many present day sewage treatment schemes, choosing a sitecompatible with the existing sewers is a maKor consideration. In addition to the incomingsewers a further consideration is the siting of the outfall from the works. 0 river is able toassimilate a certain amount of polluting matter before flora and fauna are seriously affected,and it is on this basis that the >oyal !ommission standards were fixed. 0fter an effluent isdischarged into a river the biochemical processes which remove the remaining nutrientscontinue for some distance downstream. It is desirable where several works discharge intoone river, that the discharges are reasonably evenly spaced to spread the load over theriverHs natural assimilative capacity.

In practise the managers responsible for river quality management in the A0 L the ;ater

!ompany will stipulate the standard of effluent to be achieved at the works, and this willdepend upon the classification of the river, whether the river water is used for abstraction,the riverHs amenity value, and its assimilative capacity.

@. %rovision of access and services

0 sewage treatment works requires supplies of electricity, water and possibly gas.Alectricity is required for pumping and, with activated sludge systems, for aeration. 8ightingand buildings also consume electricity.

0ccess is required for operating staff, maintenance vehicles, and initially for constructionplant. 0 further consideration in the siting of a works is therefore that it should not be too

remote.

). Anvironmental impact

0 new sewage treatment works will have effects of various kinds on the environment, andthese must be considered when assessing the suitability of a site, the effect on the receivingwaters has already been mentioned, the other maKor effect is the nuisance to thesurrounding population. ;e have seen that works need to be sited reasonably close to thepopulation which they serve for ease of access and servicing, and economy of sewerage.This can cause odour and noise problems for those in close proximity to the plant. 0ctivatedsludge systems in particular have noisy machinery and produce aerosols which are likely tobe considered as obKectionable.9isually, a sewage works is unlikely to be hailed as a maKor aesthetic achievement andscreening may be necessary. !aution should be exercised in planting trees as the leaves

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can cause problems if they are able to fall into the plant, particularly with percolating filters./bKections may also be raised to the movement of plant and vehicles both duringconstruction and operation of the works and particularly so with a substantial sludgedisposal operation. In any civil engineering proKect, environmental impact assessment mustnow be seen as a necessary part of proKect evaluation.

F1OW ASSESSMENT FORM A2AI1AB1E DATA

&' Introdu!tion

In the hydraulic and process design of sewage treatment works the 5ry ;eather Glow(5;G of sewage is the most fundamental parameter used and it is important that it shouldbe assessed as accurately as possible.

5;G is defined in 1


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(b records of daily flows which have been integrated by the flow recording mechanism(OintegratorH data. /n older systems the integrator is read, in theory, at exactly thesame time each day. In practice this does not always happen and caution shouldbe exercised in interpretation of individual data. /n modern electronic systems thedata is recorded automatically so that truly accurate information on $ hour flows isproduced.

$.$ Inter"retation of -hart Data$.$. 5etermination of the ratios of daily peak=daily average flow.

The daily variation will be less for larger works because of the attenuating effect on flow ofthe sewerage systems due to its longer time of concentration. The information is of value inassessing daily maximum and minimum flows for hydraulic design.

$.$.$ The effect of pumped surges. The need for flow balancing may be indicated ifsewage delivered in pulses from a sewage pumping station causes the storm separation

weir (-5;G to overflow in dry weather.

$.$.- 0 high base flow after long wet periods, or due to snowmelt may cause continuousoverflow to the storm tanks and this phenomenon will be apparent from the charts.

$.$. Quantification of the wet weather regime " the duration, volume, and frequency ofwet weather flows received at the works. Such an assessment can be used to evaluate theoperation of the sewerage system, the effect of storm overflows, and determination of theneed or otherwise for storm tank capacity at the treatment plant.

$.$.3 5etermination of peak flows to works resulting from very intense storms. The chartswill show the maximum instantaneous flow that can arrive at the works under extreme

conditions.

$.$.@ 5etermination of the minimum night flow and from it the rate of infiltration, I. Ideallythis determination is made when a very dry period, conforming to the 5;G definition,coincides with a holiday period. :nder these conditions there will be no seasonal infiltrationand the effects of commerce and industry will be at a minimum thus causing lessuncertainty. The basic components of flow are shown diagrammatically below.

$


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T3$I-A1 F1OW -HART SHOWING -OM$ONENTS OF F1OW

In order to assess infiltration it is necessary to estimate the minimum night flow of domesticsewage. /ne water company uses a figure of *.$3%< which is equivalent to about -*l=hd=d. Some indication is given from surveys carried out on household water usage where ithas been found that night usage is of the order of 3#* l=hd=d. Thus minimum nightdomestic flow is likely to lie in the range *#-* l=hd=d and is likely to be highest for very largecatchments with a long time of concentration or where there is $ hour shift working.

$.- Inter"retation of Integrator Data

$.-. 0n annual graphical plot of daily flows. >efer to the two examples appended to thishandout. These graphs give an immediate and valuable appreciation of the characteristicsof seasonal variation in flow received at the works. They show important characteristics ofthe operation of the sewerage system, e.g. the effects of low set overflows, seasonalinfiltration, blockages, etc.

$.-.$ 5etermination of 5;G

a Grom the I;A& definition. The daily data is taken for the selected 3 day or ) dayperiod and averaged to give 5;G.

b Grom a graphical plot. It is quite possible for the sewage flow to fall to its 5;G levelduring periods which, because of the occurrence of small amounts of rainfall in thepreceding week, do not conform to the I;A& definition. The graphical plot will identify suchperiods.

$.-.- 5etermination of 0verage Glow

The average flow of sewage is defined as the total annual flow of sewage received at thesewage treatment works divided by -@3. It is an important parameter because, unlike 5;G,it includes the effects of rainfall. In the design process it is used, for example, in theassessment of biological loadings derived from analysis of operational data. 0t worksreceiving a significant proportion of trade effluent during the working week the average flowshould be calculated from weekday flows.

The ratio of average flow7 5;G describes the degree to which the quantity of sewage isaffected by rainfall. This ratio depends upon annual rainfall, impermeable area, stormoverflow settings and the amount of seasonal infiltration. 0 typical ratio is .-* with anapproximate range of .$* to .@*.

$. Esti0ation of Fow -o0"onents fro0 Measured Data

5;G # Grom the integrator data described in $.-.$ above.

% # 0s outlined in -. in the next section.

< # 0s outlined in detail in -.- in the next section.

Take the current water company domestic water consumption figure (approx *#3*=hd=d for purely domestic catchments. 0dd up to $3 =hd=d (for the total catchmentpopulation where there is significant commercial=industrial activity.

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A # This is the average daily flow of trade effluent discharged byconsented traders to the sewerage systems.

The existing value of A should be established by reference to the records kept for chargingpurposes by the water company. The annual flow data will need to be factored to allow forthe length of the working week (e.g. a 3R day working week is equivalent to $)3 workingdays per annum.

I # 4aving established 5;G, %< L A, calculate by deduction7

I B 5;G " (%< C A

Then check against minimum night flow as described in $.$.@. If there is poorcorrelation between the two methods, check the accuracy of the flow recorder, and recheckthe assumptions made regarding %, < and A.

+' Fow Assess0ent fro0 S%nthesised Data

+'& $o"uation $

5ata on current population and future population trends should normally be obtained fromthe planning sections of the water companies. 5esign population figures used for seweragedesign should not be used for sewage treatment design due to the longer design used forsewers.

%opulation nationally is at an almost static level. 4owever in some areas there aresignificant shifts of population from urban to rural areas.

-.. !ensus 5ata. 0 national census is held by the /ffice of %opulation !ensuses L

Surveys (/%!S once every * years. The most recent census was conducted in $** andthe data is now generally available.

The census statistics give detailed information on population and households. Note thatactual OdwellingsH or OhousesH are not enumerated and there will be a slight differencebetween OhouseholdH units and OdwellingH units.

-..$ !oastal and 4oliday >esorts. >esorts can experience wide seasonal variations inpopulation resulting from holiday#making. /ne definition that is used for establishing thetotal design population is7

% B resident population plus number of bed spaces available for visitors plus four times thenumber of caravans and camping site spaces for those properties=sites connected to theworks.

+'( Infitration4 I

;here no flow records exist, the quantity of infiltration water must be estimated. 0bnormallevels of infiltration may be indicated if the sewage is significantly weaker or stronger thannormal.

Gor new sewers it is recommended that the infiltration rate should be taken as *litres=head=day. i.e. even new sewers are assumed to leak Gor an average situation, forexample, * year old sewers and no particular evidence of any abnormality in thecomposition of sewage, and allowance of @* litres=head=day is appropriate. ;ith older

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sewerage systems, particularly in low lying flat areas where the maKority of the sewers arebelow the water table, it is possible for very high rates of infiltration to occur, eg. $*litres=head=day. ;here mining subsidence has caused cracking of sewers laid above thewater table exfiltration of sewage may occur.

+'+ Water -onsu0"tion4 G

In the late ?)*s a number of detailed studies were undertaken in the :P and it was foundthat the average national domestic (household water consumption was about $*litres=head=day. Since then consumption has increased to about 3* litres=head=day in$**. There is considerable variation from region to region The following is a typicalbreakdown of domestic consumption7

Toilet Glushing -$+Dathing and Showering )+!lothes washing machine $+8uxury appliances +

/utside use -+&iscellaneous -3+**+ (3* l=hd=d

&iscellaneous statistics7

0 shower uses about 3 litres=minute and a sprinkler can use m-=hour. -+ of householduse is for drinking, that is about * litres=day per household.

5ata on existing water consumption and forecasts of future increases should always beobtained from water undertakingsH planning sections.

In forecasting future water consumption the water industry has a history of over estimatingtrends of growth in water use. %rogressively, over the last twenty years or so estimates offuture growth rates have been reduced. !urrently ($**@ consumption is forecast toincrease at about 6+ per decade although this figure is likely to vary from region to region.

;idescale adoption of domestic water metering would probably result in a small reduction inconsumption.

In sewerage and sewage treatment design it is assumed that the per capita discharge ofdomestic sewage is the same as the per capita water consumption. In a purely residentialcatchment water consumption,


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Trade

Grom trade effluent consents. !onvert !./.5. data to D./.5. estimate by using a factorderived from !./.5.=D./.5. relationships for similar wastes. ;astes which have similarbiological treatment characteristics to those for domestic sewage will probably have a!./.5=D./.5. ratio of between .3 and $.*. 4igher ratios indicate increasing difficulties forbiological treatment.

Guture developmentsIf processes known, estimate by analogy with similar operations elsewhere. If unknown,treat as if average strength sewage.

WORKS DESIGN $ARAMETERS

&' Inet Wor6s

a' -on/entiona Bar S!reens

4and raked " submerged area B *.m$=***pop=day between raking.

5imensions of chamber for mechanically raked bar screens.

!alculate fromVD

F x

S

S+B=W

where7#

; B channel width (mD B width of each bar (mm

S B width of each bar space (mmG B maximum rate of flow (m-=s9 B maximum velocity through screen (m=s5 B depth of flow in approach channel at maximum rate of

flow (m

9elocity in approach channel minimum B *.-m=s9elocity through screen maximum B *.?m=s

Dar spacing (works screens B ) to $3mmDar thickness (works screens B ? to $mm

Dar spacing (coarse screens B )3 to 3*mmDar thickness (coarse screens B $3 to -*mm

Screenings quantity " range B *.**3 to*.*-m-=day=***

pop.

Screenings quantity " average B *.*$*m-=day=***pop.

;eight of screenings " range B )$* " 63* kg=m-

5' $ro"rietar% S!reen Designs!onsult manufacturers catalogues.

$*


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!' Grit Re0o/a

&aximum surface loading B .*3m=min (*.$mm grit(!rossflow B .3-m=min (*.-mm grit&aximum velocity through unit B *.-m=s5etention period (%ista B -* sec at maximum flow5iameter (%ista B .6 to 3.)m


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+' Bioogi!a Fitration

Gor temperature variation of settled sewage7 ;inter 3V! " Summer $*V!.

a' Singe Rate Fitration

&edia " 3*m.m. slag (* " @- mm range.&edia depth " .6 to $.*m

/rganic loadings7 (0verage conditions " settled sewage>ange *.*) " *.$ kg D./.5. =m-media=day!onventional norm *. kg D./.5.=m-media=day (non#nitrifying

*.*6 kg D./.5.=m-media=day (nitrifying

4ydraulic loading7 (0verage flow rates&inimum wetting rate *.3 m-=m-media=day (non#nitrifying

*.-3m-=m-media=day (nitrifying

&aximum 4ydraulic load at average flow *.)3 to .3 m-=m-media=day

5' Re!ir!uation

%rinciple use to cure ponding due to excessive film growth at the expense of loss ofnitrification.

0lso used to maintain irrigation rates on smaller schemes where infiltration is lowand on plastic filter media which require high irrigation rates.

/rganic loading7 (0verage conditions " settled sewage

!onventional norm *.3 kg D./.5.=m-media=day (non#nitrifying*.* kg D./.5.=m-media=day (nitrifying

4ydraulic loading7 (0verage flow rates with minimum of 7 recirculation rate

&inimum wetting rate *.@* m-=m-media=day (non#nitrifying*.* m-=m-media=day (nitrifying

c' Hu0us Tan6s

>etention time B .3 to - hours at maximum treatment rate. 0ctual value chosenincreases as effluent consent standard becomes more rigorous.

Surface loading rates not to exceed 3 m-=m$=day (with tanks out of service.

(&ay be F -* m-=m$=day for high quality effluents and to give increased security ofconsent compliance.

Sludge production B *. to *.3 kg dry solids=kg D./.5. removed during filtration.

Documents

PJ4 the Basis of Sewage Treatment Works Design