Lecture 2 Sewage

8/3/2019 Lecture 2 Sewage

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Lecture Notes for Session 3Week 15


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Sewage and Wastewater Treatment

The development of urban sewerage systems has been fundamental tothe enhancement of public health throughout the world. The objectiveof such systems is to convey wastewaters to a sewage treatment works(or a watercourse) at minimum cost consistent with safeguards forpublic health, water pollution and amenity. The sewerage system andassociated treatment works are extremely expensive to build andmaintain, and there is, therefore, considerable interest and research into

the design and safe operation of such infrastructure.This unit is designed to give an understanding of the differing types of,and need, for urban sewerage systems and the basic design criteriauniversally employed for sewer flows. The characteristics of bothdomestic and trade effluents are described and the unit processfunctions of typical preliminary and primary wastewater treatment areexamined. The various forms of settling characteristics encountered inprimary sedimentation are developed and the three basic types ofsedimentation tank or basin are discussed.


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1. O RIGINS OF P RESENT D AY PRACTICE Most nineteenth century urban and industrial towns were without anyproper provision for waste disposal or water supply. It was, therefore,inevitable that water-borne diseases were endemic in such populatedareas and it became obvious by 1820 - 30 that centralised approachesneeded to be introduced to safeguard public health. One of the earliestand most significant campaigners for such facilities in the UK wasEdwin Chadwick, Secretary to the Poor Law Commission. His work ledto a 1841 government report on The Sanitation of Towns which linkeddiseases such as cholera with bacteria in contaminated waters. In turn,this led to the enactment of the first Public Health Acts of 1848 and thelater River Pollution Acts of 1876 and 1890 which established a pipedsewage system of foul sewers to convey foul sanitary wastewater andeffluent waters from domestic and trade premises for treatment incentral sewage treatment works (STWs).

This sewerage system was soon extended to also carry storm rainfall such systems comprising combined sewers. It was clearly uneconomicto make combined sewers large enough to take all possible range offlows given that, even in the UK where maximum rainfall intensities areonly moderate, the maximum surface runoff rate can be up to 150 timesthe average wastewater flow. Thus, most urban interceptor sewers havebeen provided with overflow devices (see section 2.3 below).

Since the early 1950s, further sewer systems have been developed tohandle only rainfall-runoff from impermeable roof, paving and roadsurfaces. These separate stormwater sewer systems , primarily fornew residential and commercial developments, were intended to takeexcess discharge loading from the combined system and, as it wasconsidered that such surface runoff was relatively unpolluted, theseparately piped flows could be discharged directly to the nearestwatercourse without any prior treatment.

The two sets of separate and combined sewers are often trench laid side-by-side, with the combined sewer set at a lower level than the surfacewater sewer so that side connections can be accommodated.

There are also some partially separated sewer systems where only aspecific proportion of surface runoff, such as roof drainage, is allowedinto a foul or sanitary sewer in order to enhance the hydraulicperformance and self-cleansing capacity of the foul sewer.

Table 11 overleaf provides some brief comparative details of the threebasic types of sewer system.


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Table 11: A comparison of sewer system types

Advantages Disadvantages

Combined Systems

Large enough for storm flows in thesame pipe.

Involve considerably less cost inconstruction and maintenance.

No connections to the wrong pipesystem.

Often excessively large and subject tosedimentation during dry weather.

Large quantities of clean storm waterare passed to the treatment works forcostly and unnecessary treatment.

May have been provided ad-hoc and soage, condition and capacity not known,but often decaying.

Overflow discharges to receivingwatercourses prejudice water quality.

Often subject to high infiltration andexfiltration rates.

Separate Systems

Design of hydraulic capacity for surfacerunoff is accurate.

Smaller pipes require less space.

Sewage treatment costs minimised.

Greater infrastructure cost incurredthrough a dual system.

Foul sewage and greywaters can beconnected into the surface watersystems in error.

Lower flows in the upper reaches (orhead) of a foul sewer may causesedimentation.

The first-flush from surfacestormwater sewers is often heavilypolluted.

Where the sewers downstream arecombined, some of the benefits of theseparate system are lost.

Partially separate systems

Storm drainage assists foul sewers toself-cleanse and aids the resuspension ofany deposited material.

Greater flexibility in design andtherefore can be cost-effective.

Not popular in many areas and seen ascompromise solution.

Not used to any considerable extent fornew developments.

The quality of the discharge will be veryvariable and the operation of the oldersystems which incorporate river outfallsmay not be well understood orcontrolled.


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(Note that the first two disadvantages given, in the table, for partiallyseparate systems are not operational ones. Thus, the water industry isnow recognising that, used appropriately, such systems may have agreater role to play than previously thought.)

Today, some 96% of the UK population and 75% of the Hong Kongpopulation is connected to a mains sewer line and, in the UK, some 70%of the sewers are of combined type and 25% are separate sewers.However, about a third of the total sewer system in the UK is nearly 100years old and subject to crumbling, collapse, exfiltration and infiltration.This has led over the past 10 to 15 years to substantial sewerrehabilitation and renewal involving a high investment by the sewerageutility companies approaching some 500M per annum.


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2. S EWAGE FLOWS AND D ESIGN C RITERIA The design of a sewage system is a function of the flows in terms ofboth their nature and volume with which it must cope

2.1 Sewage strengthThe strength of a sewage effluent is normally expressed in terms of theBOD5 per population served and is generally known as the populationequivalent or PE Value.

It is commonly accepted that the average per capita daily BOD 5 production is about 55 - 60 g/m 3 (or mg/l), generating an average dailyload to the STW of between 0.05 to 0.07 kg BOD/head/day.

Thus, a small residential development producing 800 m 3/day sewageeffluent and having an average BOD 5 concentration of 700 g/m 3 willhave a BOD loading of:

800 x 700 = 560,000 g BOD/day.

This would correspond to a PE value of:

55000,560 = 10,182 PE.

Some typical PE values for differing industrial enterprises are given inTable 2-1.

Table 2-1: Examples of industrial PE values

Industry PE Value

Brewing

Dairy plant

Abattoir

Paper mill

Tanning

Laundry

150 - 350

25 - 70

5,000 - 8,000

200 - 900

1,000 - 4,000

20 - 100

2.2 Sewage flowsSewage effluent is delivered to about 8,000 sewage treatment works inthe UK, with about 1,000 of these serving a population of more than10,000.


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In general terms, the design range for domestic sewage flows isuniversally considered to lie between 150 and 300 litres/head/day(l/hd/d, or l/hd.day as it is often expressed), largely depending ongeographic location and cistern design. However, there are substantialvariations in wastewater flow volumes throughout the day with the

diurnal flow peak (Q pk ) being about 2 to 3 times the minimum DryWeather Flow (DWF) rate (see Figure 21). There is also a ratio betweenminimum and maximum flows on a daily basis that can vary between4:1 and 10:1.

Figure 21: Diurnal variations in dry weather flow(Data for an industrial town of 85,000 population)

DWF is normally defined as the average daily flow in a sewer overseven consecutive days without rainfall and where the previous sevenday period received less than 0.25 mm on any day.

A minimum sewer flow velocity of about 0.75 - 1.0 m/s is required inorder to ensure a satisfactory self-cleansing sewer under DWFconditions. Reductions in velocity below this threshold self-cleansingvalue lead to deposition of solids on the sewer invert. This is aparticular problem in lengthy sewer systems and those having lowgradients, and leads to the production of septic sewage. Septic sewagemay produce gases, including potentially explosive methane and H 2S

which can cause corrosion of the sewer walls and ring joints.Given the variation in diurnal flows, the pollution load carried throughto the STW is also subject to daily variation, irrespective of anysuperimposed storm flows that may occur in a combined system.However, flow rates and pollutant concentrations occur together inunison which implies that both hydraulic and organic loadings areexerted on the STW at the same time.

0

200

400

600

800

1,000

8 12 16 20 24 4 8

Time of day

R a t e o f

f l o w

( m 3 / h )

Peak flow

Day timeaverage Daily average

(24 hrs)Night time

average

Minimum nighttime flow


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2.3 Design criteriaThe 1970 UK Ministry of Housing and Local Government TechnicalCommittee on Sewage Disposal recommended that all sewer (foul andcombined) flows up to 5 - 6 DWF (roughly equivalent to 35 - 45 x 10 3

m3

/day effluent) should be conveyed and treated at STWs. Excesssewer flows above this (which could reach 15 or 20:1 DWF) were to bediverted untreated to receiving waters via overflow structures.

Thus, traditionally, the Combined Sewer Overflow (CSO) setting of5 - 6 DWF has been based on the desired carry-on flow in the sewer i.e. the effluent flow to be diverted to and treated at the STW. This fixedDWF approach makes no allowance for diurnal or regional variations insewage strength and flow rates or of the actual dilution capacityavailable in the receiving water when the overflow is operating. Arough operational rule-of-thumb would suggest that at least aminimum 8 to 10:1 dilution capacity should be available at low receivingwater levels to prevent the river water DO level falling below 4 mg/l.The working assumption in this design practice is that spillage fromoverflow structures should only operate during rainfall events whenreceiving water levels are high providing sufficient dilution andassimilative capacity.

Formula AThe 1970 Technical Committee also produced what is called the Formula A approach which is still a widely accepted and used rule adoptedinternationally to derive the sewer overflow setting and sewer flowdesign. This is expressed as:

Q = DWF + 1,360P + 2E ( Equation 1)

where: Q is the required carry-on flow to the STW (in l/d);

P is the population served; and

E is the trade effluent;

and where DWF is defined as:

DWF = PG + I + E ( Equation 2)

where: G is the average daily domestic water consumption (l/hd/d);and

I is the infiltration rate into the sewer.

The rate of flow at which CSO spillage will first occur into thereceiving water should then be :

60)x60x(242E)1,360P(DWF ++ (l/s) ( Equation 3)

Whatever the answer to equation 3, it should not exceed the 8 to 10:1dilution ratio and, before the overflow structure operates, the sewage


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effluent should be diluted by 1.36 m 3 per head and any industrialeffluent by a factor of at least two.

Unfortunately, few of these anticipated safeguards are rarely met. CSOsare rarely monitored for compliance as it is assumed that, on average,they will only operate some 2% to 5% of the time and thus would falloutside the regulatory 95% compliance requirement standard. ManyCSOs operate (or spill) at levels below the design level of 5 - 6 DWF,principally due to the fact that settings on overspill weirs have not beenadjusted to cater for new urban development or suburban expansionwhich generates new and increased sewer flows. Thus, settings todayare rarely at the 5 - 6 DWF design level as they might have been whenthey were first designed and installed. In any case, Formula A entirelydisregards the sensitivity of any receiving water in both chemical andecological terms.

As much as 20 to 30% of CSOs in the UK (and in Europe and the USA)are deemed to be unsatisfactory in terms of premature spillage or

receiving water quality.

2.4 Using Formula AWe can illustrate the application of Formula A by considering thefollowing example.

Example 1

In a residential area of 100 persons/hectare, the per capita daily wateruse is 220 litres/head.day. Compare the foul sewage flow with aseparate stormwater runoff flow produced by a rainfall event withintensity (In) of 40 mm/hour given that the catchment area (A) has arunoff coefficient (Rc) of 0.35 (35%).

To calculate the stormflow discharge (Q) utilise the rational formula:

Q = Rc x In x A

Solution

For the foul sewage:

DWF = G x P (i.e. there is no I or E value)

= 220 x 100 l/hd.day

= 22 m3/hd.day

However, we need to compare this with the separate sewer which willonly flow during wet weather conditions, so assume that the foulseweris at full flow and therefore at 6 DWF design capacity. The totalcapacity, then, is:

6 DWF = 22,000 (litres) x 6

= 132,000 l/hd.day


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flow rate =60)x60x(24

132,000

= 1.5 l/s/hd.

For the separate stormwater sewer:

Q = C x In x A

= 0.35 x

60x6040 x 104 x (10-3).

The last two terms are required to transform, respectively, hectares to m 2 and l/s to m 3/s to maintain the correct units. Therefore:

Q = 0.039 m 3/s or 39 l/s per head.

Thus, the ratio of foul sewage to stormwater effluent is:

1.5l/s/hd to 39 l/s/hd, or 1:26


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3. T RADE EFFLUENT Where industrial wastewaters are to be discharged into a public sewer,strict consent conditions have to be met given the potential toxicity ofthe effluent and the problems that this might pose for the treatmentworks.The consent and charging criteria for trade effluents is based on theCOD load rather than BOD, as the former includes both oxidisableorganic and inorganic material as well as being a simpler and quickertest procedure. The following generic formula defines the typicalcharging approach (per m 3) for trade effluents under average flowconditions:

C = R + V + B

OsOt + S

SsSt (Equation 4)

where: C = total charge

R = reception and conveyance charge

V = volumetric and primary treatment cost

B = biological oxidation costs for settled effluent

Ot = the effluent COD following one hour quiescent settling andwith pH adjusted to 7.0

Os = the COD of the settled sewage

S = treatment and disposal costs of primary sewage sludge

St = TSS (total suspended solids) of the effluent

Ss = TSS of the crude sewage

As indicated by equation 4, the charge is only levied for those treatmentelements actually applied at the STW. Thus, if a trade effluent receivesonly a primary treatment, the biological part of the equation (B) isignored. Further, if the trade enterprise makes a capital contribution inany form for example, financial support for an extension to the STW then a reduction in the trade effluent charge is made.

The sewage undertaker (or sewerage utility company) is not required toaccept the full trade effluent discharge and the industrial enterprisemight, therefore, have to carry out some pre-treatment of its effluentprior to discharge into the sewer system. This is particularly the case fortoxic substances which might seriously prejudice either the STW consentdischarge conditions to the receiving water or final sludge disposalstandards. This is illustrated in the example overleaf.


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Example 2

A 1,400 m3/day trade effluent containing 200 mg/l of chromium isdischarged into the public sewer. The DWF sewer flow to the STW is60,000 m3/day and 70% of the chromium settles out in the primary

sedimentation tanks and is left in the sludge. A further 80% is removedin the secondary treatment stage with 50% of the activated sludge takenoff as surplus sludge and mixed with the primary sludge for digestion.The receiving water immediately downstream to the STW final effluentis abstracted for potable water supply and this has a regulatory standardfor chromium of 0.05 mg/l.

(a) What weight (kg) of Cr per day will be present in the total sludgegoing to the digester and eventually either to land or sea?

(b) What will be the Cr concentration in the STW final effluent?

(c) If the receiving water provides a four-fold dilution and has azero chromium background level, what will the Cr concentrationbe after the final STW effluent mixes in the river water?

Solution

(a) Trade discharge is 1.4 x 10 3 m3/d, and the Cr content is 200 mg/l.

The weight of Cr discharged per day is, therefore:

1.4 x 103 x 310200 = 280 kg

However, only 70% of the Cr appears in the primary sludge,amounting to:

280 x 10070

= 196 kgTherefore, the quantity of Cr passing to secondary activatedsludge process (ASP) treatment is:

280 - 196 = 84 kg

In the ASP unit, 80% of the Cr is removed, amounting to:

84 x10080 = 67.2 kg

Therefore, the total Cr in sludge sent to the digester is:

196 + (67.2 x 50%) = 229.6 kg


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(b) The quantity of Cr in the final STW effluent is:

84 - 67.2 = 16.8 kg

The total STW discharge is given as 60 x 10 3 m3/d. Therefore, theCr concentration is:

3

3

10x6010x16.8 = 0.28 g/m 3 (or mg/l)

(c) With a four-fold dilution, the Cr concentration in the receivingwater is:

428.0 = 0.07 g/m 3 (or mg/l)

As the UK drinking water standard for chromium is set at 0.05mg/l, the trade effluent would need to have a lowerconcentration to be allowed to discharge to the public sewer.Thus, some form of source pre-treatment on the premises of the

industrial factory would need to be installed.


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4. S EWAGE COMPOSITION ANDCHARACTERISTICS The typical composition of raw domestic wastewater is given in Table 4-

1, expressed in terms of concentration, with Table 4-2 showing typicalsolids content distributions for domestic sanitary sewage.

Table 4-1: Typical composition of untreated domestic wastewater

Constituent(mg/l, except where stated)

Concentration

Strong Medium Weak

Total suspended solids (TSS) 500 250 100

Total dissolved solids (TDS) 850 500 250

Settleable solids (ml/l) 20 10 5

BOD5 450 220 110

COD 1,000 500 250

Total nitrogen 85 40 20

Free ammonia (NH 3) 50 25 12

Nitrite 0 0 0

Nitrate 0 0 0

Total phosphorus 15 8 4

Chlorides* 100 50 30

Alkalinity (as CaCO 3) 200 100 50

Grease 150 100 50

* Values should be increased by the amount in domestic water supply

Table 4-2: Solids type and concentration (mg/l) in domestic sewage

Mineral Organic Total BOD 5

Total suspended solids (TSS)Settleable solidsNon-settleable solids

11055

220110

330165

10565

Total dissolved solids (TDS) 275 275 550 130

Totals 440 605 1,045 300


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The four basic chemical parameters conventionally used to describesewage composition are BOD 5, COD, TSS and NH 4 (ammonium), withthe latter being used as a basic index of toxicity.

Wastewater can only be treated biologically if sufficient carbon andnutrients are present in the effluent. Normally, there is a surplus, butSTW managers confirm this excess in terms of the C:N:P ratio whichshould be optimised at 100:5:1 (BOD 5 : N total : Ptotal in mg/l). Rawdomestic sewage has a ratio of approximately 10:17:5, so both N and Pare present in excess. However, it is the C:N ratio which is really criticaland this must remain less than about 18 - 20:1.

It is trade effluent wastes that cause most problems as they come to theSTW in a nutrient-deficient (and often toxic) state and so must be mixedwith the sanitary waste to bring it up to appropriate nutrient levels tofacilitate bacterial oxidation.

Raw sewage possesses a strong odour due to H 2S, sulphur, chlorine,ammonia and fatty acids. This is possibly the most objectionablecharacteristic parameter of sewage to the public and odour control istreated as a high priority in most STWs. Sewage tends to be yellowish incolour, having a fairly uniform temperature of around 12 - 15C with aneutral pH of 7.0 - 7.5 and a DO of 1 - 2 mg/l. However, sewage canbecome anaerobic (and smelly as a consequence) very quickly indeed ifthe forward flow of the effluent becomes retarded or blocked for anyreason.

Figure 4-1 provides an alternative view of sewage composition whichemphasises its physico-chemical structure and the operationalrequirements in terms of unit treatment processes.

This is followed by two tables which set out the inorganic and organicconstituents of domestic sewage Table 4-3 provides a detailedelemental listing of the inorganic constituents, whilst Table 4-4 indicatesthe proportion of the principal organic fractions found.


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Figure 4-1: The composition of domestic sewage

Domestic/crude sewage

99.9% Water 0.1% Solids(300 - 400 TSS mg/l)

70% Organic 30% Inorganic

Grit Salts Metals

Protein(60%)

Equally insolution andsuspension;

derived fromanimal and

plant foodstuffs

Carbohydrates(25%)

Comprise oforganic load;

starch, glucose,cellulose, lignin ;

mainly insolution

Fats(8%)

Not easilydegraded;

esters, glycerol,grease

Amino acids(4%)

VitaminsHormonesPesticides

SurfactantsHydrocarbons

(1%)

Table 4-3: Inorganic constituents of domestic sewage

Constituent US Sewage(Soft water area;

mg/l)

UK Sewage(Hard water area;

mg/l)

Cl 20.10 68.00

Al 0.13 -

Ca 9.80 109.00

K 5.90 20.00

Na 23.00 100.00

Cu 1.56 0.2

Pb 0.48 0.08

S 10.30 22.0

P 6.60 22.0


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Table 4-4: Organic constituents of domestic sewage

Constituent

In solution In suspension

Concentration(mg/l)

ProportionC of total C insolution (%)

Concentration(mg/l)

ProportionC of total C in

suspension (%)

Fats - - 140 50

Carbohydrates 70 31.3 34 6.4

Free and boundamino acids

18 10.7 42 10

Volatile acids 25 11.3

12.5 2.3Non-volatileacids 34 15.2

Detergents 17 11.2 5.9 1.8

Uric Acid 1 0.5 - -

Creatine 6 3.9 - -

Amino sugars - - 1.7 0.3

Amides - - 2.7 0.6

Organic carbon 75.6 84.1 151 71.4


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5. P RELIMINARY TREATMENT Preliminary treatment comprises screening the raw sewage and gritremoval.

5.1 Screening Initial treatment of the raw sewage entering into a STW consists of theremoval and/or disintegration of large solids and detritus. This isnecessary to prevent damage to and blockage of equipment downstreamwithin the treatment plant. This unit process is carried out using screensand/or comminutors.

ScreensThese generally have steel bars set 10 to 15 mm apart which may behand or machine raked. The forward velocity of wastewater flow

through the screens should not be less than 0.3 - 0.5 m/s to avoid settlingout of grit and other coarse solids.

Screenings may be disposed of by incineration or burial, but are oftendisintegrated on-site and then returned into the incoming sewage flow.

ComminutorsThese combine the processes of screening and disintegration. A varietyof systems are available to handle effluent flows of between 350 to100,000 m3/day.

The screen has horizontal slats and is rotated by means of an electricmotor. As the process is carried out under continuous flow conditions, itis highly efficient. The sewage passes through the screens, down thecentre of the comminutor (or detritor as it is sometimes called) and intothe downstream channel by means of an inverted syphon. Solids areheld against the outside surface of the comminutor drum by thepressure of the liquid flow.

Head losses through comminutors are usually higher than throughscreens and they can become easily blinded.

5.2 Grit removalCoarse inorganic gritty material must be removed from sewage to avoid

abrasive damage to pumps and machinery in the STW as well as toavoid silting-up of various parts of the treatment works. The gritremoval equipment should also be designed to reduce the organiccontent of the grit to not more than 15%. Additionally after removal, thegrit should be washed and the washings returned to the main sewageflow.


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Because grit is denser than the organic solids, its removal is achieved bycontrolling the velocity of sewage flow to a critical value at which onlythe coarse grit will settle out. This critical threshold velocity is 0.3 m/s,but the range of 0.2 - 0.45 m/s will generally allow a satisfactoryoperation.

The operation takes place in either a constant-velocity grit channel or inproprietary grit tanks.

Constant-velocity grit channelsTheoretically, these should be parabolic in cross-section, althoughtrapezoidal shapes are quite frequently used with side slopes greaterthan 45 degrees to encourage grit to fall down to the invert. At least two(parallel) channels should be provided (with a total capacity of 6 DWF) allowing for one channel to be emptied for grit removal, whilst the otherremains on-line. Channel length should be about 20 x depth, withchannel width being equal to

nV3

where: n is the Manning n roughness (0.012 for concrete); and

V is average flow velocity.

Grit removal can be effected by automatic or manual methods usingdredging, conveyors or suction pumps. Automatic control and re-direction of wastewater flow may be carried out by means of rectangularstanding-wave flumes connected to automatic penstocks upstream of thegrit channels.

Grit tanksThere are three basic types of grit tank.

Detrition

In this type of tank, entry is over a full-width weir, flowingthrough a square tank to a narrow exit channel. The forwardvelocity through the tank is kept low to enable grit to settle out.It is then swept to one side of the tank by a centrally mounted,continuously-rotating set of three arms, from where it falls to asloping channel and is carried upwards and discharged. At thesame time, clean water flows down the channel and washes thegrit.

Pista grit trap

Here, the sewage enters tangentially into a circular tank and isswept by a set of rotating paddles and leaves at the same level.The velocity in the tank is then reduced and the grit settles to thetapered bottom of the tank. The grit is washed in-situ by a


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counterflow of air and water in the sump before beingdischarged by a pump.

Spiral flow tanks

In these, the sewage enters orthogonally to the direction of flow

along rectangular tanks. Air diffusers along the sides create aspiral flow and the grit settles down the sloping floors into grithoppers from where it can be pumped out.


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6. P RIMARY T REATMENT

6.1 Stormwater treatment

Since it is uneconomic to fully treat all flows which reach the STW, andsecondary biological processes are adversely affected by widely varyinghydraulic and organic loadings, full treatment can only be given at anyone time to about 3 DWF. Flows between 3 DWF and the design level of5 - 6 DWF are, therefore, subject to only preliminary and stormwatertreatment on the basis that the majority of the flow is likely to consist ofsurface runoff. Storm treatment consists of screening, grit removal and afinal sedimentation in stormwater holding tanks before discharge to thewatercourse. This is considered acceptable as the rivers are normallycarrying higher flows during rainfall periods and hence will be able todilute the effluent effectively.

To divide the incoming flow, a separation weir is provided afterpreliminary treatment to divert flows in excess of 3 DWF and up to 5 - 6DWF to the stormwater sedimentation tanks. The use of the weirensures that the quantity passing through the penstock is independent ofthe head over the weir. This is important since otherwise the capacity ofdownstream sections in the treatment works may become overloaded.

The stormwater tanks fill up one at a time and no discharge is madeuntil all tanks are filled and then discharge to the watercourse may takeplace. After the rainfall event has ceased, the tanks should be graduallyre-routed back to the works inlet as soon as possible to enable fulltreatment up to the 3 DWF capacity. The sludge which settles in thestorm tanks is removed by means of mechanical scrapers after the

stormwater has been removed from the tanks and is then discharged tosludge treatment works.

6.2 Primary sedimentationPrimary sedimentation is intended to remove as large a proportion aspossible of the coarse settleable solids in sewage in order to allow laterbiological processes to work efficiently. Effective sedimentation canremove up to 90% of the Total Suspended Solids (TSS) and reduce theBOD5 by up to 40%. The basic principle of sedimentation is simple inthat any liquid containing heavy particles in suspension will clarify ifallowed to stand.

Four different types of process may apply, depending on the nature ofthe wastewater.

Type I: Discrete particle settlementThe process of discrete particle settlement (or Type I settlement) underlaminar flow conditions is described by Stokes Law which balances


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gravitational forces of particle settlement with the drag or frictionalresistance forces:

Vs =18g

( 1 - )

2d (Equation 5)

where: Vs is particle settling velocity; 1 and are particle and fluid density respectively;

g is gravitational acceleration (9.81 m/s 2);

d is particle diameter; and

is the kinematic viscosity (1.01 x 10 -6 m2/s at 20C).

The basic design approach for primary sedimentation tanks or basins isto select a design particle having a particular terminal or settling velocity(Vs) say, for example, coarse sand of 0.25 mm diameter with a Vs of 0.5m/s and then to design the tank so that all particles (i.e. 100%) havinga terminal velocity equal or greater than Vs will be captured andremoved in the tank.

The effect of particle size on settling velocity can be illustrated byreference to two particles with diameters of 0.01 mm and 0.06 mm, andhaving a density (or Specific Gravity, SG) of 2.5, settling in water havinga kinematic viscosity of 1.0 x 10 -6 m2/s:

For the 0.01 mm particle:

Vs =1881.9 (2.5 - 1.0)

)10x01.1()10x1(

6-

2-5

= 0.81 x 10-4 m/s

= 0.08 mm/s

The same calculation for the 0.06 mm diameter particle derives asettling velocity of 29.2 x 10 -4 m/s (or 2.92 mm/s).

The only watchpoint in the calculation is to ensure that the variousparameters are all entered into the equation in similar units.

In actual STW practice, the Vs value corresponds to medium sand sizesof about 0.15 mm diameter and is achieved under loading rates in thegeneral range of 15 - 25 m 3/m 2.day.


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The suspended solids will not settle if the upward velocity exceeds thesettling velocity, i.e.:

if Vs 0.3 m/s) through the tank to minimise disturbance ofthe settled sludge and help establish a uniform forward velocity. Withmechanical de-sludging, a detention period of 6 - 10 hours may beadopted or 8 - 15 hours for manual de-sludging, and the maximum weirloading rate is about 20 - 30 m 3/m 2.day to limit the influence ofdrawdown currents.

The most common unit process uses mechanical de-sludging. Thesludge scraper is suspended below a power-driven bridge which travelsalong rails either side of the tank. The scraper travels slowly against theeffluent flow and deposits the sludge in a hopper below the floor of thetank from which it is pumped to the sludge treatment works. Thebridge returns with the scraper raised above the water level. The rate oftravel of the scraper must be limited to 1.2 m/min.

Upward flow tanksThese are normally square in plan with deep hopper bottoms and arevery common in small STWs. Their main advantage is that sludgeremoval is carried out entirely by gravity and no mechanical parts arerequired for cleaning.

Sewage enters the tanks below the top water-level, but above the sludgelevel. As the sewage flows upwards and outwards, the velocitydecreases and the sludge flocculates and aggregates, with the largesludge flocs settling to the bottom of the tank where it is removed underhydrostatic head or by pumping. Upward velocity is usually 1.2 -1.8 m/hour at maximum flow with the required upward flow beingmaintained by weir troughs adjacent to the stilling well in addition tothose at the tank perimeter. The sides of the tank slope in a conical formwith side slopes steeper than 60 degrees to concentrate the sludge to the


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hopper base although this means that scum removal has to be a dailymanual operation. The capacity of the tanks is about 5 - 6 hours averageDWF volume.

True upflow tanks have a disadvantage in that hydraulic overloadingmay have more serious effects than in a horizontal flow tank. Anyparticles with a velocity lower than Vs = Q/A will not be removed in anupward flow tank, but will escape in the clarified effluent. In ahorizontal flow tank, assuming that such particles were uniformlydistributed throughout the flow, particles with Vs 1 < Q/A would still beremoved in the proportion of Vs 1 / Vs.

Radial flow tanksThese need very careful baffling as the radial flow pattern can easily setup excessive turbulence around the central sludge hopper, causing re-suspension. It is, therefore, normal to have a central stilling box to givestable flows in the tank.

In circular radial flow tanks, sewage enters at the centre stilling well andflows radially out over an outer perimeter weir into a channel. Thefloors of the tank slope (at between 1 - 15 degrees) to the centre of thetank. Sludge is guided down the slope by means of continuously-rotating scrapers or chains to a central sludge hopper which isintermittently discharged by pumping. Scum removal is carried out bya surface skimming board attached to the sludge scraper mechanism andpositioned so that scum is moved towards a collecting hopper at thesurface.

Radial tanks are designed on the principle of limiting the surfaceloading to 35 - 45 m 3/m 2.day with:

Surface loading =)(mareaTank

)(mhours24inflowDaily2

3and

Discharge overflow rate =(m)lengthWeir

areaTankxloadingSurface

The weir length around the tank perimeter is usually sufficient to give asatisfactory weir loading rate at maximum flow, but at low flows, verylow flow depths may occur. To overcome the sensitivity of these tanksto such variations in level, as well as for errors in weir level and windeffects, it is common to have serrated or notched weirs.


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Figure 61: Typical primary sedimentation tanks

(a) Rectangular horizontal flow tank

(b) Upward flow tank

(c) Circular radial flow tank

Effluent

Sludge

Stilling box

Influent

Scum baffle

Scum pit

Scum baffleStilling box Rotating arm

Sludgescraper blades

Influent

Effluent

Sludge

Scum trough

Skimming blade

Influent Effluent

Sludge

Sludge scrapers

Baffle

Scum

Scum removal

trough

Effluent decantingweirs

Documents

Lecture 2 Sewage