sludge treatment and disposal 1

SLUDGE TREATMENT AND DISPOSAL

ENVIRONMENTAL ENGINEERING II

2SLUDGE TREATMENT

GENERALThere are two end products of the waste water treatment.

The treatment plant effluentDirectly discharged in the natural water sources or into

receiving soils.The sludgeContains highly putrescible substances and pathogenic

organisms.Treated further for safe disposal

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SLUDGE AND ITS MOISTURE CONTENTRAW SLUDGE

Deposited in a primary sedimentation tank.Odorous and contains highly putrescible matter – very

objectionable.Moisture content 95% .

SECONDARY SLUDGEDeposited in a secondary clarifier.contains putrescible matter, but less objectionable

compared to raw sludge.Moisture content 96% - 98% (trickling filter) and 98% -

99% (activated sludge plant)

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Problem : A sedimentation tank is treating 4.5 million litres of sewage per day containing 275 ppm of suspended solids. The tank removes 50% of suspended solids. Calculated the quantity of sludge produced per day in bulk and weight, if (a) moisture content of sludge is 98% ;(b) moisture content of sludge is 96% .

SLUDGE AND ITS MOISTURE CONTENT

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Solution: Given: volume of sewage treated = 4.5 x 106 L/day Suspended solids present = 275 ppm

= 275mg/L= 275 x 10-6 kg/L

ie., In 1 L of sewage 275 x 10-6 kg of suspended solids is present.

therefore, in 4.5 x 106 L the mass of suspended solids present = 275 x 10-6 (kg/L) x 4.5 x 106 (L/day)= 1237.5 kg/day

Also given, % of solids removed in sedimentation tank = 50 % Thus, the mass of solids removed in sedimentation tank

= 50% of 1237.5 kg/day= 618.75 kg/day


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Solution: Case (a) : moisture content of sludge is 98%ie., 2kg of solids or dry sludge makes 100 kg of wet sludge.Thus, mass of wet sludge made by 618.75 kg of solids

= 30937.5 kg ~ 30940 kg

Assume, specific gravity of wet sludge = 1.02Unit weight of water = 1000kg/m3

So, unit weight of wet sludge = 1.02 x 1000 kg/m3 =

1020kg/m3


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Solution: Case (a) : moisture content of sludge is 96%Weight of sludge = 15470 kg of wet sludgeVolume of sludge = 15.17 cu.m

Assignment Problem : There is a sewage sludge with volume containing a

certain moisture content p1%. What will be the volume of this sludge if its moisture content is reduced to p%?

Solution:

V = vol. of sludge produced at p% V1 = vol. of sludge produced at p1%


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The process of stabilization of sludge withdrawn from the sedimentation basin.By decomposing the organic matter under controlled

anaerobic conditions.40% - 60% of organic solids converted into CO2 and

CH4 gas.Remaining organic matter will be chemically stable

and odourless with 90% - 95% of moisture content.This process reduces the sludge into three forms

Digested sludgeSupernatant liquorGases of decomposition

SLUDGE DIGESTION PROCESS

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Digested sludgeStableHumus like solid materTarry black in colourMusty earthy odourWith reduced moisture contentVolume= one third times the undigested sludgeFree from pathogenic bacteriaBut may contain cysts and eggs of bacteria, worms and

protozoaDewatered, dried up and used as fertilizer


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Supernatant liquorLiquified finely divided solid matterBOD about 3000ppmContains 1500ppm to3000ppm of suspended solids, so

re-treated with the raw sludge.

Gases of decomposition65-70% CH4

30% CO2

Traces of inert gases like N, H2SCollected and used as fuel


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STAGES IN SLUDGE DIGESTION PROCESSAcid fermentation/Acid productionAcid regressionAlkaline fermentation


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STAGES IN SLUDGE DIGESTION PROCESSAcid fermentation/Acid productionAnaerobic and facultative bacteria (acid formers) acts.Acid formers stabilize the organic solids through

hydrolysis.Soluble products fermented to volatile acids and organic

alcohols of low molecular weight.o Propionic acid, acetic acid etc

Evolution of CH4 , CO2 and H2S gases.Lowers pH value to less than 6 – highly acidic in nature.Evolution of highly putrescible odours.Continues for 15 days.


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STAGES IN SLUDGE DIGESTION PROCESSAcid regression

Volatile organic acids and nitrogenous compounds of the first stage acted upon by bacteria.

Forms acid carbonates and ammonia compounds.Evolution of H2S and CO2 gases in small amount.Offensive odourpH value rises to 6.8Entraps of gases of decomposition and forms formy scum

layer.Continues for 3 months.BOD remains high.


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STAGES IN SLUDGE DIGESTION PROCESSAlkaline fermentation

Proteins and organic acids attacked and broken up by anaerobic bacteria (methane formers).

Forms ammonia, organic acids and gases.Liquid separates out from solids and the digestive sludge (or

ripened sludge) is formed.Digestive sludge collected at the bottom of digestion tank.Alkaline in nature – pH value rises little above 7.Large amount of CH4 and small amount of CO2 and N gases

are evolved.Continues for 1 month.BOD rapidly falls down.


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FACTORS AFFECTING SLUDGE DIGESTION PROCESSTemperature pH valueSeeding with digested sludgeMixing and stirring of raw sludge with digested sludge


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FACTORS AFFECTING SLUDGE DIGESTION PROCESSTemperatureRate of digestion increases at higher temperature.

Fig 9.27


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FACTORS AFFECTING SLUDGE DIGESTION PROCESSTemperature

i. Zone of thermophilic digestion High temperature zone – 400C to 600C. Acted upon by heat loving thermophilic organisms. Optimum temperature is about 540C – digestion within 10

– 15 days.ii. Zone of mesophilic digestion

Moderate temperature zone – 250C to 400C. Acted upon by mesophilic organisms. Optimum temperature is about 290C – digestion in 30

days.


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FACTORS AFFECTING SLUDGE DIGESTION PROCESSpH value

Should not lower below 6.5Optimum value – 7.2 to 7.4 (in the final stage)Acidity increases due tooOverdosing of raw sludgeoOver withdrawal of digested sludgeoSudden admission of industrial wastesRemedy: Add hydrated lime – 2.3 to 4.5 kg per 1000

persons to the raw sludge.Raw sludge of about 3- 5% of the weight of the digested

sludge should only be added daily.


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FACTORS AFFECTING SLUDGE DIGESTION PROCESSSeeding with digested sludge

Seed with digested sludge from another tankSpeeds up the digestion process

Mixing and stirring of raw sludge with digested sludgeProper agitation methods mixes raw and digested sludge to

form homogeneous mass of sludge.Bacterial enzymes present in digested sludge gets

thoroughly mixed up – helps in better decomposition.


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SLUDGE DIGESTION TANK OR DIGESTORS.


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SLUDGE DIGESTION TANK OR DIGESTORS.Design considerationsCylindrical in shape, circular in plan – dia 3 to 12mSlope of bottom hopper floor – 1:1 to 1:3Depth of digestion tank – 6mExcept in large plants not more than 2 units are

provided.The capacity provided ranges from 21 to 61 lpcd.


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SLUDGE DIGESTION TANK OR DIGESTORS.Design considerationsIf the progress of sludge digestion is assumed to be

linear, then capacity of digestor (in cu.m) is

Where,V1 = raw sludge added per day,

cu.m/dV2 = equivalent digested sludge

produced per day, on completion of digestion V2 = (V1/3)

t = digestion periods in days


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SLUDGE DIGESTION TANK OR DIGESTORS.Design considerationsWhen the daily digested sludge could not be removed,

even though digestion gets completed , then consider separate capacity . (Monsoon Storage)

Thus total capacity ,


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SLUDGE DIGESTION TANK OR DIGESTORS.Design considerationsRealistic case: When the change during digestion is

assumed to be parabolic then the average volume of digesting sludge

Then, total capacity without monsoon storage

And total capacity with monsoon storage


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SLUDGE DIGESTION TANK OR DIGESTORS.Design problemDesign a digestion tank for the primary sludge with the

help of following data:i. Average flow = 20 Mldii. Total suspended solids in raw sewage = 300mg/Liii. Moisture content of digested sludge = 85%Assume any other suitable data required.


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SLUDGE DIGESTION TANK OR DIGESTORS.Solution Assumptions made% of solids removed in primary settling tank =65%Moisture content of fresh sludge =95%Sp.gravity of wet sludge =1.02Digestion period, t =30daysDepth of digestor =6m


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SLUDGE DIGESTION TANK OR DIGESTORS. Solution Average flow =20 Mld = 20 x 106 L/dayTSS in raw sewage =300mg/L = 300 x 10-6 kg/L∴ Mass of TSS in 20 Mld of sewage per day

=20 x 106 L/day x 300 x 10-6 kg/L=6000 kg/day

Mass of solids removed in primary settling tank = mass of solids forming the raw sludge= 65% x 6000 kg/day = 3900 kg/L

∵ Moisture content of fresh sludge = 95%Mass of wet sludge made by 3900kg/L of dry solids

= 78000 kg


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SLUDGE DIGESTION TANK OR DIGESTORS. Solution Density of wet sludge = sp.gr x density of water

= 1.02 x 1000kg/m3

= 1020 kg/m3

Volume of raw sludge, V1 = (mass / density)=(78000kg/d) / (1020kg/m3)=76.47 m3/d

Volume of digested sludge,V2

= 25.49m3/d


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SLUDGE DIGESTION TANK OR DIGESTORS.Solution Capacity of digestor ,

= 1274.5 cu.m ~ 1275 cu.m ∵ Depth of tank = 6mCross sectional area of the tank = (1275 cu.m / 6m)

= 212.5 sq.mDia of tank

=16.5m


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SLUDGE DIGESTION TANK OR DIGESTORS.Solution Provide a cylindrical digestion tank of 6m deep and

16.5m diameter, with an additional hoppered bottom of 1:1 slope for collection of digested sludge.

Sketch the section with necessary dimensions.

Design problemDesign a sludge digestion tank for 40,000 people. The

sludge content per capita per day is 0.068kg. The moisture of the sludge is 94%. The sp. gravity of the wet sludge is 1.02 and 3.5% of the digestor volume is daily filled with fresh sludge, which is mixed with the digested sludge.


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The digested sludge from the digestor contains lots of water which should be removed by dewatering and then disposed off.

Sludge drying bedsMechanical methods

Dewatering, drying and disposal of sludge by sludge drying beds.

Drying of the digested sludge on open beds of land – sludge drying beds.

Suitable for hot countries – India.

DEWATERING OF SLUDGE

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Sludge drying beds→15m x 30m in plan→45 to 60 cm deep→Area – 0.05 to 0.2sq.m per

capita.→Bottom layer –thick graded

layers of gravel or crushed stone – size varying from 15cm at bottom to 1.25cm at top.

→Top layer – 10 to 15cm thick coarse sand layer.

→Open jointed under drained pipe – 15 cm dia – 5 to 7 cm spacing – below gravel layer in valleys


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Sludge drying beds→Surrounded by brick wall

rising about 1m above the sand surface.

→Distribution troughs - Opening:15 cm x 20 cm – spacing: 2m

→Sewage sludge from digestion tank is spread over the top to a depth of 20 to 30cm.

→2 weeks - 2 months to dry the sludge.


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Mechanical methods of dewatering sludgeBy vacuum filtration or by high speed centrifuges.50% moisture removedHigh speed centrifuges requires only a small area.Before vacuum filtration the sludge is washed by the process of

elutriation to remove the organic and fatty acids.


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Disposal by dumping into sea. Disposal by burial into trenches. Disposal by incineration.

Multiple hearth furnace Fluid bed furnace Flash type furnace Infra red (Electric or radiant heat) furnace.

The process of reducing the moisture content in the sludge. Using sludge thickener or concentrator unit. Three types of thickening units.

Gravity thickeners Flotation thickeners Centrifugal thickeners

SLUDGE DISPOSAL

SLUDGE THICKENING

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Completely covered, horizontal continuous flow type primary sedimentation tank.

Detention period =12 to 36 hrsHas an extra provision for digestion of the settled sludge.Works under the principle of anaerobic decomposition.Removes 60-70% of the dissolved matter in it.

SEPTIC TANK

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SEPTIC TANK

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SEPTIC TANK

DESIGN CONSIDERATIONS

A septic tank should be capable

of storing the sewage flow

during the detention period and

an additional volume of sludge

for 6 months to 3 years,

depending upon periodicity of

cleaning.

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DESIGN CONSIDERATIONS

Water closets only, then sewage

flow = 40-70lpcd

8 to 10 persons: 1400L

(min.cap. of septic tank)

Water closets and sullage, then

sewage flow = 90-150lpcd

8 t0 10 persons: 2250L

(min.cap. of septic tank)

Rate of accumulation of sludge

= 30 L/person/year

Freeboard : 0.3 to 0.5m

SEPTIC TANK

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DESIGN

CONSIDERATIONS

Inlet and outlet baffles

Extended upto 20-22cm above

top sewage line

Atleast 7.5cm below top

covering slab

Inlet penetrating level: 30cm

below the top sewage line

Outlet penetrating level: 40%

of the depth of sewage.

SEPTIC TANK

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DESIGN

CONSIDERATIONS

Detention period:

commonly adopted is 24 hrs.

Length to width ratio

L= 2B to 3B

B not less than 90cm

Depth = 1.2 to 1.8m

SEPTIC TANK

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DESIGN PROBLEM

Design the dimensions of a septic tank for a small colony of 150

persons provided with an assured water supply from the municipal

head-works at a rate of 120lpcd. Assume any other data needed.

Assumptions made% of water supplied that becomes sewage = 80%Detention time = 24hrsRate of deposited sludge = 30L/capita/dayPeriod of cleaning = 1 yrDepth = 1.2 to 1.8m L = 2B to 3B

SEPTIC TANK

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DESIGN PROBLEM

Solution

The quantity of water supplied = per capita rate x population

= 120 x 150L/day = 18000L/day

The quantity of sewage produced = 80% x 18000 = 14400L/day

The quantity of sewage produced during the detention period

= the capacity of the septic tank

= (14400 L/day) x detention time

= (14400 L/24hrs) x 24hrs

= 14400L

SEPTIC TANK

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DESIGN PROBLEM

Solution

The volume of sludge deposited

=rate of sludge deposition x no.of persons x period of

cleaning

= 30L/capita/year x 150capita x 1yr

= 4500 L∴ Total required capacity of the tank

= volume of sewage + volume of sludge

= 14400 + 4500 = 18900L = 18.9 cu.m

SEPTIC TANK

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DESIGN PROBLEMSolution

Take depth of tank = 1.5m

The surface area of tank = (18.9/1.5)sq.m = 12.6 sq.m

Take L:B = 3:1L x B = 12.6 sq.m→ B = 2.05m ~ 2.1m→L = (12.6sq.m/2.1m) ~ 6m

Provide free board = 0.3m

→Depth = 1.5 + 0.3 =1.8m∴ Dimension of septic tank = 6m x 2.1m x 1.8m

SEPTIC TANK

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DISPOSAL OF EFFLUENT FROM THE SEPTIC TANK.

Effluent contains 200 to 250mg/l of putrescible organic matter.

BOD is high – 100 to 200mg/L

Three methods of disposal

1. Soil absorption system

2. Biological filters

3. Upflow anaerobic filters.

SEPTIC TANK

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Soil absorption system

Disposal of effluent on land.

Only adopted when sufficient land is available.

The soil must be sufficiently porous.

Percolation rate not more than 60minutes per cm.

Types - seepage pit or soakpit

- dispersion trench

SEPTIC TANK

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1. Soil absorption system

Seepage pit or soak pit

Circular covered pit.

Effluent allowed to be soaked or absorbed into the surrounding soil

Either empty or filled with stone or brick aggregates.

Effluent pipe at a depth of 0.9m from top – anti mosquito measure.

Percolation rate not more than 30min per cm

SEPTIC TANK

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DISPOSAL OF EFFLUENT FROM THE SEPTIC TANK.1. Soil absorption system

Seepage pit or soak pit: empty and lined

SEPTIC TANK

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DISPOSAL OF EFFLUENT FROM THE SEPTIC TANK.2. Soil absorption system Seepage pit or soak pit: unlined and filled with stone or

brick

SEPTIC TANK

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DESIGN PROBLEM

a. Design a septic tank for the following data:No. of people = 100Sewage/capita/day = 120LDe-sludging period = 1 yearLength:width = 4:1b. What would be the size of soak well if the effluent from this septic

tank is to be discharged in it. Assume percolation rate through the soak well to be 1250 l/cu.m/day.

SEPTIC TANK

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DESIGN PROBLEM

Assumptions made

For septic tankDetention time = 24hrsRate of deposited sludge = 30L/capita/dayPeriod of cleaning = 1 yrDepth = 1.2 to 1.8m

SEPTIC TANK

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Solution

Design of septic tank : 6m x 1.5m x 1.8m

Design of soak pit

Sewage outflow =12000L/d

Percolation rate =1250L/cu.m/day

Volume of filtering media for the soak well

= outflow/ Percolation rate = 9.6cu.m

Take depth = 2m

Area of soakwell = 4.8 sqm dia = 2.5m

SEPTIC TANK

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Dispersion trench

Effluent from septic tank to a masonry chamber - distribution box.

From distribution box effluent is uniformly distributed through an

underground network of open jointed pipes into absorption trenches

– dispersion trenches.

Dispersion trenches – filled with gravel and well graded aggregate.

Not used where plants with fibres roots are grown – causes

blockage.

Percolation not more than 60minutes per cm.

SEPTIC TANK

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SEPTIC TANK1. Soil absorption system

Dispersion trench

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SEPTIC TANK1. Soil absorption system

Dispersion trench

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Dispersion trench

Minimum absorption area pits or trenches can be calculated on the

basis of maximum allowable rate of effluent application (q in

l/m2/day) given as q = 130√tWhere, t = standard percolation rate in minutes.According to IS:2470 (Part II) 1985 q = 204/√tShall not be installed closer than 18m from any source of drinking

water.

Not closer than 6m from any habitable building.

SEPTIC TANK

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DISPOSAL OF EFFLUENT FROM THE SEPTIC TANK.Design problemEstimate the size of a septic tank (length to width ratio = 2.25,

liquid depth = 2m with 300mm freeboard), desludging intervals in years and the total trench area (sq.m) of the percolation field, for a small colony of 300 people. Assume water supply of 100 lpcd, waste water flow at 80% of water consumption, sludge production of 0.04 cu.m per capita per year, and the retention time of 3 days at start up. Desludging is done when the tank is one-third full of sludge. A percolation test indicated an allowable hydraulic loading of 100L per sq.m per day.

SEPTIC TANK

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DISPOSAL OF EFFLUENT FROM THE SEPTIC TANK.Given:

L/B = 2.25

Dw = 2mFree-board = 0.3mPopulation = 300Water supply = 100 lpcdWaste flow = 80% of water suppliedSludge production = 0.04m3/c/yearHydraulic loading = 100L/m2/day

SEPTIC TANK

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DISPOSAL OF EFFLUENT FROM THE SEPTIC TANK.Solution Water supply to the colony =100 lpcd x 300persons =30000 L/dSewage produced per day = 80% x 30000 L/d =24000L/dSewage produced in 3 days of retention period

=3d x 24000L/d = 72000L=72m3

Desludging is done when the tank is filled upto of its capacity ⅓(C).Hence, sludge volume collected is C/3.∴ Capacity (C)= max. sewage volume retained + sludge volume retained⇒C = 72m3+ C/3⇒C = 108m3

∵C = L x B x Dw

108m3 = (2.25B) x B x 2m

SEPTIC TANK

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DISPOSAL OF EFFLUENT FROM THE SEPTIC TANK.Solution ⇒ B = 4.9m⇒ L = 2.25 x 4.9m = 11.1m⇒ D = Dw + freeboard = 2.3m

∴ Tank size = 11.1m x 4.9m x 2.3mSludge volume removed while desludging

= C/3= (108m3/3) = 36m3

Sludge producd per year =0.04m3/capita/year x 300persons=12m3/year

∴ 36m3 of sludge will be produced in = (36m3)/(12m3/year) = 3years

∴ Desludging period = 3years

SEPTIC TANK

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DISPOSAL OF EFFLUENT FROM THE SEPTIC TANK.Solution

∴ Trench area required = 240m2.Design problemDesign the absorption field system for the disposal of septic tank

effluent for a population of 100 persons with sewage flow rate of 135lpcd. The percolation rate for the percolation test carried out at the site of the absorption field may be taken as 3 minutes.

SEPTIC TANK

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DISPOSAL OF EFFLUENT FROM THE SEPTIC TANK.2. Biological filters Used where soil has high percolation rate (> 60min/cm) Used in water logged areas Septic tank effluent treated further by coating with organic

medium. Much of the polluting matter gets oxidized. Requires ample ventilation and an efficient system of under

drainage system.3. Upflow anaerobic filters Used where soil has high percolation rate (> 60min/cm) Used in water logged areas

SEPTIC TANK

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DISPOSAL OF EFFLUENT FROM THE SEPTIC TANK.SEPTIC TANK

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ADVANTAGES & DISADVANTAGES OF SEPTIC TANK.AdvantagesEasily constructedNo skilled supervision requiredNo moving parts so no maintenance problemsReasonable cost compared to the sanitation provided.Considerably reduce the SS & BOD Sludge volume to be disposed off is quite less compared to normal

sedimentation tank. Volume – about 60%, Weight – about 30%.Effluent can be disposed off without much trouble.Best for isolated rural areas, isolated hospitals, isolated buildings

etc.

SEPTIC TANK

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ADVANTAGES & DISADVANTAGES OF SEPTIC TANK.Disadvantages If not functioning properly, effluents will become very foul.Size increases in case of serving many people.Leakage from top cover may cause bad smell and environmental

pollution.Periodical cleaning, removal and disposal of sludge remains a

tedious problem.Working of a septic tank is unpredictable and non uniform.

SEPTIC TANK

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Designed by Mr. Karl Imhoff Improvement over septic tank.Incoming sewage and sludge produced not allowed to get mixed up.Effluent not allowed to carry with it a large amount of organic load.Also known as two storey digestion tanks.Sludge removed is dried up and then disposed off.

IMHOFF TANK

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IMHOFF TANK

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CONSTRUCTION DETAILS &WORKINGDouble chamber rectangular tank.Upper chamber: sedimentation chamber or flowing through chamber.

›Sewage flow at a very low velocity – solids settle down to the bottom of sedimentation chamber.

›Bottom side slope: 1.25V:1H

›Entrance slot at the lowest point of this chamber.Lower chamber: digestion chamber.

›Sludge gets digested due to anaerobic decomposition.

›Divided into a number (3 to 4) of inter connected compartments

IMHOFF TANK

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CONSTRUCTION DETAILS &WORKINGLower chamber: digestion chamber.

›Hoppered bottom: 1:1 side slopes – sludge concentrates here.

›Digested sludge removed, with flow under hydrostatic pressure of 1.2 to 1.8m, periodically from the hoppered bottom through the cast-iron desludging pipes.

›Only completely digested sludge removed, left out sludge used for seeding.Gas vent or scum chamber:

›Provided above the digestion chamber and along side the sedimentation chamber.

IMHOFF TANK

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CONSTRUCTION DETAILS &WORKINGNeutral zone:

›Provided to prevent the particles of sludge or scum from entering into the sedimentation chamber from the digestion chamber.

›Scum and sludge maintained atleast 45cm above and below the slots, respectively.

IMHOFF TANK

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IMHOFF TANK

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DESIGN CONSIDERATIONSSEDIMENTATION CHAMBERRectangular shapeDetention period = 2 to 4 hrs, usually 2hrFlow through velocity ≯ 0.3m/min Surface loading ≯ 30000L/m2 of plan area/day. Length of tank ≯ 30mLength to width ratio = 3 to 5Depth = 3 to 3.5mFreeboard = 0.45m

IMHOFF TANK

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DESIGN CONSIDERATIONSDIGESTION CHAMBERCapacity = 57 litres per capita. But, in warmer climate capacity = 30 to 40 litres per capita. GAS VENT OR SCUM CHAMBERSurface area = 25 to 30% of the area of horizontal projection of the top of the digestion chamber.Width of a vent ≥ 60cm

IMHOFF TANK

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ADVANTAGES AND DISADVANTAGES OF IMHOFF TANK

Advantages Combines the advantages of septic tank and sedimentation

tank.Do not require skilled supervision while operating.60 to 65% removal of solids.30 to 40% removal of BOD.

IMHOFF TANK

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ADVANTAGES AND DISADVANTAGES OF IMHOFF TANK

Disadvantages depth of tank is more – makes it uneconomical.May give out offensive odours, when improperly operated.Unsuitable where sewage is highly acidic.Have a tendency to foam or boil – causes scum to go up –

force the sludge particle to enter the sedimentation tank.No control over the operation – unsuitable for large

treatment plants – useful only for small cities and institutions.

Mostly they have become obsolete these days.

IMHOFF TANK

sludge treatment and disposal 1

Engineering