OTHER TREATMENT PROCESSES

Other processes include:Septic tankImhoff tank PondsLagoonsDitches

TREAT SEWAGE SO THAT I T I S MADE HARMLESS.

SEPTIC TANKProcess:

Horizontal continuous flow bed tank (with longer detention time)

Extended sedimentation tank 12-36 hours detentionClosed tank – Anaerobic

decomposition of settled sludgeThree purposes

SedimentationDigestion tankStorage of digested sludge

Digestion - Evolution of gases – gas ventCleaning/ Removing digested sludge – once in a

year (6 months to 2 years) Thus Two processes-Sedimentation and

Anaerobic Sludge digestion

Construction details: Long, continuous flow tank of RCC and sides

water proof.Facility for gas vent through vent pipe.Bottom (slope) to facilitate pumping of sludge.Manhole with for removal of sludgeInlet and outlet, Tee’s will be providedSometimes baffles to reduce disturbances.

Septage:Effluent from septic tank disposalSoak pit/ Cess pool

A closed circular or rectangular pit through which effluent will be soaked or absorbed into the surrounding soil

Two- Filled or Empty typeUnder drain /absorption /dispersion trenches.

3-4 distributions@2m spacing, L shall not be more than30cm

Design criteria(Septic Tank): Quantity of sludge -30 l/capita/ years -

Sludge storage Depth - 1.2 – 1.8 mLength: Breadth - 2-4 timesMin width - 120cmOutlet ~ in let level difference = 5-7.5 or up

to 15 cmDetention time = 24 hoursFree board: 30cm

Design criteria for absorption system:Calculation of Total subsurface soil area

required for soak pits/ dispersion trenchesLq = 130/√t

t- standard percolation rate for the soil min/cm

Lq-Maximum rate of effluent application in Lpd/m2 of leaching surface

Total trench area required = Total flow per day/ Lq

COMPONENTS OF A SEPTIC SYSTEM

Septic TankDistribution Box

Leach Line

Problem:Design a septic tank for the hostels of

the Engineering college: Number of Hostels = 3Students / Hostel= 200Water supply = 120 lpcdAssume Detention period 24 hours and

Sludge volume = 30l/cap/year

Solution: Design for one hostel:Waste water = 200x120x80/100 = 19.2 m3

Let us assume that sludge withdrawal once in a year

Volume required for sludge storage for one year = 200x30 = 6000l= 6m3

Capacity of septic tank = 25.2 m3

Assuming a depth of 1.8 m , Area = 25.2/1.8=14 m2

3B2 = 14m2, B2 = 14/3, B = 2.16 say 2.20m

Length = 6.60m

Dimension = 6.6x2.2x2.1m (1.8m+0.3m free board)

26.16m3 > 25.2 m3

Assuming 1.5m depth, Area = 16.8 m2

3B2= 16.8, B= 2.36 say 2.4 m Provide 7.2x2.4x1.8m septic tank

Design of absorption system: Lq = 130/√tSay t=20min/cmLq = 130√20 = 29lpd/m2

Area required = 200x120x0.8/29 = 662m2

L = 30mW=1m

TREAT SEWAGE SO THAT I T I S MADE HARMLESS.

DI SPOSE OF SEWAGE (MAKE I T GO AWAY)