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)