1. OTHER TREATMENT
PROCESSES
Other processes include:
 Septic tank
 Imhoff tank
 Ponds
 Lagoons
 Ditches

2. TREAT SEWAGE SO THAT IT IS MADE HARMLESS.

3. SEPTIC TANK
Process:
 Horizontal continuous flow bed tank (with
longer detention time)
 Extended sedimentation tank
 12-36 hours detention
 Closed tank – Anaerobic decomposition of
settled sludge
 Three purposes
Sedimentation
Digestion tank
Storage of digested sludge

6.  Digestion - Evolution of gases – gas vent
 Cleaning/ 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 sludge
 Inlet and outlet, Tee’s will be provided
 Sometimes baffles to reduce disturbances.

7. Septage:
 Effluent from septic tank disposal
 Soak 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 type
 Under drain /absorption /dispersion trenches.
 3-4 distributions@2m spacing, L shall not be
more than30cm

9. Design criteria(Septic Tank):
 Quantity of sludge -30 l/capita/ years - Sludge
storage
 Depth - 1.2 – 1.8 m
 Length: Breadth - 2-4 times
 Min width - 120cm
 Outlet ~ in let level difference = 5-7.5 or up to 15 cm
 Detention time = 24 hours
 Free board: 30cm

10. Design criteria for absorption system:
 Calculation of Total subsurface soil area required for
soak pits/ dispersion trenches
 Lq = 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

11. COMPONENTS OF A SEPTIC SYSTEM
Distribution Box
Septic Tank
Leach Line

12. Problem:
 Design a septic tank for the hostels of the
Engineering college:
 Number of Hostels =3
 Students / Hostel= 200
 Water supply = 120 lpcd
 Assume Detention period 24 hours and
 Sludge volume = 30l/cap/year

13. 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

14.  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

15. Design of absorption system:
Lq = 130/√t
Say t=20min/cm
Lq = 130√20 = 29lpd/m2
Area required =
200x120x0.8/29 = 662m2
L = 30m
W=1m

16. TREAT SEWAGE SO THAT IT IS MADE HARMLESS.
DISPOSE OF SEWAGE (MAKE IT GO AWAY)