This document describes various sewage treatment processes including septic tanks, Imhoff tanks, ponds, lagoons and ditches. It provides details on the process, components and design of septic tanks. Septic tanks use sedimentation and anaerobic digestion to treat sewage. The design criteria includes detention time, tank dimensions, sludge storage volume and absorption field sizing based on percolation rates. An example problem demonstrates how to design a septic tank and absorption field for a hostel.

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)