SEPTIC TANK

2. What is a Septic Tank?
A Septic Tank is a below ground watertight box
(concrete, plastic or fiberglass), often about 9
x 5 x 6 feet. It separates the liquids and solids,
provides digestion of some organics (mainly by
bacteria which live without oxygen) and
storage. It discharges partially filtered and
clarified effluent to the drainfield for final
treatment.

4. Who has a Septic Tank System?
If a building (with plumbing facilities) is
not on a city sewer system, then in most
cases it has a septic tank system (also
known as an On-site Wastewater
Disposal System, or Subsurface
Wastewater Disposal System) to dispose
of the wastewater

5. What is an On-site Wastewater System?
An On-site Wastewater Disposal System collects,
treats and disposes of all the wastewater produced
in a building. A conventional system includes a
Septic Tank, a Distribution Device and a Drainfield

6. Typical Septic System

7. SEPTIC TANK
 It is a primary horizontal continuous flow
type of sedimentation tank having extra
provision to digestion of settled sludge .
 Properties
 Detention time- 12 to 36 hr.
 It removes 60% to 70% of dissolve
matters.
 Cleaning period-6 month to 3 years.

8. DESIGN OF SEPTIC TANK
 DETENTION PERIOD METHOD
 RATIONAL METHOD
A. Detention period method
1. Detention period (t) is assumed in between 12 to 36
hrs (common value = 24 hrs).
2. Assuming rate of sewage flow (lpcd) [90 to 150 lpcd]
and knowing population to be served; total discharge
(Q) is determined.
3. Volume is calculated by V=Q. t
4. Depth is assumed 1 to 2 m.
5. Surface area is calculated by As = V / depth.
6. Assuming L/B ratio in between 2 to 4 length
and breadth can be fixed.

9. Rational Method
1. Settling zone is kept min 30 cm deep.
2. Digestion zone is provided as 0.032
m3/capita.
3. Sludge storage zone of 7.30 m3/100
persons for cleaning interval of 1 year is
provided.
4. Surface area of 0.92 m2/10 lpm of flow is
provided.
5. Free board of 0.3 m is provided. ( depth 1 to
2 m, L/B ratio 2 to 4).

10. Anaerobic Digestion
ORGANIC MATTER GASES + HUMUS
CO2
CH4
H2S
NH3

11. Advantage Disadvantage
1. It can be easily
constructed.
2. No maintenance
problem.
3. It excellently remove
BOD.
4. Very less amount of
solids are produced.
5. Low cost
1. Its size should be very
large to serve many
people.
2. Smell problem
3. It needs periodic
cleaning.