A septic tank is a buried, water-tight unit designed for sludge digestion and sedimentation, appropriate for populations up to 300. It allows solids to settle, while scum floats, and the effluent is later released to percolate into the soil, removing harmful contaminants. The design and maintenance of septic tanks have advantages like low construction costs and easy maintenance, but also face challenges such as potential environmental pollution and the need for regular sludge removal.

1. SEPTIC TANK
L-33
Unit-III
Environmental Engineering – II

2. Working of Septic tank
 Septic tank is sludge digestion cum
sedimentation tank with large detention time.
 It is suitable for population up to 300.
 The septic tank is a buried, water-tight container
usually made of concrete, brick masonry,
fiberglass or polyethylene.
 Its job is to hold the wastewater long enough to
allow solids to settle down to the bottom (forming
sludge), while the oil and grease floats to the top
(as scum).

3.  Compartments and a T-shaped outlet prevent
the sludge and scum from leaving the tank and
traveling into the drainfield area.
 The liquid wastewater (effluent) then exits the
tank into the drainfield.
 Finally, the wastewater percolates into the soil,
naturally removing harmful coliform bacteria,
viruses, and nutrients.

4. LAYOUT OF
SEPTIC
TANK
TREATMENT

5.  Following reaction occurs:
 Organic materials +Nutrients = CH4 +
CO2 +NH3 + H2S + Biomass
 Sulfates, under the anaerobic conditions of septic
tanks, are reduced to hydrogen sulfide (H2S), a
pungent and toxic gas.
 Nitrates and organic nitrogen compounds are reduced
to ammonia (NH3).
 Because of the anaerobic conditions, fermentation
processes take place, which ultimately generate
carbon dioxide and methane.
 Methane, a potent greenhouse gas. Waste water also
contain phosphate and phosphate.

10. DESIGN OF SEPTIC TANK
A. DETENTION PERIOD METHOD
B. RATIONAL METHOD

11. 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.

12. 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.

13. B. 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)

14. Estimated peak discharge
No. of users Peak discharge
(lpm)
100 240
150 360
200 480
300 720

17. Advantages
1. Ease in construction.
2. No maintenance problem(except periodical
cleaning).
3. Cost of construction is less.
4. Effluent can be discharged without any
trouble.
5. It is suitable for isolated community.
6. Very little attention and skilled attendance is
needed.

18. Disadvantages
1. If ST is not properly functioning, effluent will
be very foul & dark.
2. Leakage of gases from top cover of ST may
cause bed smells & environmental pollution.
3. Periodical cleaning, removal and disposal of
sludge remains a tedious problem.
4. Unpredictable functioning.
5. Size required is large for larger communities.
6. Effluent contains high BOD.
7. Problem of mosquito nuisance.
8. Problem of ground water contamination

19. Disposal Of Effluent
 Ways Of Disposal Of Effluent Are:-
1. Soak Pit.
2. Dispersion trenches
3. Sub-Surface Irrigation
4. Leaching Cess-Pools.
5. Treated In Trickling Filters
6. Recycling.

20. Soak Pit
 Soak Pit does not provide adequate treatment
for raw wastewater and the pit will clog quickly.
 A Soak Pit should be used for discharging pre
settled black water or grey water .
 Soak pits are appropriate for rural and
suburban settlements.
 They depend on soil with a sufficient absorptive
capacity.
 Soak pits are used for soils where percolation
rate is below 25 min/cm.

21.  They are not appropriate for areas that are
prone to flooding or have high groundwater
tables.
 The total subsurface soil area required for
soak pits and dispersion trenches is given by
q = 130/(√t)
Where,
q = Max rate of effluent application in Lit/d/m2
of leaching surface.
T= Standard percolation rate for the soil in
min/cm

22.  Total trench/soak pit area required can be
calculated by
A = (Q/q)
Where,
Q = total flow = Population x Sewage flow
rate.

28. Advantages
1. Can be built and repaired with locally
available materials.
2. Small land area required.
3. Power conservative.
4. Can be built and maintained with locally
available materials.
5. Simple technique for all users.

29. Disadvantages
1. Pre-treatment is required to prevent
clogging, although eventual clogging is
inevitable.
2. Negatively affects soil and groundwater
properties.

30. Dispersion trenches

33.  Trenches are 0.5 to 1 m deep and 1 m wide.
 Maximum length of trench is 30 m.
 Spacing between trenches is 2m.
 One distribution box is provided for every 3 to 4
trenches

35. Cleaning of septic tank

37. Objective questions
1. Septic tank is ________ cum _______
tank.
2. Detention period adopted in septic
tanks is __ to __ hrs.
3. Septic tank can be used for population
up to __ users.

38. Theory questions
Q1] Explain working of septic tank with
neat sketch.
Q2] write short note on
1. Design of septic tank
2. Septic tank Effluent disposal