Domestic septic tanks are underground concrete structures that hold wastewater from homes long enough for solids to settle and be broken down by bacteria. They consist of compartments to prevent sludge and scum from exiting, and effluent flows into a drain field. Septic tanks are designed based on detention time (12-36 hours typically) or rational methods considering settling, digestion and sludge storage zones. Effluent can be disposed of via soak pits, trenches, or irrigation. Septic tanks require periodic cleaning to remove accumulated sludge but provide inexpensive wastewater treatment for small communities if properly maintained.

1. Domestic Septic Tanks
Ar. Niharika
Choudhary
M.Arch(Const.Mg
mt.)

2. What Is A Septic Tank??
• Septic Tank is in the form of Concrete box
which has their unique designs.
• The size depends on the number of bedroom
in the house
Aim Of Septic Tank “Septic” refers to the an
aerobical bacteria which digest the solids.
• It helps in removal of waste generates from
bathroom, kitchen, laundry etc.

3. TREAT SEWAGE SOTHAT IT IS MADEHARMLESS.

4. 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,
fibreglass 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).
• Compartments and a T-shaped outlet prevent the
sludge and scum from leaving the tank and
travelling into the drain field area.
• The liquid wastewater (effluent) then exits the
tank into the drain field.
• Finally, the wastewater percolates into the soil,
naturally removing harmful coli form bacteria,
viruses, and nutrients.

5. LAYOUTOFSEPTICTANKTREATMENT

6. 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
• 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

7. 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.
Design criteria(Septic Tank):
• Quantity of sludge -30 l/capita/ years -
Sludge storage
• Depth - 1.2 – 1.8 m
• Length: Breadth - 2-4 times
• Outlet ~ in let level difference = 5-7.5 or up to 15
cm
• Detention time = 24 hours
• Free board: 30cm

12. DESIGN OF SEPTIC TANK
A. DETENTION PERIOD METHOD
B. 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.

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

16. 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.

17. 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

18. Disposal Of Effluent
Ways Of Disposal Of Effluent Are:-
.Soak Pit.
.Dispersion trenches
.Sub-Surface Irrigation
.Leaching Cess-Pools.
.Treated In Trickling Filters
.Recycling.
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

19. • 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
• Total trench/soak pit area required can
be calculated by
• A = (Q/q)
• Where,
• Q = total flow = Population x Sewage
flow rate.

22. Advantages
• Can be built and repaired with locally
available materials.
• Small land area required.
• Power conservative.
• Can be built and maintained with locally
available materials.
• Simple technique for all users.
• Disadvantages
• Pre-treatment is required to prevent clogging,
although eventual clogging is inevitable.
• Negatively affects soil and groundwater
properties.

23. Dispersion trenches

26. • 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

27. Cleaning of septic tank