The document discusses septic tanks, which are underground structures for processing wastewater. Septic tanks work through sedimentation and anaerobic sludge digestion over 12-36 hours. The wastewater separates into scum, sludge, and effluent. Effluent exits into absorption fields while sludge digests. Septic tanks are suitable for up to 300 people. Design considerations include capacity, dimensions, and detention time. Effluent can be dispersed into soak pits or trenches filled with gravel.

1. SHEMEERA K.H
ASSISTANT PROFESSOR
DEPARTMENT OF CIVIL ENGINEERING
IIIT NUZVID
Low cost sanitation: Septic tanks
Low cost sanitation: Septic tanks by Shemeera is licensed under a Creative Commons Attribution- ShareAlike 4.0
International License.

2. SEPTIC TANK

4.  Process:
 Horizontal continuous flow sedimentation tank
(with longer detention time)
 12-36 hours detention
 Closed tank – Anaerobic decomposition of settled
sludge
 Thus Two processes-Sedimentation and
Anaerobic Sludge digestion
 It is suitable for population up to 300.

5. Purposes :
 Sedimentation
 Digestion tank
 Storage of digested sludge

7. WORKING
 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).
 Compartments and a T-shaped outlet prevent the
sludge and scum from leaving the tank
 The liquid wastewater (effluent) then exits the
tank into the drainfield.
 From there it percolates into the soil, naturally
removing harmful bacteria, viruses, and nutrients.

9.  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.

10. 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 for removal of sludge
 Inlet and outlet, Tee’s will be provided
 Sometimes baffles to reduce disturbances

11. Construction details

12. DESIGN OF SEPTIC TANK
Design criteria:
 Capacity of septic tank: capacity for sewage+
capacity for sludge
 Quantity of sewage :40-70litres/capita/day (if
only water closets are connected to septic
tank),90-150litres/capita/day if sullage is also
discharged into septic tank.
 Quantity of sludge accumulated -30 l/capita/
years
 Depth - 1.2 – 1.8 m
 Length to Breadth ratio: 2-3

13.  Min width – 90 cm
 Outlet ~ in let level difference = 5-7.5 or up
to 15 cm
 Detention time = 24 hours
 Free board: 0.3-0.5m

14. DESIGN STEPS
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 for sewage storage ,V1 is calculated by
V=Q. t
4. Assuming rate of accumulation of sludge 30 l/capita/
years and period of cleaning as 1 year, total sludge
deposited,V2 is determined
volume of sludge deposited in liters =30 l/capita/
years *population*1
5. Total Capacity of septic tank: V1+V2
6. Depth is assumed 1 to 2 m

15. 7. Surface area is calculated by
As = V / depth
8. Assuming L/B ratio in between 2 to 4, length and
breadth can be fixed.

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 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.Size required is large for larger communities.
5.Effluent contains high BOD.
6.Problem of ground water contamination

18. Disposal Of Effluent
Effluent from septic tank can be disposed by soil
absorption systems and anaerobic filters.
The soil absorption system may be of the
following types:
1. Soak pit/ Cess pool
2.Under drain /absorption /dispersion trenches

20. SOAK PIT/SEEPAGE PIT

21. SOAK PIT/SEEPAGE PIT
 A closed circular or rectangular pit through which
effluent will be soaked or absorbed into the
surrounding soil.
 Soak pit may be either be filled with stone
aggregates or may be kept empty.
 Soak pits are used for soils where percolation
rate is below 30 min/cm.
 They are not appropriate for areas that are prone
to flooding or have high groundwater tables

22. SOAK PIT

23. SOAK PIT

24. DISPERSION TRENCHES

25. DISPERSION TRENCHES
 Effluent is allowed to enter into a masonry
chamber(distribution box) from where it is
uniformly distributed through an underground
perforated pipes into absorption trenches which
are filled with gravel and well graded aggregate.
 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
 Suspended organic matter will be absorbed in the
trenches where as clear water will seep down to
the water table.

26. DISPERSION TRENCHES

27.  They are not recommended in areas where
fibrous roots of trees or vegetation are likely to
penetrate the system and case blockages.
 Dispersion trenches are used for soils where
percolation rate is below 60 min/cm.

28. problem