Septic tank

Low cost sanitation,septic tank:principle,operation,design and applications

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

3. 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.

4. Purposes : Sedimentation Digestion tank Storage of digested sludge

5. 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.

6. 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.

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

8. Construction details

9. 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

10. 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

11. DESIGN STEPS Detention period (t) is assumed in between 12 to 36 hrs (common value = 24 hrs) Assuming rate of sewage flow ( lpcd ) [90 to 150 lpcd ] and knowing population to be served; total discharge (Q) is determined. Volume for sewage storage ,V1 is calculated by V=Q . t 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 Total Capacity of septic tank: V1+V2 Depth is assumed 1 to 2 m

12. 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.

13. 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.

14. 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

15. 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

16. SOAK PIT/SEEPAGE PIT

17. 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

18. SOAK PIT

19. SOAK PIT

20. DISPERSION TRENCHES

21. 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.

22. DISPERSION TRENCHES

23. 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.

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