Septic tank is a water tight tank where the sewage undergoes anaerobic decomposition. They are designed only to treat human excreta and urine. The liquid from septic tank should not be allowed in to open drains and it has to be capable of undergoing secondary treatment.

Design aspects- It is designed only to treat common excreta and urine and not the village. Sewer pipes should be at least 100 mm in dimension and radii of the bend pipes should be long with cleaning eyes. The flow direction should not be greater than 45 degrees. They should be constructed in a area open to sky and possibly not closer to the building walls. A detention period of 24 to 40 hours depending on the average flow is allowed. The sedimentation depth should ne minimum 250 to 00 mm. the raw sewage entering should not disturb he settled sludge and surface scum. Outlet is in the form of dip fixed inside the tank. A minimum free board of 500 mm is given.

Sludge Removal- The digested sludge is removed once or twice in water. The scum in the first should not be disturbed much while removing the sludge. Care should be taken that the tank is not over loaded.

Biological action in a septic tank - The sewage is retained in the septic tank for a specified period of time. During the period, the suspended solids settle down and under goes anaerobic action. Thus they digested and reduced in volume.

Seepage pit or Soak Pit- The fulfillment from the septic tank carries a portion of suspended and organic matters. A seepage pit is a pit in to which the effluent from the septic is allowed to seep or seek in to surrounding soil. It can be constructed in any shape with a minimum cross sectional dimensions of 90 cm. the pit is usually lined with stone, bricks or concrete blocks with open joints.

Dispersion Trench- Dispersion trench is the trench in which the sewage pipes are laid with open joints. The aggregates surround’s the pipes and the effluent from the septic tank through the open joints and is absorbed in the surrounding soil. They are not suitable where there are trees or any other vegetation due to presence of roots.

Advantages of septic tank:

>Easy distribution

>Not costly

>Effluent can be treated easily

>Maintenance is easy.

>Digested sludge gets reduced in volume considerably

>Does not require skilled supervision

Disadvantages of septic tank

>Escape of foul gas may pollute the air.

>Removal of digested sludge is laborious.






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