Environmental Engg.

Lesson-22 Design of septic tank

Septic tank is just like a plain sedimentation tank.  But in septic tank, the bio-chemical reactions by anaerobic bacteria take place as in case of sludge digestion tanks.  During the detention period, sewage is purified and the effluent is taken to soak pits for disposal.  Bad smells occur during the digestion period of sludge and hence, the septic tanks are provided with cover at top.

Constructional features.

1. Septic tanks should be constructed of materials which are resistant to corrosion.  The tanks are constructed watertight.

2. Construction of septic tank should be such that direct currents are not established between the inlet and outlet.  This is achieved by using submerged pipe tees or by baffle walls near the inlet and outlet ends.  Scum boards may be provided near the inlet and outlet ends to prevent the escape the scum. Level of outlet is about 15 cm lower than that of inlet level

3. Septic tank should be properly ventilated by the provision of air vent pipes

4. Top cover of septic tank is usually made of R.C.C. and a manhole is provided in R.C.C. slab for the purpose of inspection and cleaning of tank.  If necessary, cast-iron steps may be provided in the tank to facilitate descent in the tank.

5. Sludge is allowed to be accumulated at the bottom of tank and it is removed at intervals either by manual labour or by pumping.  For large septic tanks, sludge removal pipe is provided and it leads the sludge to the nearby sump from where it is periodically pumped and removed.

6. Tank should be filled with water at the time of putting a into working condition.  Effluent of tank should be properly disposed off.  The direct discharge of such effluent into natural waters should be discouraged.

7. Septic tanks may be constructed in series to act like two-stage sludge digestion tanks.  But single-stage septic tanks are very popular

8. A septic tank thus combines the functions of a sedimentation tank, a sludge digestion tank and a sludge storage tank.

9. Accumulation of sludge at the bottom of tank decreases its storing capacity and hence, the septic tanks should be cleaned every 6 to 12 months.  But this period should not preferably exceed 3 years in any case.

Design aspects

Capacity:  Volume of septic tank is decided by taking into consideration the quantity of flow and the detention period.  It can also be designed on per capita basis which varies from 60 to 110 litres per person to be served by the septic tank.  The space for sludge is kept usually at the rate of 15 to 45 litres per capita per year.

Detention period:  The detention period varies from 12 to 72 hours, common being 24 hours

Freeboard:  This should be about 40 cm to 60 cm

Shape:  Septic tanks are generally rectangular in shape.  The ratio of length to width is about 2 to 4

Advantages:

• It does not practically require any special attention or skilled supervision.  It can be constructed easily

• Cost is reasonable as compared to the advantages it offers

• Performance of a properly constructed septic tank is very good.  It can remove about 90 per cent of B.O.D. and about 80 per cent of suspended solids

• There is absence of any moving parts

• There is reduction in the volume of sludge.  As compared to sludge of plain sedimentation tank, it is about 60 per cent less in volume and about 30 per cent less in weight

• Sludge, effluent and scum obtained from the septic tanks can be disposed off easily without causing serious nuisance

Disadvantages

• If the tank is not properly functioning, the effluent is dark and foul-smelling.  It is even worse than the influent

• It requires excessive large size for serving more persons

• Leakage of gases through the top  of septic tanks leads to air pollution

• Occasional removal of sludge adds to its maintenance cost and it is very tedious job

• Working of septic tank is unpredictable and non-uniform

• Use

• Septic tanks are at present are not generally recommended for treating sewage on a large scale.  They however are still useful for isolated structures or localities where drainage connection to municipal sewers is not possible.