Septic tank
In rural areas, in factories
and in unsewered urban and semi-urban areas where adequate water supply is
available from pipe, well or any other source, septic tank is suitable for
disposal of night soil. Water is required for the flow of the night soil from
latrine to the septic tank and for the functioning of septic tank. Septic tank
is so designed that the sewage is retained in the tank for 24 hours during
which period certain biological decomposition by the action of anaerobic
bacteria takes place which breaks and liquifies the night soil leaving small
quantity of solid which settles in the form of sludge at the bottom of the tank
and clear water flows out of the septic tank. The effluent from the domestic
septic tank is usually disposed by absorption in soil through soakpit or
subsoil drains. In bigger septic tank the effluent from the tank should further
be treated or purified by sprinkling or contact bed filter or by aeration and
then let off into drain.
The effluent of the septic tank may also be discharged into open drain after disinfection by treating with chlorine or bleaching powder in a small chamber outside the septic tank. No disinfectant as bleaching powder, phenyl, etc., should be used in cleaning latrines as the disinfectant entering the septic tank kills the bacteria growth and retard the action of biological decomposition. After every use of the latrine it should be flushed with about 14 litres of water from a flushing cistern or by sudden hand pouring of a bucket of water. Domestic water, paper, kitchen water and surface water should not be allowed to drain into the septic tank.
The effluent of the septic tank may also be discharged into open drain after disinfection by treating with chlorine or bleaching powder in a small chamber outside the septic tank. No disinfectant as bleaching powder, phenyl, etc., should be used in cleaning latrines as the disinfectant entering the septic tank kills the bacteria growth and retard the action of biological decomposition. After every use of the latrine it should be flushed with about 14 litres of water from a flushing cistern or by sudden hand pouring of a bucket of water. Domestic water, paper, kitchen water and surface water should not be allowed to drain into the septic tank.
Fig.
Size of septic tank
The capacity of the septic tank
depends on the interval of sludge removal or cleaning. Normally sludge may be
removed once in every two years and the liquid capacity of septic tank may be
taken as 0.13 cu.m. (130litre) per head (5 cu.ft. per head) to 0.07 cu.m.
(70litre) per head (2.5 cu.ft. per head). For small number of users 0.13 cu.m.
(130litre) per head and for large number of users 0.07 cu.m. (litre) per head
may be taken as capacity of the tank. Septic tank shall have a minimum width of
60 cm (2’-0”) and a minimum liquid depth of one meter (3’-3”) below water level
with a minimum free board (open space) of 30 cm (1 ft) above water level. The
septic tank is usually consists of two chambers (compartments) with a partition
wall at a distance of about ½ length of the tank from the inlet end. The
partition wall is raised above the liquid surface by 15 cm (6”) and a hole of
15 cm x 15 cm (6” x 6”) is made at a height of 45 cm (1’-6”) from the bed of
the tank to connect the two chambers. Small tank may be made of one chamber
with hanging baffle wall at a distance of ½ to 1/5 distance from the inlet end.
Total combined length of the tank shall be 2 to 4 times the breadth. Design
calculation of septic tank for 25 users and 20 users in metric system.
Septic tank is usually consists
of brick wall in cement mortar not less than 20 cm (9”) thick and the
foundation floor are of cement concrete 1:3:6 or 1:2:4. Both inside and outside
faces of wall and floor are plastered with a minimum thickness of 12 mm (1/2”)
thick cement mortar 1:3 and all inside corners are rounded. Floor should be
given a slope be given a slope of about 1 in 20 for the convenience of
collection and removal of sludge. Septic tank may also be built with stone
masonry, precast or cat in situ cement concrete 1:2:4 proportion. The cover of
septic tank is of R.C.C. slab with suitable circular openings with cast iron
manhole cover of cleaning and inspection. For small septic tank precast R.C.C.
slab in strips may be provided, one or two pieces may be removed for cleaning
and refitted.
Connecting pipe should be 100
mm (4”) minimum diameter and may be of S.W. pipe, R.C.C. or Hume pipe or cast
iron pipe. Inlet and outlet may be made through T-junction pipe or baffle wall
of precast R.C.C. may be provided at a distance 1/5 of length of the septic
tank so that inlet sewage may not disturb the working of the tank. Ventilation
pipe of 50 mm (2”) minimum diameter are provided up to a height of 1.80 meter
(6 ft). If the septic tank is within 15 meter (50ft) of a habitable building,
the ventilating pipe should be carried to a height of 1.80 meter (6 ft) above
the roof of building.
Soak pit or seepage pit
Fig.
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Soak pit shall be less than 90
cm (3 ft.) in diameter and not less than 1.5 m (5’) in depth below invert level
of the inlet pipe. The pit is constructed with lining of dry brick or stone
with open joints backed with at least 7.5 cm (3”) thick coarse aggregate. The
upper portion of the lining up to the inlet level at least 45 cm (1’-6”) in
height should be made with masonry in mortar for strengthening, for preventing
surface water to enter into the pit and to prevent rat throwing earth into the
pit. The inlet pipe should be fixed in the wall of the pit with mortar. The pit
should be covered with precast or laid in situ R.C.C. slab. The absorption area
of soak pit should be 1 sq.m. (10 to 15 sq.ft.) per head of user depending on
the nature of the soil. The soak pit should be raised above ground level and no
surface water should be allowed to enter into it.
