Sanitary, Water Supply and Septic Tank

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SANITARY AND WATER SUPPLY WORKS

SANITARY WORKS
Sanitary works usually consists of providing flush type latrines and connecting with Sewer lines of septic tank. For estimating the numbers of different fittings are found out and rates are taken per number for supply and fixing in position. The latrine seat with flushing cistern, flushing pipe, etc., are usually taken as one set, and rate per set for the complete work is taken in the estimate. Flushing cistern, with flushing pipe may also be taken separately. Wash-hand basins, Bath tubs, Urinals, etc. are also estimated per number for the complete work. The fittings as Mica valve, Cowl, Gully trap Master trap, etc., are also estimated number wise. The pipe lines of different materials of different diameters are estimated on running metre (r ft) basis for the tomplete work, supplying and fixing in position including excavation, refilling, jointing etc. together with lime concrete bedding as per specification. Fittings as bends, junctions etc. are not measured separately. Length shall be measured along the centre line of pipes and fittings. Masonry Manholes and Inspection Chambers are estimated per number for different sizes fitted with G.I. manhole for the complete work. Detailed estimates for Manholes and Inspection chambers may also be prepared if required. Latrine seat, syphon, footrests, flushing cistern, flushing pipe etc. each may be taken separately and estimated per number.

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 ‘nala’ or 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 (3 gallons) 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.

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 cum (130 litre) per head (5 cu ft per head) to 0.07 cum (70 litre) per
    head (2.5 cu ft per head). For small number of users 0.13 cu m (130 litre) per head and for large
    number of users 0.07 cu m (70 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 metre (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
    1½ 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″x6″) is made at a height of 45 cm (1′-6″) from the bed of the
    tank to connect the two chambers. Small tanks may be made of one chamber with hanging baffle
    wall at a distance of 1/2 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
    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 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 cast
    in situ cement concrete of 1:2:4 proportion. The cover of the septic tank is of R.C.C. slab with
    suitable circular openings with cast iron manhole cover for 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 metre (50 ft) of a
    habitable building, the ventilating pipe should be carried to a height of 1.80 metre (6 ft) above the
    roof of the building.
    Starting of new septic tank. A septic tank should be filled with water to its outlet level before
    night soil as allowed to enter the tank. It should then be seeded with sludge obtained from the septic
    tank in the neighbourhood. In the absence of the sludge a small quantity of decaying organic matter
    such as decomposed cowdung may be introduced.
    Desludging. Removal of sludge from the septic tank is required once in a year or once in
    every two years. The quantity of dry sludge is about 70 gram per head per day.

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