In areas having sewerage facility the wastewater from the house is carried by the sewerage system and treated at the sewage treatment plant. In un-sewered areas, this wastewater is treated in the septic tank. The various parts of the septic tank are properly designed as per the standard laid.
Effluent from the septic tank is not pure thus needs further treatment for which it is sent to the soak pit. The septic tanks are designed for foul sewage ( containing fecal matter and urine ).
Sullage wastes (waster from bathroom, kitchen, etc) may be applied directly to the gardens or grassed areas. It may be drained to a seepage pit or dispersion trench from which it overflows into or is absorbed by the surrounding soil.
Parts of Septic Tank
- The design of septic tank inlets should be done in such a way that incoming sewage creates the least possible disturbance to the settled sludge.
- For tanks up to 1200 mm width, a T-shaped dip pipe having a diameter not less than the incoming drain diameter may be provided.
- The pipe shall be fixed in such a way that the top of the pipe should rise above water level at least 300 mm. The bottom limb extends about 300 mm below the top water level.
- For tanks having a width greater than 1200 mm two or more than two submerged inlets are required.
- A baffle wall should be provided at a distance of 150 mm from the inlet end of the tank. It should have a depth of 150 mm below the invert level of the inlet pipe and 150 mm above the top water level.
- For tanks of less than 1200 mm width, the outlet pipe dia should not be less than 100 mm.
- The top limb should rise above the water level and the bottom limb should be at least D/3 below the top water level.
- The center line of the outlet pipe should be 40 mm below the centerline of the inlet pipe.
- If the tank is to be designed for a capacity of 2000 liters or above it should be divided into two chambers.
- The partition should be done in such a way that the capacity of the first chamber is twice that of the capacity of the second chamber.
- Opening of dia 100 to 150 mm may be provided in the partition at a depth of 300 mm below top water level.
- Min. freeboard of 30 cm should be provided.
Access Openings and Cover
- Each compartment of a septic tank shall be provided with a rectangular or circular access opening of 455x 610 mm or 500 mm diameter.
- It should be covered with the reinforced concrete or a cast iron lid.
- Septic tank should be provided with ventilating pipes of min 50 mm diameter.
- It should be extended to such a height where it will not cause smell nuisance to the surrounding area. It should be at least 20 m from the nearst builidng
Points to consider in working of septic tank
- Surface or subsurface water should not enter into the septic tank.
- The effluent coming from a septic tank should not be allowed into an open channel drain or water body without secondary treatment because this effluent is high in organic matter.
- Wastewater containing excessive detergents, grease, and disinfectants should not be treated in the septic tank. IF done so they can adversely affect the anaerobic decomposition.
- If the slope of the incoming drain (carrying foul sewage) is steep due to the unavoidable site conditions, the last section of the drain, (12m portion) should not be laid at a gradient steeper than 1: 50. This ensures the minimization of turbulence in the tank due to high-velocity water.
- Sewage from the pump should not be discharged directly into the septic tank for this a separate tank should be constructed from where sewage should be fed to the septic tank.
The layout of sewage system
- The layout of the sewage system should be as simple and direct as possible.
- The pipes should be laid out in straight lines both vertically and horizontally. however, where bends are absolutely necessary, it should be a long radius bend with cleaning eyes. irregulars shapes that are possessing a hindrance to the flow should be avoided.
- At junctions of pipes, (in manholes) direction of flow from a branch connection should not make an angle greater than 45 degrees with the main pipe.
Location of Septic Tank
- Septic tank should be located preferably at a place open to the sky. away from the exterior of the wall of the building.
- It should not be located in areas prone to flooding.
- Septic tank should also be accessible for cleaning. For this, access- hole should be provided
Dimensions of Septic Tank
- It should have a minimum width of 75 cm.
- minimum depth of 1m below water level and a minimum liquid capacity of one thousand liters.
- In case of rectangular septic tanks, the length of the tank shall be at least 2 to 4 times the width.
- For circular tanks, the minimum diameter shall not be less than 3.55 m and operating depth shall not be less than 1.0 m
Indian Standard for Septic Tank Design
IS 2470-1985 Part-1 is used for the design of the septic tank.