Code of practice for installation of septic tanks, Part II: Secondary treatment and disposal of septic tank effluent
1985 Edition

According to IS 2470 Part 2 (1985), the suitability of soil for septic tank effluent disposal depends on key parameters including soil type and permeability (porous, dense, or clayey), the percolation rate measured as the time taken for water to drop 25 mm during testing, and the groundwater table depth relative to the surface. Soils with percolation rates up to 30 minutes are generally suitable for dispersion trenches or seepage pits. Dense or clayey soils with percolation rates exceeding 60 minutes require biological or upflow anaerobic filters. Additionally, if the groundwater level is within 1.8 meters from the surface, mound or above-ground disposal systems are recommended to avoid contamination.

The percolation rate measures the soil’s permeability to ensure it can effectively absorb septic tank effluent. The procedure involves filling a test hole with 300 mm of water over gravel and allowing 24 hours for soil swelling. After this, water depth is adjusted to 150 mm and the drop in water level is measured over a fixed interval, typically 30 minutes. For sandy or porous soils, measurements may be taken every 10 minutes for an hour. The percolation rate is calculated by dividing the time interval by the drop in water level, indicating the soil’s absorption capacity. This information guides the design of soil absorption systems, ensuring environmental safety and system longevity.

IS 2470 Part 2 recommends three types of upflow anaerobic filters: single chambered rectangular, double chambered rectangular, and circular types. Effluent enters these filters from the bottom and passes upward through a media bed of 20 mm coarse aggregate that supports microbial growth for organic matter degradation. The double chambered design includes two compartments with specified media depths and uses perforated trays for flow distribution and valves for sludge removal. Biological filters utilize stone or sand media to support microbial films and typically achieve about 70% BOD reduction. Proper ventilation, air vents 150 mm above ground, flexible pipe joints, and controlled effluent distribution mechanisms are essential for optimal performance.

Per IS 2470 Part 2 Clause 5.3, absorption trenches must maintain a minimum distance of 18 meters from drinking water sources such as wells to prevent microbial contamination. Additionally, a buffer of at least 6 meters is required from habitable structures to avoid potential structural damage. These distances may be adjusted depending on soil percolation rates and load-bearing capacity. The standard also advises avoiding absorption systems in geologically sensitive areas like limestone or crevice rock formations due to contamination risks, recommending alternate treatment methods where soil conditions are unfavorable.

The standard specifies that effluent dispersion methods depend on subsoil water depth, soil texture, and percolation rates. For subsoil water levels within 1.8 meters, porous soils with percolation rates of 30 minutes or less should use dispersion trenches constructed as mounds above ground. Dense or clayey soils with percolation exceeding 60 minutes require biological or upflow anaerobic filters with effluent discharged to surface drains or used for gardening. For subsoil water deeper than 1.8 meters, porous soils can use seepage pits or dispersion trenches, while dense soils still use dispersion trenches. Effluent discharged to open drains must be disinfected to prevent contamination. Even distribution and proper ventilation are crucial for biological filters to function effectively.