Ultimate disposal of sewage on land and water bodies

ULTIMATE DISPOSAL OF SEWAGE

There are two general methods of disposing effluents:

1. Dilution (disposal in water) and
2. Effluent irrigation or Broad irrigation or Sewage farming (disposal on land)

Disposal by dilution is the process where the treated sewage or effluent from sewage treatment plant is discharged into a river or stream or large body of water such as a lake or sea. The discharged  sewage is purified by a process called self purification of natural waters. The degree and amount of treatment given to raw sewage before disposing it into any water body depends on the quality of raw sewage and the intended use of the water body. The sewage can be disposed into water bodies if:

• The sewage is comparitively fresh and free from floating and settleable solids
• The receiving water has a high dissolved oxygen content
• The receiving waters are not used for navigation purposes or water supply for a reasonable distance on the downstream side from the point of disposal
• The flow currents of the diluting waters are favourable and do not cause deposition, nuisance or destruction of aquatic life. 
• The waters should have swift forward currents that carry away the sewage to points of unlimited dilution. 
• The receiving waters should not have slow back currents that cause sedimentation resulting in large sludge deposits.

The Bureau of Indian Standards (BIS) has laid down guidelines for discharge of domestic sewage effluents in IS:4764-1973 and for industrial effluents in IS:2490-1974
These tolerance limits are national guidelines for guiding various state pollution control boards for prescribing legally enforceable standards.

DISPOSAL ON LAND
In this method, the sewage effluent (treated or diluted) is disposed off by applying it on land. The percolating water may either join the water-table or is collected below by a system of under-drains. This water can then be used for irrigating crops. This method may help in increasing crop yield as sewage contains fertilising minerals and other elements. When raw or partly treated sewage is applied on land, a part of it evaporates and the remaining percolates through the soil. While percolating, the suspended particles in the sewage are caught in soil voids. By maintaining proper aeration of these voids, the organic sewage solids caught in these voids get oxidised aerobically. This is possible if soil is sufficiently porous and permeable. However, if the soil is made up of heavy, sticky and fine grained material, the void spaces get choked resulting in non-aeration of voids leading to anaerobic decomposition of organic matter and evolution of foul gases. Excessive clogging may result in ponding of sewage and breeding of mosquitoes. The degree of treatment will depend on type of soil on land.