The following paragraphs briefly review the important points of various widely practised modes of disposal of liquid effluents:
Disposal on Land:
The disposal of liquid effluents on land is a common practice, but if any stagnation of liquid effluents on land is allowed, mosquitoes and flies will breed on these, resulting in health problems.
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The careful and well-designed disposal of liquid effluents on land helps to achieve economic drop-irrigation and soil filtration, depending on water-holding capacity of the soil, land topography, general climate and particular crop in question. Excessive application of nutrient-rich water on land, however, may pose the following problems:
1. Concentration of various constituents, causing deleterious effects on crop growth.
2. Decreased soil permeability
3. Sewage sickness, and
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4. Ground-water contamination.
With a view to maintaining the quality of effluents at par with irrigation water so as to avoid any adverse effect on the crop, soil and ground water, the industrial effluents discharged on land should not contain constituents that are likely to or do pollute ground water to an extent that it (the ground water) does not remain fit for human or animal consumption.
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Disposal into Public Sewers:
“The presence of certain constituents in industrial effluent may choke or damage sewers, impair the proper working of the sewage treatment plant, and where land irrigation is adopted for final disposal of the sewage, the vegetation and the soil may be adversely affected Large amounts of sulphur compounds in effluents, for instance, would tend to form sulphuric acid under certain conditions, which would corrode the sewer.
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Toxic elements such as chromium or others with a high acidity or alkalinity level and the high temperature of the effluents are, inimical to biological activity in the treatment plant.
The presence of flammable and toxic volatile matter may cause explosions and be a serious hazard to the life of persons working in sewers or in the sewage treatment plants. Oils, tar and grease may stick to the sides of sewers and reduce their hydraulic capacity.
It is, therefore, necessary to limit the concentration of such undesirable constituents in industrial effluents discharged into public sewers”.
The disposal of solids such as ash, cinder, sand, tar, lime residue, lime slurry, and garbage into public sewers is also inadmissible. In order to prevent such ill effects of untreated industrial effluents on sewers, sewage treatment plants and sewage forms, it is necessary to limit certain constituents present in the industrial effluents before allowing them to be discharged into a sewer.
Disposal into Inland Surface Waters:
For the prevention and control of water pollution, the Indian Standards Institution, now called Bureau of Indian Standards (BIS), has prescribed “certain standards for the purity of water into which effluents have been discharged so that such receiving water courses could be safely used for various purposes”.
The water polluting contaminants of industrial effluents, discharged into land surface water, may be an acid, alkali, oil or grease, a floating suspended or dissolved organic matter, a toxic or poisonous substance or a colour as per definition laid down by the Indian Standards Institution, “By inland surface water is meant rivers, estuaries, stream, big lakes and reservoirs, including rivers liable to seasonal drying”.
The BIS further notes that “pollution of inland surface water with sewage, effluents and industrial water is, an important problem associated with industrial development and concentration of populist ion in cities”.