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Vol. 6 No. 1 - Millennium Issue - January 2000

Microbiologically treated wastewater for irrigation

By: M.R. Suseela

Population explosion and rapid industrialization have resulted in deterioration of natural water bodies. Despite the abundance of natural water bodies, water has become scarce. Wastewater, the major contaminant includes, domestic sewage, agricultural wastes and industrial effluent. This wastewater is released into the rivers, lakes and ponds. It may also be disposed, directly in open land, where part of it evaporates and part of it percolates. All such direct disposals involve serious risks to health and sanitation, besides, valuable resources of manure and water are wasted. As a potential carrier of pathogenic microorganisms, water can endanger health and life of biological systems. It may also lead to the evolution of bad odour and spread of enteric diseases (cholera, typhoid fever, dysentery), and fish mortality in the receiving water. It is, therefore, necessary to employ treatment facilities that purify wastewater prior to its disposal into water bodies. Wastewater treatment procedures, both natural and artificial, are largely dependent upon microbial activity.

Pollutants in wastewater may be chemical and microbial. Sewage consists of approximately 99.9 percent of water, 0.02 to 0.03 percent suspended solids, and other soluble organic and inorganic substances. On a percentage basis, the solids appear small. However, the tremendous volume of material handled (several hundred million gallons), contains solids to the tune of hundreds of tonnes. The chemical constituents are extremely important and are subject to variations, between communities, as well as within a community, even from hour to hour. Inorganic chemicals initially present in the water-supply will likewise be present in the sewage. Organic compounds are contributed through human excreta and other domestic wastes. However, both organic and inorganic compounds are added by industrial wastes. For example, sugar factories, paper mills, tanneries, distilleries add organic substances, while mines and metal industries contribute acids and salts of metal and other inorganic wastes.

Since the composition of wastewater varies, it is to be expected that the types and number of microbes will fluctuate. Fungi, protozoa, algae, bacteria and viruses are the main groups of microorganisms. Raw sewage may contain millions of bacteria per ml including the coliforms, streptococci, anaerobic spore-forming bacilli, the proteus group and other types originating in the intestinal tract of humans. Sewage is also a potential source of pathogenic protozoa, bacteria and viruses. The causative agents of dysentery, cholera and typhoid may occur in sewage. The poliomyelitis virus and infectious hepatitis virus are excreted in the faeces of infected hosts and thus may appear in sewage.

Pollution of water is measured in terms of biochemical oxygen demand (BOD). One of the primary reasons for treating wastewater prior to its being returned to the water resource is to reduce the drain on dissolved oxygen supply of the receiving body of water. The magnitude of the BOD is related to the amount of organic material in the wastewater i.e., the more oxidizable organic material, the higher the BOD.

The composition of the micro-flora and micro­fauna of a stream is a good indicator of pollution. Presence of Sphaerotilus natans indicates considerable organic pollution and the smell of hydrogen sulfide is evidence of anaerobic sulfate reduction and should serve as an alerting signal.

Underground sources such as wells and springs provide most of the water for individual homes in rural areas. Surface water, however, should not be used for drinking purposes unless it is subjected to purification, since there is constant danger of contamination and consequent transmission of disease. As water penetrates through the layers of soil, water from wells and springs undergoes filtration, which removes suspended particles, including microorganisms. It is of prime importance that the supply of groundwater selected be located at safe distance from possible sources of contamination, e.g., septic tanks, cesspool.

The main operations involved in a large-scale water purification plant to produce water of a quality safe for human consumption are sedimentation, filtration and dis-infection. Sedimentation occurs in large reservoirs, where the water remains for a holding period. Large particulate matter settles to the bottom. Sedimentation is enhanced by the addition of alum (aluminum sulfate) at the treatment plant, which produces a sticky flocculent precipitate. Many microorganisms, as well as finely suspended matter, are removed as this precipitate descends through the water in the settling basins. The water is next passed through sand filter bed, which removes 99 percent of the bacteria. The majority of large­scale water treatment facilities employ chlorination for dis-infection.

Wastewater treatment is necessary before it can be disposed off without producing significant undesirable or even harmful effects. However, some communities still dispose off inadequately treated wastewater into natural bodies of water either because they are indifferent to consequences or because it is assumed that the body of water is sufficiently large and so located that dilution prevents hazards. Communities can no longer rely on disposal of wastewater by dilutions. There is an increasing demand for domestic and industrial water necessitating more reuse of water that receives wastewater.

Dr. (Ms.) M. R. Suseela is a scientist at the National Botanical Research Institute, Lucknow - 226001 (India).

This article has been reproduced from the archives of EnviroNews - Newsletter of ISEB India.

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