VPE 511: Environment and Environmental Hygiene (2+1)

CONTROL OF WATER POLLUTION

Treated Wastewater Disposal and Effluent Reuse

Urban sewage

• With urbanization the water consumption increases with larger proportions of industrial water needs. 80 to 90 percent of the water consumed ends up as sewage, with its large pollution potentials.
• Untreated sewages find their way into natural streams, lakes , sea and ground water, destroying the water quality in these natural water bodies and upsetting the ecological balances. Thus shortage of good water resources, death of aquatic life, health hazard to man and animal, corrosion of structures and aesthetic degeneration result. To avoid such consequences the following steps are necessary:
• Consumption of water by urban communities should be reduced, by avoiding wastages and leakages , by adopting less water consuming industrial processes and by treating and reusing wastewaters.
• Wastewaters should be treated with the objectives of reducing environmental pollution, enabling the reuse of treated wastewater and recovering the useful matter present in the wastewater.
• Water quality modeling and management of the receiving natural water bodies should be undertaken to preserve their quality while availing their waste (self) purification potentials thereby reducing the treatment costs.

Useful Constituents in Sewages

• More than 99% of sewage is mere water that is polluted the 1% impurities with all the evil attributes. Hence water is the most abundant component of the wastewaters to be put to reuse. The other components of wastewaters and their potentials for recovery and reuse as follows:
  • Organic matter is abundant in municipal sewage and some industrial wastes, particularily distilleries, sugar factories , food processing industries, slaughter houses, dairies etc. From this organic matter, manure and fuel gas can be realized. Anaerobic digesters constructed in distilleries may recover the entire cost in two, three years due to generation bio-gas that can be replace the huge conception of coal. Also, as bio-gas is a clean fuel, pollution problem from the industry can be minimized.
  • Chemicals in industrial wastes can be varied depending on the industry. Some chemicals will have recovery value and can partly pay for the waste treatment processes. However the emphasis should be to reduce the concentration of the effluents by suitable process alternations in the industry.

Methods of Recovery

• Organic matter in sewage can be recovered by aerobic and anaerobic bacterial applications. Setting tanks, activated sludge process, anaerobic sludge digesters, biogas plants etc, can enable the recovery of manure and fuel gas from organic components of sewages. The chemical in sewages are recovered by precipitation, adsorption, volatization, chelation, solvent extraction, ion filters, reverse osmosis, etc.

Reuse of Treated Wastewaters

• Some of the possible reuses of treated wastewaters are as follows:
• Where water scarcity is acute, the treated wastewater may be given further treatment (often called tertiary treatment) to make it potable. Even today many places treat sewage and reuse it for domestic purposes including drinking.
• Cooling towers in industries for heat exchange can use treated sewages.
• Irrigation and development of greenery can be largely benefited by treated sewages due to the micro nutrients like N,P,S , mineral salts etc, present in treated effluents. Development of sewage sickness of the soil and spread of sewage borne communicable diseases are the inherent hazards of sewage farming, necessitating, a careful management of the systems.
• Treated effluents can be used to recharge the ground waters . Soil systems have an excellent purification potential with rational management. The ground water resources can be saved from depletion by artificial recharging with sewage effluents, treated to the needed extent. Surface spreading, percolation tanks and injection wells are the methods of recharging.
• Pisiculture is possible in ponds formed by treated sewage effluents, with careful management.
• Sewage can be treated to any required quality and can be used for secondary purposes for industries. To cite an example, Madras Refineries Limited treats the municipal sewage of Madras by several advanced treatment processes to reuse it as the process water.