When run-off from land containing nutrients reaches water bodies like lakes, it results in dense growth of plant life. This phenomenon is called Eutrophication. Natural aging of lakes also leads to nutrient enrichment of its water. In a lake, the water is cold and clear (oligotrophic stage), supporting little life. With time, streams draining into the lake introduce nutrients such as nitrates and phosphates, which encourage the growth of aquatic organisms. Aquatic plants and animal life grow rapidly, and organic remains begin to be deposited on the lake bottom (mesotrophic stage) (Fig. 13.5).
Pollutants from anthropogenic activities like effluents from the industries and homes can radically accelerate the aging process. This phenomenon is known as Cultural or Accelerated Eutrophication.
Nutrients stimulate the growth of algae, water hyacinth and can cause clogging of canals, rivers and lakes as well as, displacing native plants. It causes unsightly foam and unpleasant odours, and deprives the water of dissolved oxygen.
Wastewater or sewage originates from domestic waste waters, industrial wastes and animal wastes. Realizing the importance of clean potable water, the Government passed the Water (Prevention and Control of Pollution) Act in 1974, which made it mandatory to treat wastewater in treatment plants. The treatment can be carried out by three ways:
1. Physical methods
2. Chemical methods
3. Biological methods
Wastewaters containing insoluble substances or colloids are treated through processes such as flotation, sedimentation, filtration and centrifugal separation.
Chemical methods of wastewater treatment include:
• Generation of insoluble solids.
• Produce an insoluble gas.
• Produce biologically degradable substances from a non-biodegradable substance.
• Oxidize or reduce to produce a non-objectionable substance.
(1) Bioremediation of wastewater includes the aerobic treatment (oxidation ponds, aeration lagoons) and anaerobic treatment (anaerobic bioreactors, anaerobic lagoons).
(2) Phytoremediation of wastewater includes constructed wetlands, Root Zone Wastewater Treatment (RZWT), and Decentralized Waste Water Treatment System (DEWATS) (Fig. 13.6 a).
Case Study: Auroville, located in South India near Puducherry has been experimenting with natural wastewater recycling systems (Fig:13.6a). Such treatment plants have now also been implemented in Aravind Eye Hospital, Puducherry (Fig.13.6 b) and the Chennai Mathematical Institute, Siruseri IT Park, Chennai.
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