12.7 Eutrophication

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. . ).

Pollutants from anthropogenic activities like effluents from the industries and homes Fig. . Stages of Eutrophication Environmental Issues greater the BOD of the waste water more is its polluting potential. Once the BOD of sewage water is reduced significantly, the effluent is then passed into a settling tank where the bacterial “flocs” are allowed to sediment.

This sediment is called activated sludge. A small part of activated sludge is pumped back into the aeration tank to serve as the inoculum. The remaining major part of the sludge is pumped into large tanks called anaerobic sludge digesters. Here, the bacteria which grow anaerobically, digest the bacteria and the fungi in the sludge.

During this digestion, bacteria produce a mixture of gases such as methane, hydrogen sulphide and CO . These gases form biogas and can be used as a source of energy. Tertiary treatment Tertiary treatment is the final process that improves the quality of the waste water before it is reused, recycled or released into natural water bodies. This treatment removes the remaining inorganic compounds and substances, such as nitrogen and phosphorus.

UV is an ideal disinfectant for wastewater since it does not alter the water quality – except for inactivating microorganisms. UV is a chemical- free process that can completely replace the existing chlorination system and also Primary treatment Primary treatment involves the physical removal of solid and particulate organic and inorganic materials from the sewage through filtration and sedimentation. Floating debris is removed by sequential filtration. Then the grit (soil and small pebbles) are removed by sedimentation.

All solids that settle form the primary sludge and the supernatant forms the effluent. The effluent from the primary settling tank is taken for secondary treatment. Secondary treatment or biological treatment The primary effluent is passed into large aeration tanks where it is constantly agitated mechanically and air is pumped into it. This allows vigorous growth of useful aerobic microbes into floc (masses of bacteria associated with fungal filaments to form mesh like structures).

While growing, these microbes consume the major part of the organic matter in the effluent. This significantly reduces the BOD (Biochemical oxygen demand or Biological oxygen demand). BOD refers to the amount of the oxygen that would be consumed, if all the organic matter in one litre of water were oxidized by bacteria. The sewage water is treated till the BOD is reduced.

The Pretreatment Waste water from cities and towns Remove coarse solids sand and grit Air - % Organic solids Recirculated to aeration tank Primary sludge Production of biogas Anaerobic sludge digester Secondary sludge Primary settling tank Aeration zone (air + liquid water) Secondary settling tank Disinfection zone Treated sewage for dispose/reuse Preliminary treatment Tertiary treatment Primary treatment Secondary or biological treatment Fig. . Sewage treatment process Environmental Issues inactivates chlorine-resistant microorganisms like Cryptosporidium and Giardia.