Page 6 - LN-CH-16

• Organic remains deposit at the bottom of the lake and with time, water becomes warmer.
Eventually, floating plants develop in the lake, finally converting it into land. This accelerated
ageing of lakes due to sewage, agricultural and industrial wastes is called cultural or
accelerated eutrophication.
(c) Biochemical Oxygen Demand (BOD) is the amount of oxygen required for microbial
breakdown of biodegradable organic matter. It is higher in polluted water and lesser in clean
water.

(d) Algal bloom is excessive growth of planktonic (free-floating) algae in aquatic bodies.
• In domestic sewage, nutrients like nitrogen and phosphorus favours the growth of algal
bloom.
• It causes fish mortality and deterioration of water quality.
Example, Excessive growth of water hyacinth (Eichhornia crassipes). It is the most
problematic aquatic weed, also called Terror of Bengal.
It grows abundantly in eutrophic water bodies and imbalances water ecosystem.
(vi) A Case Study— Integrated Wastewater Management
(a) Wastewater including sewage can be treated in an integrated manner by utilising a mix of
artificial and natural processes.
• One such example is town of Areata situated along Northern coast of California. An
integrated wastewater treatment process was developed with the help of biologists from
Humboldt State University in this town.
• The cleaning occurs in two steps:
— The conventional sedimention, filtrating and chlorine treatment is carried out.