When sewage, having biodegradable organic matter is released in water body, micro-organisms involved in biodegradation of organic matter in the receiving water body consume a lot of oxygen to decompose the sewage and as a result there would be a sharp decline in dissolved oxygen downstream from the point of sewage discharge, and biological oxygen demand (BOD) would increase. Presence of large amount of nutrients in water causes excessive growth of planktonic (free floating) algae, called algal bloom. Algal bloom causes deterioration of water quality and fish mortality.