Aerobic Waste Water Treatment - Waste waters contribute organic materials and inorganic nutrients to river waters; this is called eutrophication. This promotes microbial and plant growth, which depletes oxygen from the water resulting in death of fish and other fauna.

Sometimes algal and plant growth may be so profuse as to physically choke the river. Waste waters may also add inert particulate matter, which settles in the river; sometimes it may be hot and raise the river water temperatures. All these factors influence the composition and the density of flora and fauna of the rivers.