zone de dégradation

CONT: Zones of Water Pollution. Discharge or mixing zone is where pollutants enter the stream or the river. If the waste is dark, it will make the water dark, increase its temperature and subsequently oxygen levels will drop. The rate of discharge should not be stronger than flow of the stream. In zone of degradation oxygen levels drop from BOD (biological/biochemical oxygen demand). Zone of active decomposition is characterized by the following changes: 1) dissolved oxygen (DO) will be at its absolute minimum; 2) odors and gases bubbling up. In the zone of recovery the river will slowly restore its oxygen through aeration process. The anaerobic conditions will disappear. Nitrites will be converted into nitrates and stimulate growth of algae, which will produce oxygen through process of photosynthesis. In zone of clean water oxygen levels are really elevated. And BOD is almost satisfied.