Explanation

Biochemical Oxygen Demand (BOD) in a river water increases when sewage gets mixed with river water ‘Whanever untreated sewage are disposed into natural waters such as streams, ponds, lakes, etc., the normal amount of dissolved oxygen, present in water, gets quickly utilized by microorganisms. The oxygen demand for oxidation of organic matter present in swage is increased’. This, high value of BOD means the water is highly polluted by organic matter