The higher amounts of waste increase the rates of decomposition and O2 consumption, thereby decreases the DO content of water.  

• The demand for O2 is directly related to increasing input of organic wastes and is empresses as biological oxygen demand (BOD) of water. 
• Water pollution by organic wastes is measured in terms of Biochemical Oxygen Demand (BOD).  

• BOD is the amount of dissolved oxygen needed by bacteria in decomposing the organic wastes present in water. It is expressed in milligrams of oxygen per litre of water. 
• The higher value of BOD indicates low DO content of water. Since BOD is limited to biodegradable materials only. Therefore, it is not a reliable method of measuring pollution load in water. 
• Chemical oxygen demand (COD) is a slightly better mode used to measure pollution load in water.  
• It is the measure of oxygen equivalent of the requirement of oxidation of total organic matter (i.e., biodegradable and non-biodegradable) present in water. 

Source: Shankar