BOD refers to the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria. The microbes biodegrade the major part of the organic matter in the effluent by using the dissolved oxygen. It reduces the BOD of the effluent. When the BOD of sewage is reduced significantly the effluent is passed into a settling tank where the bacterial floes are allowed to sediment forming the activated sludge. Thus, higher the value of BOD, more is the water polluted and more is the amount of biodegradable matter.