(b) Sewage is a biodegradable pollutant. Those pollutants which can be broken down into simpler,harmless substances in nature in due course of time (by the action of micro-organisms like certain bacteria) are called biodegradable pollutants. DDT, plastic, polythene,bags, insecticides, pesticides,mercury, lead, arsenic, metal article like aluminum cans, synthetic fibers, glass objects, iron products and silver foils are non biodegradable pollutants.