Lagoons used for purification of polluted water

• are large shallow artificial lakes also known as clarification lakes, maturation ponds or oxidation ponds.
• use micro-organisms/bacteria in presence of dissolved oxygen.
• gives an excellent final effluent (with 3 to 4 lagoons arranged in series) having suspended solid < 1 mg/litre and BOD = 3.8 mg/litre.
• all (a), (b) & (c).