This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Environment and health, Subject name: Social & Preventive Medicine
Trickling filter is used in - 
Select and explain. 
A. Primary treatment of sewage, B. Secondary treatment of sewage, C. Oxygen ponds, D. Sewage farming
B: Secondary treatment of sewage
Answer: B: Secondary treatment of sewage 
Explanation: Modern sewage treatment include purification of sewage by action of anaerobic and aerobic bacteria.Include primary and secondary treatment. Primary treatment include screening,gritchamber and primary sedimentation. Secondary treatment is for the fuher treatment of effluent from primary sedimentation tank .It may still contain organic matter and living organisms which increase the demand for oxygen and cause pollution.Aerobic oxidation can be carried out by Trickling fliter method and activated sludge process.Trickling fliter is a bed of crushed stones.As effluent percolates through filterbed,it get oxidised by bacterial flora in zoological layer.Action of filter is purely biological. Parks textbook of preventive and social medicine.K Park. Edition 23.page no:763,764