<prompt> Trickling filter is used in:Primary treatment of sewageSecondary treatment of sewageOxygen pondsSewage farming  <response>: Ans. is b ie secondary tt of sewage Modern Sewage treatmentThe treatment of sewage may be divided into 2 stages primary tt - secondary tt Primary tt - in primary tt the solids are separated from the sewage partly by screening and partly by sedimentation and subjected to anaerobic digestion Secondary tt - the effluent is subjected to aerobic oxidation. Secondary treatment is done by one of the following methodsTrickling filter methodActivated sludge process. 