Trickling filter is used in:

Ans. is 'b' ie secondary t/t of sewage Modern Sewage treatmentThe treatment of sewage may be divided into 2 stages primary t/t & - secondary t/t Primary t/t - in primary t/t the solids are separated from the sewage partly by screening and partly by sedimentation and subjected to anaerobic digestion Secondary t/t - the effluent is subjected to aerobic oxidation.* Secondary treatment is done by one of the following methodsTrickling filter methodActivated sludge process.