Correct answer is (a) Initial settlement
Easiest explanation: Bio filters are fed with effluent only after initial settlement of the industrial effluent to avoid clogging of pores of filters also, it would block the distributor arm, Bio-magnification is the process by which toxic substances get deposited in the food chain, Immobilization uses Bio-accumulation processes, bio-degradation is the process that leads to degradation of compounds with the help of bacteria.