The correct answer is (d) Rotating biological contractors
The best explanation: Rotating biological contractors absorb oxygen from the air and pollutants from the wastewater when the spherical disc is in contact with the air and with the waste water respectively during its rotation which can be decomposed aerobic microorganisms, new microorganisms are fed and old microorganisms with declined level of activity is taken off, high rate bio filters usually constructed as tower models treating food waste which has considerable potential to rough high strength industrial effluents prior to sewage discharge whereas, Conventional bio filters are also called as Percolating filters in which the waste trickles and is oxidized and hence removed by biological degradation.