The correct answer is: C. 60 to 80%
A trickling filter is a type of biological wastewater treatment process that uses a bed of rocks or other media to support a biofilm of microorganisms. The microorganisms in the biofilm degrade organic matter in the wastewater, reducing the biochemical oxygen demand (BOD).
The BOD of wastewater is a measure of the amount of oxygen that would be required to decompose the organic matter in the wastewater. High BOD levels can cause problems in receiving waters, such as fish kills and the depletion of dissolved oxygen.
Trickling filters are very effective in reducing BOD. In most cases, BOD can be reduced by 60 to 80% in a trickling filter.
Option A is incorrect because it is too low. Option B is incorrect because it is too high. Option D is incorrect because it is too high.