How does increasing the recirculation rate affect the affluent BOD from a trickling filter?

Increasing the recirculation rate in a trickling filter system enhances the effectiveness of biological treatment by providing a greater volume of treated water back to the filter. This process allows for more contact time between the microorganisms on the media and the organic material present in the influent wastewater. As a result, the microorganisms can more efficiently break down the organic matter, leading to a reduction in the biochemical oxygen demand (BOD) of the effluent water.

This mechanism works because higher recirculation rates help maintain optimal conditions for biological activity, supporting microbial growth and promoting more thorough digestion of organic pollutants. The enhanced microbial activity results in a lower concentration of BOD in the effluent, as more organic material is consumed during the treatment process.

While other options hint at various outcomes, they do not accurately capture the impact of increased recirculation on BOD levels, which is a direct benefit of optimizing the biological treatment process.