What is an example of a secondary treatment process?

The trickling filter is an example of a secondary treatment process in wastewater treatment. Secondary treatment is designed to significantly reduce the concentration of organic matter and other pollutants that remain after primary treatment, where solids are removed from the wastewater.

In a trickling filter, wastewater is distributed over a bed of media, usually rocks or plastic, that supports the growth of biofilm. Microorganisms in this biofilm feed on the organic pollutants in the wastewater as it trickles over the media, effectively breaking down harmful substances and facilitating their removal from the water.

Other options like a septic tank and composting toilet primarily serve as primary treatment processes. Septic tanks focus on the sedimentation of solids and initial anaerobic digestion, while composting toilets manage human waste in a way that promotes decomposition without relying on water. A gravel filter does not constitute a secondary treatment process either, as it typically acts as a filtration mechanism rather than a biological treatment method. Thus, the trickling filter stands out as a specific example of a secondary treatment process that enhances the quality of effluent before it is discharged or further treated.