Which type of effluent is typically treated by tertiary filters?

Tertiary filters are specifically designed to treat secondary effluent, which means they work with the water that has already undergone primary and secondary treatment processes. Primary treatment removes larger solids, while secondary treatment focuses on reducing organic matter through biological processes. By the time effluent reaches the tertiary treatment stage, it still may contain remaining contaminants, such as nutrients, fine suspended solids, and pathogens. Tertiary filtering aims to further purify the water, improving its quality for discharge into the environment or for reuse applications.

Secondary effluent is therefore the correct choice as it is the stage of wastewater that precedes tertiary treatment. The other types of effluents listed, such as primary effluent and untreated sewage, have not undergone the necessary secondary treatment steps, making them unsuitable for tertiary filtering processes. Stormwater runoff also typically requires different treatment approaches, as it may consist of surface runoff not related to wastewater treatment plants.