Which of the following is not a method used in biological wastewater treatment?

Filtration is not considered a biological method of wastewater treatment because it primarily involves the physical removal of suspended solids and particulates from water. In filtration processes, the focus is on the separation of contaminants from the water based on size, which does not rely on biological activity or the interaction of microorganisms with the wastewater.

In contrast, the other methods listed, such as the activated sludge process, trickling filters, and membrane bioreactors, all involve biological processes where microorganisms play a crucial role in breaking down organic matter in the wastewater. The activated sludge process utilizes aeration to facilitate the growth of microorganisms that decompose organic pollutants. Trickling filters involve the use of littered media, where microorganisms grow and treat the wastewater as it flows over. Membrane bioreactors combine biological treatment and membrane filtration, treating water by using microorganisms and separating treated effluent from solids.

Thus, the distinction between filtration and these biological methods helps clarify why filtration is not classified as a biological approach in wastewater treatment.