Which of the following describes a biological process for removing contaminants?

The activated sludge process is a biological method employed in wastewater treatment that utilizes microorganisms to break down organic matter and remove contaminants from wastewater. In this process, raw wastewater is aerated in a reactor, allowing bacteria and other microorganisms to thrive and consume organic pollutants. The mixture of wastewater and microorganisms forms a flocculent mass, which aids in the sedimentation of solids when the water is allowed to settle. The settled sludge can then be returned to the aeration tank to maintain a healthy population of bacteria, promoting ongoing treatment.

This method is particularly effective for treating sewage and other organic wastes because it relies on the natural metabolic processes of microorganisms, making it an efficient and sustainable approach to managing wastewater. Other treatment processes listed, like membrane filtration, chemical precipitation, and reverse osmosis, primarily involve physical or chemical mechanisms rather than biological ones.