Which of the following is considered a biological treatment process?

The activated sludge process is classified as a biological treatment process because it relies on microorganisms to treat wastewater. In this method, sewage is aerated in a tank where microbes digest organic matter. The aeration provides oxygen to the microorganisms, which in turn break down organic pollutants present in the wastewater, converting them into biomass and carbon dioxide. After the treatment, the mixture is allowed to settle, enabling the separation of the biomass from the treated water. This process effectively reduces the organic load and is a critical step in the secondary treatment stage of wastewater treatment plants.

In contrast, the other processes listed do not utilize biological mechanisms as their primary treatment method. Membrane filtration mainly separates solids from liquids using physical barriers; chemical precipitation involves the use of chemicals to remove contaminants from water; and sand filtration primarily relies on physical filtration where gravity pulls water through sand, trapping particles. Thus, these methods do not primarily depend on biological activity for pollutant removal, distinguishing the activated sludge process as the correct answer.