Name a physical method used in the secondary treatment of wastewater.

The activated sludge process is a physical method used in the secondary treatment of wastewater. This process involves aerating the wastewater, which encourages the growth of microorganisms that consume organic matter in the presence of oxygen. As the microorganisms multiply and form flocs, or clumps, they effectively reduce the biochemical oxygen demand (BOD) of the wastewater.

During the activated sludge process, aeration tanks are typically used where the wastewater mixes with a population of activated sludge. Once the organic material is broken down, the mixture flows to a secondary clarifier where the biomass, or activated sludge, settles out from the treated water. The settled biomass can then be either returned to the aeration tank to maintain the microbial population or removed for further treatment or disposal.

This process is preferred for its efficiency in reducing pollutants and improving water quality, making it a cornerstone of secondary wastewater treatment.

In contrast, chemical precipitation and reverse osmosis involve different mechanisms either utilizing chemicals to help settle out solids or applying pressure to separate contaminants through membranes, and aerobic digestion is a process related to the stabilization of sludge, not specifically a method of secondary treatment. Hence, the activated sludge process distinctly fits within the definition and use of a physical method in secondary wastewater treatment.