In secondary municipal wastewater treatment, what role do microorganisms play?

Microorganisms play a vital role in the process of secondary municipal wastewater treatment through their ability to consume and break down organic matter in the wastewater. This biological treatment phase is essential for reducing the concentration of pollutants, particularly biodegradable materials, that remain after primary treatment, which typically involves the physical separation of solids from liquids.

During this stage, microorganisms such as bacteria feed on the organic waste present in the wastewater, converting it into biomass (their own cellular material) and other byproducts like carbon dioxide and water. This process significantly reduces the organic load of the wastewater, making it less harmful before discharge into the environment. Moreover, the increase in microbial biomass can be further processed or removed in subsequent steps, helping to clarify the treated water.

The other options, while related to wastewater treatment, describe different functions or processes. Chemical disinfection involves the use of chlorine or other chemicals to kill pathogens, and filtering out solids typically occurs in primary treatment stages. Precipitation of heavy metals is often facilitated by chemical treatments rather than biological processes. Thus, the consumption and removal of waste by microorganisms is the primary function they serve in secondary treatment, making it the correct answer.