What process primarily occurs during secondary treatment?

The primary process that occurs during secondary treatment is the biological degradation of organic matter. This stage of wastewater treatment focuses on the removal of soluble organic pollutants, primarily through the action of microorganisms. During secondary treatment, bacteria and other microorganisms consume organic substances in the wastewater, breaking them down into simpler compounds. This biological process is crucial because it significantly reduces the concentration of biodegradable organic materials, which, if left untreated, can cause harm to aquatic ecosystems and public health.

In many treatment plants, this biological degradation occurs in aerated tanks where oxygen is supplied to promote the growth of aerobic bacteria. The overall goal of secondary treatment is to convert the organic matter into biomass, along with carbon dioxide and water, which can subsequently be removed from the treated effluent.

The other processes mentioned do not primarily characterize secondary treatment. Mechanical filtration relates more closely to physical methods of removing solids and debris from wastewater, while chemical precipitation involves the addition of chemicals to facilitate the removal of specific contaminants, often utilized in primary or tertiary treatments. Incineration of sludge is a method of reducing waste volume after treatment but does not occur during the secondary treatment itself.