Which process is typically employed during the secondary treatment stage?

The activated sludge process is a widely used method during the secondary treatment stage of wastewater treatment. This process involves aerating wastewater to encourage the growth of microorganisms, which consume organic pollutants in the water. By introducing air or oxygen into the wastewater, the microorganisms thrive and form flocs—clumps of cells and organic particles that settle out of the treated water.

During secondary treatment, the goal is to significantly reduce the biochemical oxygen demand (BOD) and suspended solids in the wastewater. The activated sludge process is effective in achieving this because the microorganisms can effectively break down complex organic materials.

In contrast, filtration and disinfection typically occur as part of the tertiary treatment or at the final stages of wastewater treatment, where finer particles and pathogens are removed. Incineration is not a secondary treatment method but rather a process used for the disposal of solid waste or sludge rather than the treatment of liquid wastewater. Flocculation, while important in some treatment processes, is mainly a part of primary treatment or can also help in tertiary treatments, but it is not the primary method used in secondary treatment to promote biological decomposition of organic material.