How does secondary treatment differ from primary treatment?

Secondary treatment is key in the sewage treatment process, as it specifically employs biological processes to enhance the removal of dissolved and suspended organic matter from wastewater. In this stage, microorganisms are introduced to consume and break down the organic pollutants that were not fully captured during primary treatment. Primary treatment primarily focuses on the physical separation of solids from liquid waste through processes like sedimentation. However, it does not significantly address the removal of dissolved organic substances, which is where secondary treatment plays an essential role.

During secondary treatment, various methods can be used, such as activated sludge systems or trickling filters, which utilize the metabolic activity of bacteria and other microorganisms to convert organic matter into more stable forms, effectively reducing the biochemical oxygen demand (BOD) of the effluent. This biological approach is vital for achieving higher levels of water quality before the treated water is released or further processed.

In contrast, other options point to characteristics that do not accurately describe the processes of secondary or primary treatment. For instance, while chemical treatments may accompany secondary processes, the hallmark of secondary treatment is its biological mechanism. The emphasis during secondary treatment is on biological action rather than solely on extended settling times or serving as the final step in sewage treatment, which would usually incorporate additional processes after secondary treatment,