Which treatment stage typically involves biological processes to reduce organic matter?

The correct answer is indeed secondary. This treatment stage plays a crucial role in wastewater management, specifically focusing on the reduction of organic matter through biological processes. During secondary treatment, microorganisms, such as bacteria, are employed to decompose organic pollutants present in the sewage. This process occurs in aeration tanks where the wastewater is mixed with air to help these microbes break down the waste effectively.

Secondary treatment is essential for achieving significant reductions in biochemical oxygen demand (BOD) and total suspended solids before the water proceeds to further stages or is discharged into the environment. The effectiveness of this stage largely depends on the type of biological processes used, such as activated sludge systems or trickling filters, which are optimized for maximizing microbial activity and efficiency.

Other stages, such as primary, tertiary, and preliminary treatment, have distinct functions in the overall wastewater treatment process. While primary treatment focuses on physical processes like sedimentation to remove larger solids, tertiary treatment aims at further polishing the effluent, often utilizing chemical processes for nutrient removal. Preliminary treatment, on the other hand, involves the initial screening and grit removal to protect subsequent treatment systems. Each stage is vital, but secondary treatment uniquely emphasizes biological decomposition to tackle organic matter in wastewater.