What is the key difference between primary and secondary sewage treatment

SouravAugust 19, 2024

Answered step-by-step

The key difference between primary and secondary sewage treatment lies in the level of treatment and the methods used to remove contaminants from wastewater.

Primary Sewage Treatment

• Objective: The main goal is to remove large solids and organic matter from sewage.
• Process: This stage involves physical processes such as screening and sedimentation. In screening, large objects (like sticks and plastic) are removed. In sedimentation, the sewage is allowed to settle in a tank, where heavier solids settle at the bottom (forming sludge) and lighter materials float to the top (forming scum).
• Outcome: It reduces the total suspended solids (TSS) and some organic material but does not significantly reduce dissolved substances or harmful microorganisms.

Secondary Sewage Treatment

• Objective: The goal is to further reduce organic matter and pollutants that remain after primary treatment, including those dissolved in the water.
• Process: This stage uses biological processes to degrade organic pollutants. It typically involves aerobic bacteria that consume the remaining organic matter. The two common methods are activated sludge (where air is supplied to promote bacterial growth) and trickling filters (where sewage is passed over microbial-covered media).
• Outcome: It significantly lowers the biochemical oxygen demand (BOD) and improves the overall quality of the treated water. Secondary treatment also reduces pathogens and dissolved organic materials, although it might not completely eliminate all contaminants.