What is the purpose of wastewater treatment?

Wastewater treatment plants have three stages: primary, secondary, and tertiary (advanced) treatment. Wastewater treatment is done to purify wastewater, even though a typical municipal (domestic) wastewater contains 99.9% water and the remaining (0.1%) portion is composed of solids, organisms, chemicals, etc.

Only physical processes (e.g., gravity) are used in primary steps (e.g., screening and sedimentation units) to remove pollutants by sedimentation, floatation, and screening.

Microbial growth is an important technique to operate secondary wastewater treatment units. In terms of mass balance, flow, substrate, and microbial mass must be known or quantified. These mass balances are interrelated. Substrate (expressed as mg/L BOD), VS (volatile solids), or VSS (volatile suspended solids) are measured to safely operate a bioreactor. Secondary units (or operations) include a biological/biochemical reactor (such as trickling filter and suspended growth tank), a secondary sedimentation tank, and a disinfection (mostly chlorination) unit. In this step, the main goal is to remove most of the BOD (biochemical oxygen demand). However, this step is ineffective to decrease nitrogen and phosphorus (these two are major contributors of eutrophication problem).

Advanced treatment, also known as tertiary treatment, is required if certain pollutants present in raw sewage cannot be eliminated by primary and secondary treatment, or if the receiving water body is susceptible to eutrophication. The primary objective of advanced treatment is to eliminate various toxic substances (such as heavy metals and other synthetic hazardous wastes) and nutrients, particularly nitrogen and phosphorus.

When it comes to wastewater treatment, the options (units selected) are nearly infinite, contingent upon factors such as land availability, energy concerns, technological accessibility, design considerations, etc.

I can give details about one design used at the fish canning industry in Turkey, where air floatation is used to treat wastewater of this kind. When designing systems for industrial wastewater treatment, consideration should be given to the types of pollutants and the sensitivity of the receiving water.

The purpose of wastewater treatment is to remove contaminants and pollutants from used water before it is released back into the environment. This process aims to protect human health, prevent the pollution of natural water bodies, and ensure sustainable water management.