What is the purpose of wastewater treatment?
Wastewater treatment is done to purify wastewater to minimize its solids, organisms and chemicals.
Wastewater (sewage) treatment is done to purify wastewater although a typical municipal (domestic) wastewater contains 99.9% water. The remaining (0.1%) portion is composed solids, organisms, chemicals, etc. Wastewater treatment plants have three stages: primary,s econdary and tertiary (advanced) treatment.
Primary steps (such as screening and sedimentation units) are operated to remove pollutants by sedimentation, floatation and screening. Only physical processes (e.g. gravity) are used for this step.
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). 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.
If raw sewage has particular pollutants that cannot be removed by the primary and secondary treatment or the receiving water bods is sensitive to eutrophication problem, advanced treatment (tertiary treatment) is needed. In advanced treatment, the main goal is to remove nutrients, especially nitrogen and phosphorus, various toxic substances (such as heavy metals and orher synthetic hazardous wastes).
Depending on land availability, energy concerns, technological availability, design considerations, etc. the choice (units chosen) is almost limitless in wastewater treatment.
For industrial wastewater treatment, design should be made according to types of pollutants and receiving water sensitivity. As an example, I can provide details in one design used at fish canning industry in Turkey. Air floatation is used to treat such wastewater.