A standard wastewater treatment train would typically consist of a primary clarifier system to remove solid and floating materials, a secondary treatment system consisting of an aeration basin followed by flocculation and sedimentation or an activated sludge system and secondary clarifiers, a tertiary biological nitrogen removal system, and a final tertiary disinfections unit.

The aeration basin/activated sludge system removes organic material by growing bacteria (activated sludge). The secondary clarifier removes the activated sludge from the water. The tertiary system is becoming more prevalent to remove nitrogen and phosphorus and do final disinfections of the water prior to its discharge to a surface water stream or ocean outfall. Before reentering into a body of water, the wastewater gets treated under a multi-stage process then only the water gets renovated and it is used for application. The process is the removal of wastewater pollutants, organic matter, disease-causing organisms, solids and nutrients.

Most wastewater is treated in industrial-scale wastewater treatment plants, which may include physical, chemical and biological treatment processes. However, the use of septic tanks is widespread in rural areas, serving up to one quarter of the homes in the U.S. The most important aerobic treatment system is the activated sludge process, based on the maintenance and recirculation of a complex biomass composed by microorganisms able to degrade the organic matter carried in the wastewater.

Anaerobic processes are widely applied in the treatment of industrial wastewaters and biological sludge. Some wastewater may be highly treated and reused as reclaimed water. For some wastewaters ecological approaches using reed bed systems such as constructed wetlands may be appropriate. Modern systems include tertiary treatment by micro filtration or synthetic membranes. After membrane filtration, the treated wastewater is indistinguishable from waters of natural origin of drinking quality.