Aerobic treatment is a biological wastewater treatment process that takes place in the presence of oxygen. Aerobic biomass converts organics in the wastewater into carbon dioxide and new biomass.

The first step in a wastewater treatment process utilizes primary or mechanical systems to remove large suspended and floating solids. In the next step, called secondary or biological wastewater treatment, high concentrations of naturally occurring bacteria are confined in treatment tankage and combined with protozoa and other microbes to form activated sludge. The activated sludge bacteria break down organic molecules into inert substances creating effluent water suitable for discharge to rivers, lakes or streams.

Aerobic treatment technologies can act as stand-alone systems for treating raw wastewater, or can be used to polish anaerobically pretreated wastewater to further remove biochemical oxygen demand (BOD) and total suspended solids (TSS). Aerobic technologies can also be used specifically as a biological nutrient removal system (BNR) to remove nitrogen and phosphorus.