The bacteria use organic pollutants to grow and transform it to energy, water, carbon-dioxide and new cell material. The bacterium is present in form of flocs which are suspended and mixed with wastewater.

Activated sludge systems are suspended-growth type and are used in conventional high-tech wastewater treatment plants to treat almost every wastewater influent as long as it is biodegradable.

The typical F/M value considered for an extended ASP process is 0.1 g/g.d. This is a process parameter that is used to characterize operating conditions. Based on this F/M value sizing of aeration tanks for extended ASP is carried out.

Volume of tank = flow x retention time considered = 500 x 8 =4000 m³. The air requirement is 0.5 m³/hr/ m³ tank volume. Therefor air requirement considered is 4000 x 0.5 = 2000 m³/hr.

EPA regulations describe stabilization ponds as providing treatment equivalent to secondary treatment removing 65 percent of BOD and suspended solids from incoming sewage and discharging approximately 50 percent higher effluent concentrations than modern bioreactors.

Exposure of water droplets to air for 2 seconds will lower CO₂ by 70-90%. If CO₂ > 10mg/L aeration is recommended. Orwise, lime addition should be used to neutralize CO₂.

The cells need oxygen for ir metabolism, air is injected from bottom of aerator. The water is well agitated by rising bubbles and creates good contact between three ingredients: cells, sewage and oxygen.

Pre-treated sludge is treated in an activated sludge reactor. This helps to reduce a load of treatment on activated sludge process.

For conversion of particulate organics through ASP, Solid retention time considered would be 2-4 days. Particulate organic carbon is defined as those materials that can pass through a filter which is of size 0.7 to 0.22 um. This usually constitutes soil organic matter that includes plant material, pollen etc.

While designing an aeration tank, volumetric organic loading rate considered is 0.3-3. The volumetric organic loading rate is defined as amount of COD/BOD applied to aeration tank volume per day. It is expressed in kg BOD/COD m³d.