When a amount of water has into colloids, suspended solids, organic and inorganic pollutant, These all pollutant should be separated from the water, which called wastewater. For wastewater treatment has three steps ;

- Physical Treatment

- Chemical Treatment

- Biological Treatment

Biological Treatment ; method of this treatment is more different than others because this treatment consists microorganisms. Biological treatment systems are divided into aerobic and anaerobic. These systems consists various of treatment modification such as ;

- Activated Sludge Process,

- Oxidation Ditch

- Sequence batch reactor(SBR)

- Contact stabilization

- Aerated lagoons

- Rotating biodisc reactor

All of methodology of biological treatment, especially aerobic systems need dissolved oxygen in the wastewater. It can use aspirating aerator, surface aerator, blower, diffuser in order to increase dissolved oxygen in water. It is important to use equipment, which increase dissolved oxygen in water. Because logic of biological treatment systems consist microorganisms, who need dissolved oxygen. These equipment should be worked good quality for better treatment.