The concentration of organic matter in most organic wastewater is relatively low, which can be effectively removed by aerobic methods. During the process, a part of the organic matter is converted into carbon dioxide through microbial metabolism, and the other part enters the microorganisms. Anaerobic methods are effective for removal. In the process, organic matter is converted into methane, carbon dioxide and microbial cells. The design of bioreactor is an important link in the development of biological treatment process. The goal is to optimize the growth environment of microorganisms and maintain a high concentration of microorganisms. The aerobic process requires continuous oxygen supply to the reactor to support microbial respiration, while the anaerobic process must be free of oxygen because oxygen is highly toxic to the methanogens in the anaerobic process.

The microorganisms in the biological aerobic treatment can be suspended growth, such as activated sludge process, or fixed growth, such as biofilm process. In suspension growth, microorganisms form a dispersed suspension system in water, and the wastewater contacts the suspended microorganisms through mixing. In stationary growth, microorganisms are attached to solid surfaces such as stones or plastics, and wastewater is in contact with microorganisms when it flows over the biofilm surface in a thin film. The microorganisms in the biological anaerobic treatment can also be suspended growth or fixed growth. However, in the anaerobic filter, the wastewater flows into the filter bed in an upward flow, which can effectively discharge the gas generated in the filter bed.