Most wastewaters have putrifying (rotting in due course) organic matter. Biological wastewater treatment systems are to covert th organic matter into easily manageable end products, such as carbon dioxide, methane and humus, which can be utilized or dispose off without affecting the environment. The microorganisms use the organic matter as food to provide energy and carbon fo cellular synthesis. Industrial fermentation uses aseptic techniques to maintain pure cultures and the environment is controlled. Biological wastewate treatment systems are only partially controlled. The wastewater (substrate or food) characteristics may change from time to time there are changes in temperature and there is always a heterogeneous inoculum of microorganisms from soil and air. This results i a variety of microorganisms participating in the wastewater biological reaction. The fittest survive and dominate the population When the compounds in wastewater are metabolized, intermediate compounds serve as food for other microorganisms. Th population of individual microorganisms and the community structure also changes from time to time reflecting the changes in environmental conditions. It is possible to zero in on groups of microorganisms participating in the wastewate biological process based on their overall biochemical reactions.