The aerobic treatment unit components — pretreatment tank, aeration chamber, air pump and settling chamber — work together to treat the water to a high quality. In the system, wastewater first enters the pretreatment tank or trash trap, which removes plastic objects and other solids that float or settle. Then it enters an aeration chamber, where oxygen supplied by an air pump allows aerobic organisms to live. Treatment in the aeration chamber is a biological process in which microbes eat the waste and their bodies transform it into nonpolluting material. The microbes change dissolved and solid pollutants into cell mass, nondegradable material and gases such as carbon dioxide, hydrogen and methane. It is important to maintain an active population of microbes in the system to break down solids. A variety of aerobic microorganisms living together in a mixed state can decompose many kinds of materials. The mixed state keeps the microorganisms and the solids suspended in the wastewater. Treated wastewater moves from the aeration chamber into a settling chamber or clarifier. The clarifier allows the cell mass and nondegradable materials to settle from the water before it leaves the treatment system. The separation of microbial cells from treated effluent is an important part of the process. Aerobic treatment processes greatly lower biochemical oxygen demand (BOD), a common measure of pollution, as well as reduce the suspended solids that do not settle to the bottom of the clarifier. This process also removes some of the nitrogen and reduces the number of disease-causing organisms in the waste.