Wastewater contains all manner of impurities. In most regions it will have physical, biological and chemical contaminants. Because of the complexity of the impurities, there are sizable number of treatments the fluid will go through before it can result in a sae fluid stream and solid waste. In the most advanced systems the fluid stream can even be converted into safe drinking water. The solid waste usually results in a combination of disposable waste and fertilizer. One of the first processes is the removal of suspended solids. This is solid waste that can clog the channel through which the wastewater is being channeled.

This is mechanical wastewater treatment usually involves sieving to trap larger solid objects and sedimentation to encourage settling by gravity. Chemicals can often be used to promote the sedimentation process. This process can help remove more than half of the suspended solid material in wastewater. The next stage involves the removal of dissolved organic material. Microbes are encouraged to consume the organic matter in the water, which they then convert into carbon dioxide, water and energy for their own reproduction. Secondary sedimentation is then carried out. This step allows for further removal of suspended solids.

The next step is the application of secondary treatments. This includes basic activated sludge process, the use of filtration systems and biological activity to further breakdown the organic matter not consumed before. The final stage involves the separation of effluent from the remaining solid waste. The effluent is often considered close to the quality of drinking water.  This tertiary stage is however rarely used in many parts of the world because of the massive investment in plants, energy and chemicals that must be applied to generate this quality of results. Another problem with the tertiary stage is that while chlorine is the best way to purify the water, it is expensive and is believed to be harmful at those levels.