Waste water treatment entails the removal or recovery of waste water constituents that are harmful to any specific point of disposal as well as the environment in general. Domestic, community, and many types of industrial waste water are treated using biological purification processes, whereas many other industrial wastes must be purified using physical/chemical treatment methods. There are at least eight typical models for biological treatment processes, as well as a number of standard modifications for these models. However, due to the wide range of parameters and specifications involved in each individual case, any raw waste water must be thoroughly investigated. Pilot studies are also conducted to determine the design parameters required for plant engineering.