Waste water treatment involves removal or recovery of waste water constituents harmful to any specific point of disposal and to the environment in general. Domestic, community and many sorts of industrial waste water are treated by biological purification processes, whereas many other industrial wastes need to be purified by physical/chemical treatment methods. For biological treatment processes, there are at least eight typical models, together with a number of conventional modifications for these models. However, because of the wide variety of parameters and specifications involved in each individual case, very careful investigations need to be conducted on any raw waste water. In addition, pilot studies are carried out to determine the necessary design parameters for plant engineering.