Satisfactory disposal of wastewater, whether by surface, subsurface methods or dilution, is dependent on its treatment prior to disposal.  Adequate treatment is necessary to prevent contamination of receiving waters to a degree which might interfere with their best or intended use, whether it be for water supply, recreation, or any other required purpose. 

Wastewater treatment consists of applying known technology to improve or upgrade the quality of a wastewater.  Usually wastewater treatment will involve collecting the wastewater in a central, segregated location (the Wastewater Treatment Plant) and subjecting the wastewater to various treatment processes.  Most often, since large volumes of wastewater are involved, treatment processes are carried out on continuously flowing wastewaters (continuous flow or "open" systems) rather than as "batch" or a series of periodic treatment processes in which treatment is carried out on parcels or "batches" of wastewaters.  While most wastewater treatment processes are continuous flow, certain operations, such as vacuum filtration, involving as it does, storage of sludge, the addition of chemicals, filtration and removal or disposal of the treated sludge, are routinely handled as periodic batch operations. 

Wastewater treatment, however, can also be organized or categorized by the nature of the treatment process operation being used;  for example, physical, chemical or biological.  Examples of these treatment steps are shown below.  A complete treatment system may consist of the application of a number of physical, chemical and biological processes to the wastewater. 

Some Physical, Chemical and BiologicalWastewater Treatment Methods

Physical

Sedimentation (Clarification) 
Screening 
Aeration 
Filtration 
Flotation and Skimming 
Degassification 
Equalization

Chemical

Chlorination 
Ozonation 
Neutralization 
Coagulation 
Adsorption 
Ion Exchange

Biological

Aerobic
Activated Sludge Treatment Methods 
Trickling Filtration 
Oxidation Ponds 
Lagoons 
Aerobic Digestion 

Anaerobic
Anaerobic Digestion 
Septic Tanks 
Lagoons