Water treatment describes those industrial-scale processes used to make water more acceptable for a desired end-use. These can include use for drinking water, industry, medical and many other uses. Such processes may be contrasted with small-scale water sterilization practiced by campers and other people in wilderness areas. The goal of all water treatment process is to remove existing contaminants in the water, or reduce the concentration of such contaminants so the water becomes fit for its desired end-use. One such use is returning water that has been used back into the natural environment without adverse ecological impact.
The processes involved in treating water for drinking purpose may be solids separation using physical processes such as settling and filtration, and chemical processes such as disinfection and coagulation.
Potable water purification 
Water purification is the removal of contaminants from untreated water to produce drinking water that is pure enough for the most critical of its intended uses, usually for human consumption. Substances that are removed during the process of drinking water treatment include suspended solids, bacteria, algae, viruses, fungi, minerals such as iron, manganese and sulphur, and other chemical pollutants such as fertilisers.
Measures taken to ensure water quality not only relate to the treatment of the water, but to its conveyance and distribution after treatment as well. It is therefore common practice to have residual disinfectants in the treated water in order to kill any bacteriological contamination during distribution.
World Health Organisation (WHO) guidelines are generally followed throughout the world for drinking water quality requirements. In addition to the WHO guidelines, each country or territory or water supply body can have their own guidelines in order for consumers to have access to safe drinking water.
Processes for drinking water treatment 
A combination selected from the following processes is used for municipal drinking water treatment worldwide:
- Pre-chlorination - for algae control and arresting any biological growth
- Aeration - along with pre-chlorination for removal of dissolved iron and manganese
- Coagulation - for flocculation
- Coagulant aids, also known as polyelectrolytes - to improve coagulation and for thicker floc formation
- Sedimentation - for solids separation, that is, removal of suspended solids trapped in the floc
- Filtration - removing particles from water
- Desalination - Process of removing salt from the water
- Disinfection - for killing bacteria.
There is no unique solution (selection of processes) for any type of water. Also, it is difficult to standardise the solution in the form of processes for water from different sources. Treatability studies for each source of water in different seasons need to be carried out to arrive at most appropriate processes.
The above mentioned technologies are well developed, and generalised designs are available that are used by many water utilities (public or private). In addition to the generalised solutions, a number of private companies provide solutions by patenting their technologies. The developed world employs a considerable amount of automation for water and wastewater treatment. The developing nations worldwide use automation along with manual operations. The level of automation is a choice of operators. The aspects that govern the choice of level of automation are capital and operating costs, skills available locally, operators comfort, integration of automation & control with rest of the component of water supply and so on.
Sewage treatment 
Sewage treatment is the process that removes the majority of the contaminants from wastewater or sewage and produces both a liquid effluent suitable for disposal to the natural environment and a sludge. To be effective, sewage must be conveyed to a treatment plant by appropriate pipes and infrastructure and the process itself must be subject to regulation and controls. Some wastewaters require different and sometimes specialized treatment methods. At the simplest level, treatment of sewage and most wastewaters is carried out through separation of solids from liquids, usually by sedimentation. By progressively converting dissolved material into solids, usually a biological floc, which is then settled out, an effluent stream of increasing purity is produced.
In developing countries 
As of 2006, waterborne diseases are estimated to have caused 1.8 million deaths each year. These deaths are attributable to inadequate public sanitation systems and in these cases, proper sewerage (or other options as small-scale wastewater treatment) need to be installed.
Appropriate technology options in water treatment include both community-scale and household-scale point-of-use (POU) designs. Military surplus water treatment units like the ERDLator are still seen in developing countries. Newer military style Reverse Osmosis Water Purification Units (ROWPU) are portable, self-contained water treatment plants are becoming more available for public use. 
In order for the decrease of waterborne diseases to have long term effects, water treatment programs implemented by research and development groups in developing countries must be sustainable by their citizens. This can ensure the efficiency of such programs after the departure of the research team as monitoring is difficult because of the remoteness of many locations.
Industrial water treatment 
Two of the main processes of industrial water treatment are boiler water treatment and cooling water treatment. A lack of proper water treatment can lead to the reaction of solids and bacteria within pipe work and boiler housing. Steam boilers can suffer from scale or corrosion when left untreated leading to weak and dangerous machinery, scale deposits can mean additional fuel is required to heat the same level of water because of the drop in efficiency. Poor quality dirty water can become a breeding ground for bacteria such as Legionella causing a risk to public health.
With the proper treatment, a significant proportion of industrial on-site wastewater might be reusable. This can save money in three ways: lower charges for lower water consumption, lower charges for the smaller volume of effluent water discharged and lower energy costs due to the recovery of heat in recycled wastewater.
Corrosion in low pressure boilers can be caused by dissolved oxygen, acidity and excessive alkalinity. Water treatment therefore should remove the dissolved oxygen and maintain the boiler water with the appropriate pH and alkalinity levels. Without effective water treatment, a cooling water system can suffer from scale formation, corrosion and fouling and may become a breeding ground for harmful bacteria such as those that cause Legionnaires' Disease. This reduces efficiency, shortens plant life and makes operations unreliable and unsafe.
See also 
- Agricultural wastewater treatment
- Carl Rogers Darnall
- Ecological sanitation
- Industrial wastewater treatment
- Peak water (water supply & demand)
- Pulsed-power water treatment
- Water pollution
- Water quality
- Water supply network
- US Environmental Protection Agency, Washington, DC (2004). "Primer for Municipal Waste water Treatment Systems." Document no. EPA 832-R-04-001.
- Metcalf & Eddy, Inc. (1972). Wastewater Engineering. McGraw-Hill Book Company. ISBN 0-07-041675-3.
- "Safe Water System", US Centers for Disease Control and Prevention, Atlanta, GA. Fact Sheet, World Water Forum 4 Update, June 2006.
- "Household Water Treatment Guide", Centre for Affordable Water and Sanitation Technology, Canada, March 2008.
- Lindsten, Don C. (September 1984). "Technology transfer: Water purification, U.S. Army to the civilian community". The Journal of Technology Transfer 9 (1): 57–59. doi:10.1007/BF02189057.
- Water Treatment and the need for Boiler and Cooling Water Treatment.[better source needed]
Further reading 
- "Water management technologies: The high tide of hi-tech." TLVInsider. Issue 3, Israel H2O, November 2009.
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- International Water Association Professional / research organization
- Nanotechnology-enabled Water Treatment-Project NeWT, Center for Biological and Environmental Nanotechnology (CBEN), Rice University
- NSF International - Independent non-profit standards organization
- Transnational Ecological Project - Industrial wastewater treatment (Russia)
- Water Environment Federation - Professional association focusing on wastewater treatment
- WHO.int, WHO Guidelines
- Safe and Sustainable Water for Haiti web site hosted by Grand Valley State University