Water purification is the process of removing unwanted chemicals, biological contaminants, suspended solids, and gases from water. The goal is to produce water suitable for specific purposes. Most of the water is purified and disinfected for human consumption (drinking water), but water purification can also be done for a variety of other purposes, including medical, pharmacological, chemical and industrial applications.
The methods used include physical processes such as filtration, sedimentation, and distillation; biological processes such as slow sand filters or biologically active carbon; chemical processes such as flocculation and chlorination; and the use of electromagnetic radiation such as ultraviolet light.
Water purification can reduce the concentration of particulates, including particulate matter, parasites, bacteria, algae, viruses, and fungi, as well as reduce the concentration of various dissolved substances and particulates.
Drinking water quality standards are generally set by governments or international standards. These standards generally include minimum and maximum concentrations of contaminants, depending on the intended use of the water.
Visual inspection cannot determine if the water is of adequate quality. Simple procedures like boiling or using a household activated carbon filter are not enough to treat all the possible contaminants that may be present in water from an unknown source. Even spring water, considered safe for all practical purposes in the 19th century, must now be tested before determining what kind of treatment, if any, is needed. Chemical and microbiological analyzes, although expensive, are the only way to obtain the information necessary to decide on the appropriate purification method.
According to a 2007 report by the World Health Organization (WHO), 1.1 billion people lack access to a better supply of drinking water; 88% of the 4 billion annual cases of diarrheal diseases are attributed to unsafe water and inadequate sanitation, while 1.8 million people die each year from diarrheal diseases. The WHO estimates that 94% of these cases of diarrhea can be prevented through changes in the environment, including access to safe water.  Simple techniques for treating water at home, such as chlorination, filters, and solar disinfection, and for storing it in safe containers, could save large numbers of lives each year.  Reducing deaths from waterborne diseases is an important public health goal in developing countries.
Groundwater – Water emerging from some deep groundwater may have fallen as rain many tens, hundreds, or thousands of years ago. Soil and rock layers naturally filter groundwater with a high degree of transparency and often do not require any additional treatment other than the addition of chlorine or chloramines as secondary disinfectants. Such water can arise as springs, artesian springs or it can be drawn from wells or wells. Deep groundwater is generally of very high bacteriological quality (i.e., there are generally no pathogenic bacteria or pathogenic protozoa), but the water can be rich in dissolved solids, particularly calcium and magnesium carbonates and sulfates. Depending on the layers the water has passed through, other ions may also be present, including chloride and bicarbonate. It may be necessary to reduce the iron or manganese content of this water to make it acceptable for drinking, cooking, and laundry. Primary disinfection may also be necessary. Where groundwater recharge is practiced (a process in which river water is injected into an aquifer to store water in times of abundance so that it is available in times of drought), the groundwater may require additional treatment as per regulations applicable state and federal.