Water purification is the process of removing unwanted chemicals, biological contaminants, suspended solids and gases from the water. The goal is to produce water suitable for specific purposes. Most water is purified and disinfected for human consumption (drinking water), but water purification can also be performed 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 a variety of dissolved material and particulate matter.
Drinking water quality standards are usually 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 deal with all possible contaminants that may be present in the water from an unknown source. Spring water, considered safe for all practical purposes in the 19th century, also now needs to be tested before determining what type of treatment, if any, is needed. Chemical and microbiological analyzes, although expensive, are the only way to obtain the information needed to decide on the appropriate purification method.
According to a 2007 report from the World Health Organization (WHO), 1.1 billion people lack access to a better supply of safe 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. WHO estimates that 94% of these cases of diarrheal diseases can be prevented through changes in the environment, including access to clean water.  Simple techniques for treating water at home, such as chlorination, filters and solar disinfection, and for storing it in safe containers, could save a large number of lives every year.  Reducing deaths from waterborne diseases is an important public health goal in developing countries.