Water treatment processes have developed over the past century in an effort to minimize waterborne disease and transmission.  In the United States, techniques have been developed in response to local conditions with various objectives besides disinfection, including color reduction, turbidity removal, softening, taste and odor control, and corrosion control.

The impetus for water treatment finds its roots in the work of Dr. John Snow, who related a disease outbreak to a London water source in the mid-19th century.  Over the next few decades, a microbiological indicator for water quality was developed.  By the early 20th century, the United States Public Health Service (PHS) established bacteriological standards for drinking water.  By the mid-20th century, the PHS water quality standards were revised to include to various chemical constituents as well as bacteriological indicators.  This was significant since it extended the concern over water quality from waterborne disease to long-term toxicological (and eventually carcinogenic) effects from long-term ingestion.

By the 1970's, with the passage of the Safe Drinking Water Act (1974), the Environmental Protection Agency (EPA) was mandated to identify substances present in drinking water that had adverse public health affects.  As a result, national interim primary drinking water regulations were established by EPA in 1977.  In 1979, a group of chlorination by-products known as trihalomethanes (THM) were also regulated.  Over the next several years, continued health effects research augmented the toxicological and carcinogenic database for various compounds.  In 1986, with the passage of the amendments to the Safe Drinking Water Act, EPA was mandated specifically to regulate microbiological constituents, inorganic and organic compounds, and radioactivity to better safeguard public health. These new regulations dramatically increased the monitoring requirements for public water systems.  83 contaminants were to be regulated in the initial stage with an additional 25 contaminants to be added every three years.  In 1988, regulatory efforts focused on lead.  Lead-containing drinking water coolers were to be eliminated in schools.

The latest amendments to the Safe Drinking Water Act occurred in 1996.  The criteria for the selection and regulation of contaminants was a key amended item.  EPA no longer has to set 25 new standards every three years. Regulated contaminants either need to have adverse health effects or are present at levels sufficiently high to warrant public concern.  Development of regulatory levels are to be based on risk assessment, cost-benefit analysis, and minimizing overall risk.  The law also takes into consideration the problems of small systems. The best treatment technologies for smaller systems will be identified with affordability as a major criterion.  Small systems are also afforded more flexibility with respect to monitoring and can be granted variances if the system cannot afford to comply with the guidelines.  Other specific areas addressed include: disinfection by-products, filtration, groundwater disinfection, arsenic, sulfate, radon, and source water protection.