Drinking Water Treatment

By Dr. Elizabeth C. Butler
School of Civil Engineering and Environmental Science
University of Oklahoma
Norman, OK  73019-0631


Overview of Regulations

Introduction.  In the United States, drinking water quality is regulated by the U.S. Environmental Protection Agency (EPA) under the Safe Drinking Water Act (SDWA).  Comprehensive information on various aspects of SDWA can be found on EPA’s website:  Under SDWA, Congress mandated EPA to establish standards that govern drinking water quality for public water supplies having at least 15 service connections or that serve at least 25 people.  These include both primary and secondary standards—primary standards are legally enforceable and based on health risks, and secondary standards are recommended guidelines for non-health-related water quality parameters related to aesthetics—for example color and odor.

Setting and complying with drinking water standards.  EPA’s first step in establishing drinking water standards is to establish Maximum Contaminant Level Goals (MCLGs), which are those concentrations in drinking water that are estimated to pose no threat to human health based on standard exposure assumptions (e.g., a 70 kg adult drinking two liters of water per day).  For carcinogens, MCLGs are typically equal to zero, and for non-carcinogens, MCLGs are typically equal to the reference dose (RfD).  EPA’s Integrated Risk Information System (IRIS) ( provides the toxicological data, including RfDs, with which MCLGs are set.  After establishing MCLGs, EPA considers various factors in setting primary drinking water standards, including best available technology and the costs of implementation.  Primary drinking water standards typically take the form of Maximum Contaminant Levels (MCLs). There are currently almost 100 primary drinking water standards that fall into the following categories: microorganisms, disinfectants, disinfectant byproducts, inorganics, organics, radionuclides (

Drinking water treatment utilities that serve the same customers year round are required under SDWA to provide to their customers annually a “Consumer Confidence Report” describing water quality monitoring results as they relate to SDWA rules.  In many Oklahoma cities, these Consumer confidence reports are posted online.  A partial list is below.

Stakeholder input is sought at various points in the SDWA rulemaking process via Federal Register notices that solicit public comments. Technical assistance is available to small drinking water systems for which complying with new standards may pose serious economic and/or technical challenges (EPA 2007).  While EPA is charged with setting drinking water standards, states and tribes are typically responsible for enforcing those standards.

Identifying candidate contaminants for regulation.  To identify contaminants for potential drinking water standards under SDWA, EPA has established three Candidate Contaminant Lists (CCLs), called CCL 1 (1998), CCL 2 (2005), and CCL 3 (2009), and is currently soliciting input for CCL 4.  Candidate contaminants are those that may occur in public drinking water supplies and may pose a health risk.  Details of the CCL process are described on EPA’s website (

State of Practice

Municipal water treatment plants are charged with treating water supplies to meet at least primary drinking water standards.  Consumers typically also desire/demand aesthetically pleasing water, which necessitates treatment for secondary drinking water standards as well.  Typically, the main function of municipal water treatment is disinfection. Particle removal may also be necessary for surface waters.  Removal of other constituents may be required based on surrounding land use and water quality, for example ground waters that have received industrial contamination may require treatment to remove volatile organics and/or metals, and ground or surface waters in agricultural or fertilizer intensive areas may require treatment for removal of nitrate and other nutrients. Communities may also have specific, naturally occurring or anthropogenic contaminants of concern that require specialized treatment, such as the high naturally occurring arsenic in some portions of the Garber Wellington aquifer that represents about 30% of the water supply in Norman, Oklahoma.

The sequential steps in drinking water treatment may include, depending on water quality and distance to the customer, screening, coagulation and flocculation, treatment with powdered activated carbon, filtration, and disinfection.  Technical details about these unit processes can be found in textbooks for scientists and engineers.  For the lay person, EPA has posted an informative virtual tour of a typical water treatment plant that describes these unit processes:

Technical Issues

Perhaps the greatest challenge facing water treatment utilities, which, unlike private business, may not have the option of easily raising their prices to consumers, is complying with drinking water standards in both an efficient and cost effective manner.  Furthermore, many chemicals on the current CCLs, some of which are likely to be regulated under SDWA in the future, are emerging contaminants that may be present in water supplies at ng/L to µg/L levels (parts per billion and parts per trillion).  These contaminants—for example pharmaceuticals and personal care products—typically require expensive and complicated analyses for detection (e.g., purge and trap gas chromatography/mass spectrometry with sample pre-concentration and/or derivitization), and are typically not effectively removed via the common water treatment practices of coagulation/flocculation, filtration, and disinfection.  Thus, if such contaminants are regulated under SDWA in the future, small utilities will require assistance in monitoring and treating these contaminants.  Coupled with this challenge are the water shortages facing many communities in the United States, which are driving increased focus on both potable and non-potable wastewater reuse, either before (grey water reuse) or after wastewater treatment.  Since many emerging contaminants are inefficiently removed by traditional wastewater treatment practices, municipalities will require advanced treatment for removal of such contaminants from wastewater and/or drinking water.


SDWA, a law passed by Congress, requires EPA to establish primary (legally enforceable) and secondary (not legally enforceable) drinking water standards for drinking water systems that have more than 15 connections or that serve more than 25 people.  States and tribes are generally responsible for enforcement of these standards.  Municipal water treatment facilities carry out the treatment processes, which almost always include at least disinfection, and, for surface waters coagulation/flocculation and filtration, to meet SDWA standards.  Customers can obtain information about their drinking water quality via the EPA-mandated Consumer Confidence Report.  As more contaminants from the CCLs are regulated under SDWA, communities will likely face increasing technical and financial challenges associated with water treatment to meet new drinking water standards.


EPA 2007, Restructuring and Consolidation of Small Drinking Water Systems: A Compendium of State Authorities, Statutes, and Regulations, Office of Water, Washington, DC, EPA 816-B-07-001, (accessed 5/29/12).



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