The Drinking Water Treatment Process

By Rick Watson


There are records of humans treating water for thousands of years. However, scientists only began understanding the sources and impacts of drinking water contamination in the late 1800s. During this time the focus remained on visible contamination because germs and their relationship to disease were not understood until the 1880s when Louis Pasteur discovered how microscopic organisms could transmit disease through water. Until this time, treatment focused on improving the aesthetic qualities of drinking water (e.g., appearance, odor, taste, etc.) mostly by reducing suspended solids. Early documented treatment methods included filtering through charcoal, exposing to sunlight, boiling, straining and settling.

During the early to mid-1800s, sand filtration was beginning to be regularly used to remove particulate matter from water. Following Pasteur’s discoveries and a gradual increase in the understanding of disease-causing microorganisms in public water, drinking water quality began focusing on microbes in drinking water. During this time (early 1900s), scientists determined source water suspended matter also contained microbial contaminants causing typhoid, dysentery and cholera epidemics. The result was increased sand filtration use for United States drinking water.

Filtration was, and continues to be, an effective treatment for reducing turbidity. However, reducing transmission of waterborne disease through turbidity reduction is limited. During the early 1900s, we learned disinfectants, like chlorine, effectively reduce drinking water microbial contamination and waterborne disease outbreaks. Drinking water filtration and disinfection are considered one of the greatest, most effective actions improving public health and premature death of all time. These two drinking water treatment technologies remain the most commonly applied treatment processes. 

The Modern Water Treatment Process

The Center for Disease Control cites the following statistics for U.S. drinking water systems:

  • There are over 150,000 U.S. public water systems. Approximately 33.5 percent are community systems and 66.5 percent are noncommunity systems (e.g., private mobile home parks, subdivisions, parks, etc.).
  • Over 286 million Americans get their tap water from a community water system.
  • Eight percent of U.S. community water systems provide water to 82 percent of the U.S. population through large municipal water systems.
  • Although the majority of community water systems (78 percent) are supplied by ground water, more people (68 percent) are supplied by community water systems using surface water.

There are important distinctions between ground water and surface water treatment systems. Typically, since ground water systems do not contain significant suspended solids, they rarely require coagulation, flocculation, sedimentation or filtration (unless metals removal—primarily iron and manganese—is necessary). Often ground water systems need only disinfection and distribution. However, surface water systems are far more susceptible to everyday environmental impacts requiring solids and turbidity removal.

While filtration and disinfection remain the cornerstone of the modern water treatment process, nearly all modern water treatment plants incorporate additional treatment steps into their treatment process. Coagulation, flocculation, and sedimentation are commonly used before filtration. This process creates larger, heavier particles. These particles are removed by settling in a clarifier prior to filtration. The removal of much of the particulate matter using flocculation and sedimentation reduces filter loading, improves treatment efficiency, and reduces the cost of drinking water treatment.

Several water treatment technologies are typically used, in sequence, to make raw surface or ground water safe for human consumption prior to discharge from the treatment system and distribution to the water system’s customers. The following treatment steps are commonly used in drinking water treatment systems.


Surface water intakes commonly use screens to remove debris from raw water (weeds, twigs or sticks, fish or other aquatic life, etc.). 

Coagulation, Flocculation, and Sedimentation

During coagulation, positively charged chemicals are added to the raw water (aluminum sulphate and ferric sulphate are two of the most common) and react with alkalinity to form metal hydroxides of aluminum and iron. Small particles in the raw water attach to these coagulants. Flocculation gently mixes the water, forming larger, heavier particles. Sedimentation then separates solids from the water. Clarifiers collect settled solids for removal and disposal and decant clarified water for further treatment. 


Clear water from the top of the clarifier is discharged to filters. Filtration removes additional solids from the water the clarifier has not removed. Typically, surface water filters are comprised of several different media types and sizes. Materials commonly used for drinking water filtration are gravel, sand and granular activated carbon (GAC). Filtration helps remove chemicals, parasites, bacteria and viruses. When the water treatment plant does not include GAC in its filters, powdered activated carbon (PAC) is sometimes added during the coagulation, flocculation, and sedimentation process to remove organic contaminants or unpleasant taste or odor. 


Following filtration, the water is disinfected and then discharged to the distribution system. Effective filtration prior to disinfection is essential because excessive suspended solids interfere with the disinfection process and make it less effective. Chlorine addition prior to distribution is the most common form of public drinking water disinfection. While not true of all states, West Virginia requires all public water systems chlorinate their drinking water. Chlorine is highly effective, relatively safe, easy to dose and feed and inexpensive. Residual chlorine in the distribution system also prevents re-contamination between the treatment facility and the consumer. While chlorine can cause formation of disinfection byproducts, trihalomethanes (TTHM) and haloacetic acids (HAA5), adequate solids removal before chlorination, proper chlorine feed rates, and residual monitoring can minimize or eliminate this problem. 

Ultraviolet (UV) light and ozone are also sometimes used to disinfect drinking water. UV and ozone do have some advantages over chlorine. These disinfection processes do not form disinfection byproducts. They can also be more effective against chlorine resistant microorganisms, such as Cryptosporidium and Giardia, and some chemical contaminants (e.g., pesticides, industrial solvents and pharmaceuticals). Among the drawbacks to UV and ozone are cost and their inability to prevent regrowth or recontamination during storage and distribution. Also, determining if disinfection has been completely accomplished is not possible because UV or ozone do not leave a measurable residual. 

Less Common Water Treatment Processes


Aeration is sometimes used in the drinking water treatment process. When used, it is typically a pretreatment step. Where gases or volatile organic compounds (VOCs) are present in the raw water, aeration is an economical, efficient method used to remove these contaminants or reduce them to acceptable levels. Aeration can also, when combined with post-aeration sedimentation and pH adjustment where necessary, be used to oxidize and remove metals from drinking water. 


Increasingly, water treatment plants are using membrane ultrafiltration rather than traditional gravel/sand filters. Ultrafiltration was, and still is, more commonly used for industrial or pharmaceutical water treatment. However, because it effectively removes bacteria, Cryptosporidium, Giardia, and potentially viruses, it is being used more frequently for drinking water treatment. Although capital and maintenance costs of ultrafiltration have decreased in recent years, this technology remains more expensive to install and maintain than traditional gravel/sand filtration. When used for drinking water treatment these technologies are most commonly applied when it is necessary to remove high concentrations of sodium and chlorides or other compounds not easily precipitated and removed using sedimentation and traditional filtration.

Richard Watson has a Master’s of Science in Engineering and spent 45 years working in drinking water and wastewater as a regulator and a consultant. 


Center for Disease Control and Prevention

A Century of US Water Chlorination and Treatment: One of the the Greatest Public Health Achievements of the 20thCentury

November 26, 2012

UnitedStates Environmental Protection Agency

The History of Drinking Water Treatment

Office of Water (4606)


February 2000


Water Treatment: Our Essential Guide to Water Treatment Technology

April 23, 2019