Water treatment

DEFINITION: Processing of raw water to make it safe for drinking

To be safe for drinking, water must be free of disease-producing bacteria, undesirable tastes and odors, color, turbidity, and harmful chemicals. The proper treatment of water supplies addresses all such possible problems and is thus essential to providing the safe drinking water that is necessary to human health and welfare.

Many substances may occur naturally in raw water that are either harmful or unpalatable to people. Human discharge of many substances into the environment also contaminates water supplies. Contaminants, either natural or anthropogenic (human-caused) in origin, can be divided into three groups: organoleptic substances that pertain to the senses of vision, taste, and odor; inorganic and organic chemical substances, which could be toxic or aesthetically undesirable, or could interfere with water treatment processes; and harmful microorganisms, which usually result from human and animal wastes.

The organoleptic parameters must be reduced to very low levels for drinking water to be acceptable for public use. Color, turbidity, and represent visual problems. Color results from that leaches from soil or decaying vegetation. Turbidity results from suspended clay or organic matter that imparts a muddy and therefore undesirable appearance to the water. Particulate matter floating in water is not only aesthetically undesirable but also may provide food for certain organisms. Decomposed organic material and volatile chemicals result in unpleasant tastes and odors in water.

Iron, manganese, and aluminum are metals that are commonly found in water. Other metals such as lead, copper, cadmium, and silver are occasionally present, as are the nonmetals nitrate, fluoride, and phenols. These chemicals have both natural and anthropogenic origins. Chemically synthesized compounds such as pesticides, herbicides, and polychlorinated biphenyls (PCBs) are particularly dangerous as they can enter the food chain and accumulate in animal tissue.

Although most bacteria are harmless and indeed essential to life, some varieties (pathogens) can cause illness and death. These waterborne diseases include cholera, typhoid, and bacillary dysentery, all of which are common in areas without properly treated water. Viruses are pathogenic organisms that are much smaller and much harder to control than bacteria. Common viral diseases include poliomyelitis and infectious hepatitis. Cryptosporidium and Giardia are protozoan waterborne parasites that are found in surface waters. They cause severe forms of gastroenteritis that can be deadly in people who have immune-suppressed systems, such as those living with acquired immunodeficiency syndrome (AIDS).

The origin and characteristics of the raw water source govern the type of treatment necessary to provide safe drinking water. For example, groundwater may require only pH adjustment and minor disinfection if the source is relatively pristine. In heavily fertilized agricultural areas and locations where soluble iron and manganese are naturally present, however, ion exchange for nitrate removal and chemical treatment for iron and manganese removal may be needed.

Surface water generally requires many more types of treatment, such as screening, sedimentation, chemical treatment, clarification, filtration, and disinfection. Installation of bar screens to block fish and debris is a standard first step in treating raw surface water. The screens must be strong enough to prevent wood, game fish, and even shopping carts from getting into the treatment plant and damaging the machinery. The next step is usually a sedimentation basin, where the larger suspended particles can settle out by gravity. This process may be accelerated through the mixing of chemicals with the water to form a flocculate precipitate, which helps settle the suspended particles. The chemical coagulation process removes natural color originating from peat, animal and vegetable debris, plankton, and other organic substances.

Even after sedimentation, some of the finer particles in the water may still be in suspension and have to be removed by filtration. Sand filters provide an inexpensive and effective medium for the removal of fine solids in either raw water or partially treated water. Many facilities have replaced sand filters with granular activated carbon (GAC) filters because these can remove a wide variety of undesirable organic compounds such as herbicides, pesticides, and chemical compounds that form naturally. They are also useful in the treatment of taste and odor. Indeed, many beverage manufacturers that make products (such as soda or beer) in which water is a major component use GAC filters. Residential point-of-use kitchen filters for drinking water incorporate GAC filters as the major treatment technique.

The final treatment process is disinfection, since pathogenic bacteria can pass through both the sedimentation basin and filtration. Government-set standards for drinking-water quality tend to require the absence of the indicator organisms fecal streptococci and the coliform group of bacteria, specifically fecal Escherichia coli, in the distributed water. Disinfection, which is the killing of harmful bacteria, is usually accomplished through chlorination. Chlorine is a very effective biocide, but one major disadvantage of its use is that it is very reactive and can produce compounds, such as trihalomethanes, that are potentially carcinogenic. Other compounds produced by chlorine have taste and odor problems. Ozone and ultraviolet light are also powerful disinfectants, but they do not have the residual properties of chlorine, which protects water from contamination as it travels through the distribution system.

Some water treatment plants built since the late twentieth century use a combination of ozonation for its effectiveness against Cryptosporidium, Giardia, and viruses; GAC filters for taste and odor control; and small amounts of chlorine as a residual biocide for the treated water in the distribution system.

In the twentieth and twenty-first century, the prevalence of artificial chemicals in the water supply, particularly per- and polyfluoroalkyl substances (PFAS), has led to a new suite of issues within the field of water treatment. Found in household items such as nonstick pans, food packaging, certain types of water resistant clothing, and also widely used in the manufacturing process, the ingestion of PFAS is harmful to humans and other living organisms; research efforts in the twenty-first century have shown that the primary source of exposure to PFAS is through drinking water sources. As a result, a number of processes have been introduced to the water treatment cycle to remove PFAS from drinking water. In a 2018 examination, the Environmental Protection Agency (EPA) found activated carbon treatment, a process that involves using activated carbon (a modified form of carbon with a large amount of surface area) to adsorb (physically and chemically bond to) PFAS, to be an extremely effective method of filtering PFAS from water. Other treatments for PFAS include ion exchange treatments that use positively charged ions to attract the negatively charged PFAS to form a chemically bonded resin that can be safely incinerated, as well as reverse osmosis and nanofiltration, both of which use permeable microscopic membranes to filter out PFAS from water.

The twenty-first century has also introduced new technologies that offer solutions to water decontamination at the household level. Point of use (POU) filters, which are water filters that are mounted directly to the faucet, have been found to be effective in filtering out water pollutants, particularly lead, without relying on local infrastructure. Whole house filtration systems also remained an option to conveniently filter water from every faucet at once.

Bibliography

Bosscher, Valerie, et al. “POU Water Filters Effectively Reduce Lead in Drinking Water: A Demonstration Field Study in Flint, Michigan.” Journal of Environmental Science and Health, Part A, vol. 54, no. 5, 2019, pp. 484–93, doi:10.1080/10934529.2019.1611141. Accessed 22 Feb. 2022.

De Silva, Amila O., et al. “PFAS Exposure Pathways for Humans and Wildlife: A Synthesis of Current Knowledge and Key Gaps in Understanding.” Environmental Toxicology and Chemistry, vol. 40, no. 3, 2021, pp. 631–57, doi:10.1002/etc.4935. Accessed 22 Feb. 2022.

Ford, Tim. “Water and Health.” Environmental Health: From Global to Local, edited by Howard Frumkin, 2nd ed., John Wiley & Sons, 2010.

"How Water Treatment Works." CDC, 6 Feb. 2024, www.cdc.gov/drinking-water/about/how-water-treatment-works.html. Accessed 12 Dec. 2024.

Manahan, Stanley E. “Water Treatment.” Fundamentals of Environmental Chemistry, 2d ed., CRC Press, 2001.

McKinney, Michael L., et al. “Water Pollution.” Environmental Science: Systems and Solutions, 4th ed., Jones and Bartlett, 2007.

"Reducing PFAS in Drinking Water with Treatment Technologies." United States Environmental Protection Agency, 23 Aug. 2018, www.epa.gov/sciencematters/reducing-pfas-drinking-water-treatment-technologies. Accessed 22 Feb. 2022.

Sullivan, Patrick J., et al. “Water Protection.” The Environmental Science of Drinking Water. Elsevier Butterworth-Heinemann, 2005.

"Water Treatment." WCS Group, www.wcs-group.co.uk/water-treatment-guide. Accessed 12 Dec. 2024.