During the nineteenth and twentieth centuries, acute waterborne diseases such as cholera and typhoid fever prompted the need for the development of drinking water and wastewater treatment to improve water quality. The regulations that govern for our water treatment today are based on both health concerns and aesthetics.
The health concerns include two categories: acute effects such as gastroenteritis from microorganisms and chronic effects, including cancer, caused by disinfection by-products. Aesthetics concerns are hardness, color, turbidity, taste, and color.
Prior to the 1970s, many of the major waterways in the United States were degraded because of unchecked pollutants released into lakes and rivers. This led to the passage of two important pieces of water quality legislation: the Clean Water Act (CWA) in 1972 and the Safe Drinking Water Act (SWDA) in 1974. The CWA and the SDWA are complementary. The SDWA prescribes approaches for treating water that is used for human consumption. This water typically comes from groundwater, rivers, and lakes. The CWA, in turn, protects all U.S. bodies of water by prescribing water quality standards and limits for pollutant discharges into eventual drinking, fishing or recreational water sources. Both are necessary and are aimed to achieve the same goal, to ensure high-quality water for human consumption, use, and recreation.
The CWA regulates the discharge of pollutants, mostly from wastewater treatment plants into rivers, lakes, estuaries, and wetlands by means of permits and standards. The main objective is to protect the surface water at a level that is considered “swimmable and fishable.” In addition, the CWA established a separate standard, the National Pollution Discharge Elimination System (NPDES), which is administered by states. This serves as a mechanism managing for the quantity and quality of wastewater effluents for each individual discharger. Because each watershed is different and has different functions for the local populations, states make determinations about the level of protection that is appropriate for the specific ecological condition, water use and pollution. For example, waterways that are used as reservoirs for drinking water will have more regulations than those used for shipping and transport.
The SDWA, enacted in 1974 and amended in 1986 and 1996, establishes water quality standards for all public water systems that serve an average of 25 or more people daily. These standards include the national primary drinking water standards and the national secondary drinking water standards. (fed reg 301). Primary standards are based on the protection of human health, setting acceptable levels of contaminants allowed in drinking water. These are identified under two categories: enforceable maximum contaminant levels (MCLs) and nonenforceable maximum contaminant level goals (MCLGs). MCLs are the maximum levels of contaminants in water delivered to any user; MCLGs are the maximum level of contaminants in drinking water that have no known or anticipated adverse health effects.
Secondary standards include potential cosmetic effects, such as skin discoloration and laundry staining, and aesthetic effects such as taste, odor, and color.
Drinking water treatment plants that use surface water (lakes and rivers) as a source must also follow the Surface Water Treatment Rule (SWTR) to protect humans from the risk of waterborne infectious disease. The SWTR prescribes treatment technologies including some combination of disinfection and filtration to achieve a minimum of 99.99 percent of viruses killed or inactive; the maintenance of strict turbidity levels; and the absence of fecal coliform bacteria.
Turbidity is the measure of relative clarity of a liquid based on the amount of light scattered by material in the water when a light is shined through the water sample. The higher the intensity of scattered light, the higher the turbidity. Examples of material that causes water to be turbid include clay, silt, finely divided inorganic and organic matter particles, algae, soluble colored organic compounds, and plankton, bacteria and other microscopic organisms.
Coliform bacteria are a group of organisms found in soil, surface water and on plants. Fecal coliforms bacteria are a subgroup and are present in the intestines and feces of animals and humans. The presence of these bacteria indicate that the water is contaminated with the waste from animals or humans and has the potential to cause disease. Some strains of
, a subgroup of fecal bacteria found in warm-blooded animals, cause illness. Through disinfection, filtration and turbidity standards, SWTR works to manage this health threat.
Many treatment technologies are designed to imitate natural processes, with the goal of functioning in an even more efficient manner. Promoting and expanding the use of proven designs are generally good practice for water quality engineering in that if a technique works in one place, it will likely work elsewhere.
Water is delivered to the drinking water treatment plant through a pipe or aqueduct. The water passes through coarse screens to remove large objects such as logs and fish. A pump or gravity them pushes it through an aeration stage to saturate the water with oxygen and to remove gases. Next, chemicals are added and rapidly mixed such as alum or ferric chloride. These two compounds cause particles to coagulate so that they are easier to remove in the following. Activated carbon may also be added to remove taste- and odor-causing molecules. The next stage is flocculation where the water is gently mixed to promotes collisions between coagulated particles, allowing them to adhere and form larger particles. In the sedimentation stage, the coarse particles are separated by gravity. In addition, the water may pass through sand filtration units for the removal of fine particles. At this point, the activated carbon is removed. If activated carbon was not added earlier, the water may be passed through a fixed bed of granular activated carbon. The treated water now passes through one more stage of chemical addition – a disinfectant such as chlorine is added to the water to kill or inactivate pathogenic microorganisms. Fluoride may be added and chemicals to adjust the pH to help prevent corrosion of pipes. The following section describes these water treatment processes in more detail.