The goal of water treatment, usually from surface sources such as lakes, reservoirs, or rivers, is to remove contaminants and organisms through a combination of biological, chemical, and physical processes to make it safe for drinking. Some of these occur in the natural environment, whereas others occur in engineered and constructed water treatment plants. The engineered processes usually mimic or build on natural processes.
History of Water Treatment
Water-treatment concepts underlying those used today were developed in Europe during the 1700s. An outbreak of cholera in London was linked to a sewage-contaminated drinking water well in 1854. John Snow was credited with this finding. At the point in which the United States began using chlorine to disinfect drinking water (1908), Europe was also using chlorine but exploring the possibility of employing ozone to treat drinking water. The U.S. Public Health Service developed the first drinking-water regulations in the United States in 1914. The U.S. Environmental Protection Agency (EPA) later assumed responsibility for this task when it was established in 1970. The Safe Drinking Water Act (SDWA) became law in 1974, and was significantly revised in 1986 and 1996. The revisions reflected improvements in analytical methods to detect contaminants at lower levels and improvements in automated monitoring used to evaluate treatment plant performance. The revisions also started to address the need to balance immediate (acute) risks versus long-term (chronic) risks. The need to disinfect water to kill pathogens to protect against acute illnesses, versus the formation of disinfection by-products and their chronic health effects is an example of this risk balance.
The United States has continued to examine water treatment practices in Europe, particularly water-quality standards established by the World Health Organization (WHO). Although there are some philosophical differences between the United States and Europe relating to the treatment of the distribution system and its operations, the United States has benefited from the European experience. One such philosophical difference is that the European water treatment community does not see the maintenance of a disinfectant residual to the end of the distribution system as a necessary public health protection measure. The United States drinking water community sees this as an important step to protect customers and the water system from bacteriological regrowth or recontamination. As the United States entered the twenty-first century, researchers were collaborating with scientists around the world to continuously improve water quality and treatment, and openly share their research findings.
Water Quality Regulations in the United States
The EPA, under the requirements of the SDWA, regulates drinking water in the United States. The EPA additionally regulates wastewater, but under the requirements of the Clean Water Act (CWA). Storm water and discharges into surface water are also regulated under the CWA.
The SDWA sets maximum contaminant levels (MCLs) and treatment techniques (TTs) that drinking water must meet to be considered safe for consumption. The list includes microorganisms, disinfectants and disinfection by-products, inorganic chemicals, organic chemicals, and radionuclides.
The Water Cycle
The requirements of the CWA and SDWA are different, but interrelated. Consider the water cycle and the water-use cycle. Water falls to the earth in the form of precipitation. It drains into rivers, lakes, and streams either naturally or via constructed storm-water-drainage systems. Industrial manufacturers and wastewater treatment plants discharge effluent from their processes into lakes and rivers. Under the CWA, these facilities have water-quality limits that their effluent must meet. These limits have been established to protect the water ecosystem and downstream users. Water suppliers withdraw water from lakes and rivers to be treated for human consumption and other uses. The water is treated and delivered to customers' taps through a system of pipes and storage facilities that make up the water distribution system. After the water is used, it is conveyed to a wastewater treatment plant and discharged back as effluent to a receiving water body. If the water is used outside, it either seeps into the ground or drains to a storm-water system, which may go to a treatment plant or directly to a river, lake, or another body of water. The cycle continues as the water flowing to the ocean evaporates, ultimately falling again as precipitation. See the illustration for a diagram of the water or hydrologic cycle.
Source Water Protection
Source water protection, often referred to as "watershed protection," is the reduction or prevention of water pollution at its source, represents a tradeoff between treatment plant construction and operation costs. This kind of protection is not always possible, but it has been very effectively implemented by several water systems. A water system that has access to a high-quality source may not need as extensive a treatment plant as a system with a poorer-quality source. This is especially true if a high-quality source, such as a reservoir in an isolated natural area, can be protected by limiting human activity close to that source. Water from such a source may not require the settling step, may involve fewer chemicals or smaller doses of them, or might be able to kill pathogens with strong disinfectants like ozone or ultraviolet light instead of providing filtration.
The Water-Treatment Process
Whether in the natural environment or a constructed water-treatment plant, there are several key processes that occur during water treatment: dilution, coagulation and flocculation, settling, filtration, disinfection, and other chemical treatments. The quality of the source water and the effectiveness of source-water protection and management have a direct bearing on the complexity of the treatment that is required. Source-water protection is the first step in water treatment, with the natural and engineered processes following. The processes in a water treatment plant are shown in the illustration.
Dilution. Prior to industrialization, the pollution of rivers and streams was not as significant a problem. Waste products were released into water bodies, but the quantity of such discharges was not as great as present-day levels. The receiving waters were large enough and the mixing or detention time was long enough that the contaminants were diluted to a level that reduced the amount of concern about risks. A common saying in the past was "the solution to pollution is dilution." This is not the most efficient treatment method because even small amounts of pollutants, such as some pesticides, can build up, or bioaccumulate, in body fat over time. It is also not the preferred approach because it sends the message that polluting the environment is an acceptable course of action.
Coagulation and Flocculation. Sometimes, the particles that need to be filtered out during water treatment are very small. This makes them less likely to settle out and less likely to be filtered out. Chemicals called coagulants and/or filter aids are added to the water and mixed in (flocculated) to make the fine particles stick together to form bigger particles that can better settle out or be filtered out more effectively. Depending on the microbial and chemical makeup of the water, different chemicals are used as coagulants. The purpose of these two steps is to improve the performance of the remaining treatment processes.
Settling. For facilities treating water that contains a lot of solids, settling or sedimentation is a common treatment step. The process slows the flow of the water in a pond or basin so heavier items can settle to the bottom. If the water is not sufficiently slowed down, these items are carried along to the next step in the process, which is not desirable. For plants treating very polluted raw water , settling may be used as the first step in the treatment plant (presedimentation) and again following the coagulation and flocculation steps.
Filtration. There are several methods of filtration used in water treatment. The selection of which type to use is generally a function of the raw water quality. As filtration implies, water flows through a material that removes particles, organisms, and/or contaminants. The material used is most often a granular medium such as sand, crushed anthracite coal, or activated carbon. Some facilities layer different types and sizes of media. Along with varying the size and type of filter media, facilities are also designed to operate at different flow rates through the filter media. Traditional filtration plants include slow sand filtration, high-rate filtration, and diatomaceous earth filtration.
Another type of filtration that was more widely used in the late 1990s and early 2000s is membrane filtration. It occurs by forcing water through a membrane barrier. A membrane is like a high-tech coffee filter. As water under pressure flows through the membrane, contaminants and organisms are captured on the membrane and not allowed to pass through. Membranes are not well suited to highly contaminated source waters because the solid materials clog up the membrane almost immediately. Membrane filtration is gaining use in the United States for special applications and in combination with other types of filtration.
Disinfection. Filtration and the steps prior to filtration focus on the physical removal of contaminants in the water. In addition to physical removal, it is still important to provide chemical disinfection. Disinfectants used include chlorine, chloramines (chlorine plus ammonia), ozone, ultraviolet light, and chlorine dioxide. Chlorine was first used in the United States in a water-treatment plant in 1908.
The advantage of chlorination is that it continues to kill bacteria as water moves through pipes to the tap. Its disadvantage is the possibility of disinfection by-products. Excess chlorine in water can combine with organic material in the water to form substances such as trihalomethanes, which can cause liver, kidney, or central nervous system problems, and are linked to an increased risk of cancer over a lifetime exposure.
Disinfection is needed to inactivate (kill) bacteria and viruses that make it through the physical removal (filtration) steps. Viruses and giardia are effectively killed by chlorine. Over time, scientists have found that some organisms such as Cryptosporidium are resistant to chlorine. Cryptosporidium rose to public attention in 1993 when it sickened over 400,000 people, killing a hundred, in Milwaukee, Wisconsin. Largely because of this scare, new or amended U.S. drinking-water regulations developed early in the twenty-first century that expanded water treatment requirements specifically to address Cryptosporidium. Although chlorine is not effective against Cryptosporidium, alternative disinfectants such as ozone and ultraviolet light do appear to be effective at killing it. In Europe, both of these disinfectants are often used without chlorination to kill bacteria in the water supply.
An amendment to the SDWA requires that all sources of potable water in the United States be filtered. In some locales throughout the nation, such as Boston and Seattle, reservoir water is essentially free of organic matter, and municipalities have been able to avoid filtration because they have extensive watershed protection and management programs in place.
Other Chemical Treatments. Chemicals are added to drinking water to adjust its hardness or softness, pH, and alkalinity. Water that is acidic is very corrosive to the pipes and materials with which it comes into contact. The addition of sodium hydroxide can reduce corrosivity and extend the service life of pipelines, storage tanks, and building plumbing systems. Pipes may also be coated with chemicals to prevent metals like copper from dissolving in the water. In addition, chemicals are used to reduce the leaching of lead from old lead pipes and lead-soldered copper supply pipes. Fluoride is frequently added to the water in many communities to improve the dental health of younger residents.
Groundwater Protection and Treatment
Wellhead protection is critical to preventing the contamination of ground-water supplies. Groundwater is pumped out of an aquifer, which is like a small underground lake surrounded by layers of rock and soil. Water from the surface flows through the rock and soil to get to the aquifer. The earth naturally provides filtration of microscopic pathogens. It does not always provide adequate protection against viruses or chemicals that are dumped on the ground. Groundwater typically contains higher concentrations of metals like iron and manganese because these metals occur naturally in the earth. Groundwater may also be much harder than surface water. Processes similar to those outlined above are also used to treat groundwater, except that the filtration steps are often focused on removing chemicals or metals rather than pathogens. Some groundwater supplies are not treated at all, while others may be filtered and disinfected. As with surface waters, the quality of the source dictates what treatment steps are required.
Regulatory Reporting and Public Education
Water systems in the United States submit reports each month to state or federal regulatory agencies, summarizing treatment-plant performance and sampling results. The majority of medium and large water systems in the United States have staff working twenty-four hours a day. If something were to go wrong at the plant, the plant operators have procedures that they would follow to shut down the plant, switch to alternate equipment, adjust chemical dosages, or collect additional samples. State and federal regulations specify when the water plant operator must notify the state or federal agency, and these requirements are built into the plant's procedures. The regulations also specify when the public must be notified. Orders to boil the water are usually jointly issued by the state health agency and the drinking-water system quickly after a problem has been discovered (most likely via telephone and radio). Public notices about problems with routine monitoring results or the failure to collect required samples would generally be distributed in the newspaper or via the water utility's annual water quality report (also called a consumer confidence report). The requirement that all water systems compile and distribute a user-friendly report began in 1998. This report provides an overview of the water-system activities and compliance with regulations for the year, as well as identifying ways that customers can get involved or acquire more information.
see also Agriculture; Cryptosporidiosis; Groundwater; Health, Human; Nonpoint Source Pollution; Snow, John; Wastewater Treatment; Water Pollution.
american water works association. (1999). water quality and treatment, a handbook of community water supplies, 5th edition. san francisco: mcgraw-hill.
american water works association and american society of civil engineers. (1998). water treatment plant design, 3rd edition. san francisco: mcgraw-hill.
peavy, howard s.; rowe, donald r.; and tchobanoglous, george. (1985). environmental engineering. mcgraw-hill series in water resources and environmental engineering. san francisco: mcgraw-hill.
symons, james m. (1992). plain talk about drinking water: answers to 101 important questions about the water you drink. boulder, co: american water works association.
u.s. environmental protection agency, office of water web site. available from http://www.epa.gov/ow.
Julie Hutchins Cairn
One of the problems in protecting drinking water is that by the time results of tests for E. coli or Cryptosporidium or even anthrax are known, an urban population can already be at risk. Inventors Gregory Quist and Hanno Ix are out to change that. They use laser beams to scan a flow of water; particles in the water scatter the light beam and each scatter pattern is different. A computer analyzes the pattern and provides continuous real-time identification of microorganisms. The system is being tested at a Los Angeles, California, water facility.
Water treatment—or the purification and sanitation of water—varies as to the source and kinds of water. Municipal waters, for example, consist of surface water and ground-water , and their treatment is to be distinguished from that of industrial water supplies.
Municipal water supplies are treated by public or private water utilities to make the water potable (safe to drink) and palatable (aesthetically pleasing) and to insure an adequate supply of water to meet the needs of the community at a reasonable cost. Except in exceedingly rare instances, the entire supply is treated to drinking water quality for three reasons: it is generally not feasible to supply water of more than one quality; it is difficult to control public access to water not treated to drinking water quality; and a substantial amount of treatment may be required even if the water is not intended for human consumption.
Raw (untreated) water is withdrawn from either a surface water supply (such as a lake or stream) or from an underground aquifer (by means of wells ). The water flows or is pumped to a central treatment facility. Large municipalities may utilize more than one source and may have more than one treatment facility. The treated water is then pumped under pressure into a distribution system, which typically consists of a network of pipes (water mains) interconnected with ground-level or elevated storage facilities (reservoirs).
As it is withdrawn from the source, surface water is usually screened through steel bars, typically about 1 in (2.54 cm) thick and about 2 in (5.08 cm) apart, to prevent large objects such as logs or fish from entering the treatment facility. Finer screens are sometimes employed to remove leaves. If the water is highly turbid (cloudy or muddy), it may be pretreated in a large basin known as a presedimentation basin to allow time for sand and larger silt particles to settle out.
All surface waters have the potential to carry pathogenic (disease-causing) microorganisms and must be disinfected prior to human consumption. Since the adequacy of disinfection cannot be assured in the presence of turbidity, it is first necessary to remove the suspended solids causing the water to be turbid. This is accomplished by a sequence of treatment processes that typically includes coagulation, flocculation, sedimentation , and filtration .
Coagulation is accomplished by adding chemical coagulants, usually aluminum or iron salts, to neutralize the negative charge on the surfaces of the particles (suspended solids) present in the water, thereby eliminating the repulsive forces between the particles and enabling them to aggregate. Coagulants are usually dispersed in the water by rapid mixing. Other chemicals may be added at the same time, including powdered activated carbon (to absorb taste- and odorcausing chemicals or to remove synthetic chemicals); chemical oxidants such as chlorine , ozone , chlorine dioxide, or potassium permanganate (to initiate disinfection, to oxidize organic contaminants, to control taste and odor, or to oxidize inorganic contaminants such as iron, manganese, and sulfide); and acid or base (to control pH ).
Coagulated particles are aggregated into large, rapidly settling "floc" particles by flocculation, accomplished by gently stirring the water using paddles, turbines, or impellers. This process typically takes 20 to 30 minutes. The flocculated water is then gently introduced into a sedimentation basin, where the floc particles are given about two to four hours to settle out. After sedimentation, the water is filtered, most commonly through 24–30 in (61–76 cm) of sand or anthracite having an effective diameter of about 0.02 in (0.5 mm). When the raw water is low in turbidity, coagulated or flocculated water may be taken directly to the filters , bypassing sedimentation; this practice is referred to as direct filtration.
Once the water has been filtered, it can be satisfactorily disinfected. Disinfection is the elimination of pathogenic microorganisms from the water. It does not render the water completely sterile but does make it safe to drink from a microbial standpoint. Most water treatment plants in the United States rely primarily on chlorine for disinfection. Some utilities use ozone, chlorine dioxide, chloramines (formed from chlorine and ammonia), or a combination of chemicals added at different points during treatment. There are important advantages and disadvantages associated with each of these chemicals, and the optimum choice for a particular water requires careful study and expert advice.
Chemical disinfectants react not only with microorganisms but also with naturally occurring organic matter present in the water, producing trace amounts of contaminants collectively referred to as disinfection byproducts (DBPs). The most well-known DBPs are the trihalomethanes . Although DBPs are not known to be toxic at the concentrations found in drinking water, some are known to be toxic at much higher concentrations. Therefore, prudence dictates that reasonable efforts be made to minimize their presence in drinking water.
The most effective strategy for minimizing DBP formation is to avoid adding chemical disinfectants until the water has been filtered and to add only the amount required to achieve adequate disinfection. Some DBPs can be minimized by changing to another disinfectant, but all chemical disinfectants form DBPs. Regardless of which chemical disinfectant is used, great care must be exercised to ensure adequate disinfection, since the health risks associated with pathogenic microorganisms greatly outweigh those associated with DBPs.
There are a number of other processes that may be employed to treat water, depending on the quality of the source water and the desired quality of the treated water. Processes that may be used to treat either surface water or groundwater include: 1) lime softening, which involves the addition of lime during rapid mixing to precipitate calcium and magnesium ions; 2) stabilization, to prevent corrosion and scale formation, usually by adjusting the pH or alkalinity of the water or by adding scale inhibitors; 3) activated carbon adsorption , to remove taste- and odor-causing chemicals or synthetic organic contaminants; and 4) fluoridation , to increase the concentration of fluoride to the optimum level for the prevention of dental cavities.
Compared to surface waters, groundwaters are relatively free of turbidity and pathogenic microorganisms, but they are more likely to contain unacceptable levels of dissolved gases (carbon dioxide , methane , and hydrogen sulfide), hardness, iron and manganese, volatile organic compounds (VOCs) originating from chemical spills or improper waste disposal practices, and dissolved solids (salinity ).
High-quality groundwaters do not require filtration, but they are usually disinfected to protect against contamination of the water as it passes through the distribution system. Small systems are sometimes exempted from disinfection requirements if they are able to meet a set of strict criteria. Groundwaters withdrawn from shallow wells or along riverbanks may be deemed to be "under the influence of surface water," in which case they are normally required by law to be filtered and disinfected.
Hard groundwaters may be treated by lime softening, as are many hard surface waters, or by ion exchange softening, in which calcium and magnesium ions are exchanged for sodium ions as the water passes through a bed of ion-exchange resin. Groundwaters having high levels of dissolved gases or VOCs are commonly treated by air stripping, achieved by passing air over small droplets of water to allow the gases to leave the water and enter the air.
Many groundwaters—approximately one quarter of those used for public water supply in the United States—are contaminated with naturally occurring iron and manganese, which tend to dissolve into groundwater in their chemically reduced forms in the absence of oxygen. Iron and manganese are most commonly removed by oxidation (accomplished by aeration or by adding a chemical oxidant, such as chlorine or potassium permanganate) followed by sedimentation and filtration; by filtration through an adsorptive media; or by lime softening.
Groundwaters high in dissolved solids may be treated using reverse osmosis, in which water is forced through a membrane under high pressure, leaving the salt behind. Membrane processes are rapidly evolving, and membranes suitable for removing hardness, dissolved organic matter, and turbidity from both ground and surface waters have recently been developed.
Industrial water treatment differs from municipal treatment in the specified quality of the treated water. Many industries use water supplied by a local municipality, while others secure their own source of water. Those securing water from a private source often treat it using the same processes used by municipalities. However, industries must often provide additional treatment to provide water suitable for their special needs, which may include process water, boiler feed water, or cooling water.
Process water is water used by an industry in a particular process or for a group of processes. The quality of water required depends on the nature of the process. For example, water used to make white paper must be free of color. In some instances, water of relatively poor quality may be acceptable, e.g., water used to granulate steelmaking slags, while other uses require water of the very highest purity, e.g., ultrapure water used in the manufacture of silicon chips.
Water used in boilers for thermoelectric or nuclear power generation must be very low in dissolved solids and must be treated to prevent both corrosion and scale formation in the boilers, turbines, and condensers. High purity process waters and boiler feed waters are typically produced using ion-exchange demineralization and special filters designed to remove sub-micron sized particles.
Cooling water may be used once (single-pass system) or many times (closed-loop system). Cooling water used only once may receive little treatment, typically continuous or intermittent disinfection to reduce slime growths and perhaps stabilization to control corrosion and scale formation. Water entering a closed-loop system may not only be stabilized and disinfected, but may also be treated to remove nutrients (nitrogen and phosphorus ) or dissolved solids. Additional treatment is usually provided in the loop to remove dissolved solids that accumulate as a result of evaporation.
See also Safe Drinking Water Act
[Stephen J. Randtke ]
Peavy, H. S., D. R. Rowe, and G. Tchobanoglous. Environmental Engineering. New York: McGraw-Hill, 1985.
Water Treatment Plant Design. 2nd ed. New York: McGraw-Hill, 1990.
"Recommended Standards for Water Works." Great Lakes Upper Mississippi River Board of State Public Health and Environmental Managers. Albany, NY: Health Education Services, 1992.
Water is treated to make it safe to drink and to use for other purposes, such as to spray on agricultural plants. Water that contains domestic and industrial waste is often required to be treated to lessen or remove the contaminants prior to the discharge of the water into a river, lake , or ocean .
Some industrial processes require water that is free of impurities and microorganisms . One example is the water used in the manufacture of pharmaceuticals. The preparation of a medicine using contaminated water could be disastrous for the patient.
The need for treatment of drinking water is becoming more urgent, even in developed countries. The increasing populations of developing countries are encroaching more on previously undisturbed watersheds. As the watershed quality deteriorates, the ability of the watershed to naturally purify the water flowing through it is lessened. As well, the increasing use of chemicals is contaminating groundwater . Watersheds that were pristine only a few decades ago are now under threat.
The treatment of water for drinking is also referred to as water purification. Purification typically involves several steps. These are designed to remove objects from the water, particularly if the water is from a surface source like a river or a lake, and also to treat the water to minimize the risk from microorganisms.
The physical removal of objects like sticks and leaves is the first step in drinking water treatment. The water is filtered and then passed into a settling tank. As the name implies, the tank allows sand and grit to settle out on the bottom. Even smaller material is next removed in a step called coagulation. Here, a chemical called alum is added. The alum forms globs that attach to bacteria , silt, and other materials. The globs subsequently sink to the bottom of the holding tank.
Water can then be treated in several ways. It can be pumped through a filter that has much smaller holes in it than the filter designed to remove large objects. The holes or pores of the filter are so small that particles as small as viruses, bacteria, and protozoa cannot pass through to the other side of the filter. The filtration is intended to mimic the movement of water down from the surface through the soil and rock layers.
Filtration has become an important way to clear water of protozoa such as Cryptosporidium and Giardia. These organisms, which are typical residents of wild animals like the beaver, are resistant to the traditional chemical treatment of water. Chemical treatment utilizes chlorine to kill susceptible microorganisms. The process of killing the microbes is referred to as disinfection.
Chlorination disinfection has the advantage that a residual amount of the chemical remains in the water as the water passes through the pipelines on its way to the tap. This property, and the efficiency of killing by chlorine and chlorine-containing compounds, has made chlorination the most popular drinking water treatment method for over 50 years. However, the method is not without drawbacks. In particular, chlorine by-products can form in the presence of organic material. These byproducts, which are known as trihalomethanes, have been linked to health problems in humans. There is concern that the long-term ingestion of trihalomethanes can be harmful to health. Increasingly, the use of chlorine dioxide, which does not form trihalomethanes, or alternatives to chlorination, either alone or as secondary treatment that permit the chlorine concentration to be lowered, are being used.
Other means of disinfection that are becoming increasingly popular include the use of ozone and ultraviolet light . Home-based ultraviolet systems that sterilize the water just prior to the tap are becoming popular.
Wastewater treatment includes domestic and industrial waters. Domestic water commonly includes water flushed down toilets and the "gray" water from bathing and dish washing. Industrial water is water that has been used in production processes. Such water can contain chemicals that are toxic or foul smelling. Processing of domestic and industrial wastewater is necessary to remove the noxious compounds and microorganisms, or reduce the amounts of these items to acceptable levels, before the water is discharged into another body of water. Increasingly, the treatment of wastewater is a legal requirement.
Like the treatment of drinking water, wastewater treatment is a multi-stage process. Initially, a pre-treatment step filters out or grinds up objects such as sticks, rags and bottles that would clog equipment further on in the process.
The primary treatment step allows materials to either settle to the bottom or, in the case of liquids such as grease or oil that do not mix with water, to float to the surface. The surface waste is skimmed off. The clarified water passes on to the secondary treatment.
Secondary treatment uses microorganisms to digest organic material in the water. This can be done in one of three ways. The first method is called the fixed film system. This was developed in the mid-nineteenth century. The film is a film of microorganisms that has grown on rocks , sand, or plastic. In the case of a film on a flat support such as a plastic sheet, as in a typical domestic septic field, the wastewater can be flowed over the microbial film. As the water slowly passes over the film, the bacteria in the film digest the impurities in the water. Alternatively, the fixed film can be positioned on an arm, which can slowly sweep through the wastewater.
A third version of secondary treatment is called the suspended film. Microorganisms are suspended in the wastewater. Over time , the microbes clump together and settle out as sludge. The sludge can then be removed. Some of the sludge is added back to the wastewater to keep the digestion process going. This cycle can be repeated on the same volume of water, in order to digest most of the impurities.
The sludge that is collected can be subsequently used as compost, or can be digested by the bacteria, which produce methane that can be collected for use as a fuel and power source.
A forth version of secondary treatment is a lagoon. Wastewater is added to a lagoon and the sewage is degraded over the next few months. The algae and bacteria that are normal residents of the lagoon will use compounds such as phosphorus and nitrogen as food sources. Bacteria will produce carbon dioxide that is used by algae. The resulting algal activity produces oxygen that stimulates growth of the bacteria. This cycle of microbiological activity can continue until the organic matter in the water is consumed.
The final treatment step removes or neutralizes bacteria and other microorganisms. This step involves the use of a disinfectant like chlorine, or the use of filters, ozone, or ultraviolet light. Neutralization of the disinfectant chemical might be necessary prior to the flow of the treated water into a river, stream, lake, or other body of water. For example, chlorine can be removed by a reaction with sulfur dioxide .
Within the past several decades, the use of treatments that rely on the presence of living material such as plants to treat wastewater has become more popular. These systems, which are known as "living machines," can produce water that meets the requirements of purity for drinking water.
American Water Works Association. Water Quality and Treatment. 5th ed. Denver: American Water Works Association, 1999.
Droste, R. L. Theory and Practice of Water and Wastewater Treatment. New York: John Wiley & Sons, 1996.
Water is treated to make it safe to drink and to use for other purposes, such as to spray on agricultural plants. Water that contains domestic and industrial waste is often required to be treated to lessen or remove the contaminants prior to the discharge of the water into a river, lake, or ocean.
Some industrial processes require water that is free of impurities and microorganisms. One example is the water used in the manufacture of pharmaceuticals. The preparation of a medicine using contaminated water could be disastrous for the patient.
The need for treatment of drinking water is becoming more urgent, even in developed countries. The increasing populations of developing countries are encroaching more on previously undisturbed watersheds. As the watershed quality deteriorates, the ability of the watershed to naturally purify the water flowing through it is lessened. As well, the increasing use of chemicals is contaminating ground-water. Watersheds that were pristine only a few decades ago are now under threat.
Wastewater treatment is typically a multi-stage process. Typically, the first step is known as the preliminary treatment. This step removes or grinds up large material that would otherwise clog up the tanks and equipment further on in the treatment process. Large matter can be retained by screens or ground up by passage through a grinder. Examples of items that are removed at this stage are rags, sand, plastic objects, and sticks.
The next step is known as primary treatment. The wastewater is held for a period of time in a tank. Solids in the water settle out while grease, which does not mix with water, floats to the surface. Skimmers can pass along the top and bottom of the holding tank to remove the solids and the grease. The clarified water passes to the next treatment stage, which is known as secondary treatment.
During secondary treatment, the action of micro-organisms comes into play. There are three versions of secondary treatment. One version, which was developed in the mid-nineteenth century, is called the fixed film system. The fixed film in such a system is a film of microorganisms that has developed on a support such as rocks, sand, or plastic. If the film is in the form of a sheet, the wastewater can be overlaid on the fixed film. The domestic septic system represents such a type of fixed film. Alternatively, the sheets can be positioned on a rotating arm, which can slowly sweep the microbial films through the tank of wastewater. The micro-organisms are able to extract organic and inorganic material from the wastewater to use as nutrients for growth and reproduction. As the microbial film thickens and matures, the metabolic activity of the film increases. In this way, much of the organic and inorganic load in the wastewater can be removed.
Another version of secondary treatment is called the suspended film. Instead of being fixed on a support, microorganisms are suspended in the wastewater. As the microbes acquire nutrients and grow, they form aggregates that settle out. The settled material is referred to as sludge. The sludge can be scrapped up and removed. As well, some of the sludge is added back to the wastewater. This is analogous to inoculating growth media with microorganisms. The microbes in the sludge now have a source of nutrients to support more growth, which further depletes the wastewater of the organic waste. This cycle can be repeated a number of times on the same volume of water.
Sludge can be digested and the methane that has been formed by bacterial fermentation can be collected. Burning of the methane can be used to produce electricity. The sludge can also be dried and processed for use as compost.
A third version of secondary treatment utilizes a specially constructed lagoon. Wastewater is added to a lagoon and the sewage is naturally degraded over the course of a few months. The algae and bacteria in the lagoon consume nutrients such as phosphorus and nitrogen. Bacterial activity produces carbon dioxide. Algae can utilize this gas, and the resulting algal activity produces oxygen that fuels bacterial activity. A cycle of microbiological activity is established.
Bacteria and other microorganisms are removed from the wastewater during the last treatment step. Basically, the final treatment involves the addition of disinfectants, such as chlorine compounds or ozone, to the water, passage of the water past ultraviolet lamps, or passage of the water under pressure through membranes whose very small pore size impedes the passage of the microbes. In the case of ultraviolet irradiation, the wavelength of the lamplight is lethally disruptive to the genetic material of the microorganisms. In the case of disinfectants, neutralization of the high concentration of the chemical might be necessary prior to discharge of the treated water to a river, stream, lake, or other body of water. For example, chlorinated water can be treated with sulfur dioxide.
Chlorination remains the standard method for the final treatment of wastewater. However, the use of the other systems is becoming more popular. Ozone treatment is popular in Europe, and membrane-based or ultraviolet treatments are increasingly used as a supplement to chlorination.
Within the past several decades, the use of sequential treatments that rely on the presence of living material such as plants to treat wastewater by filtration or metabolic use of the pollutants has become more popular. These systems have been popularly dubbed “living machines.” Restoration of wastewater to near drinking water quality is possible.
Wastewater treatment is usually subject to local and national standards of operational performance and quality in order to ensure that the treated water is of sufficient quality so as to pose no threat to aquatic life or settlements downstream that draw the water for drinking.
Glennon, Robert J. Water Follies: Groundwater Pumping And The Fate Of America’s Fresh Waters. Washington, DC: Island Press, 2004.
MHW. Water Treatment: Principles and Design. New York: Wiley, 2005.
Russell, David L. Practical Wastewater Treatment. New York: Wiley-Interscience, 2006.
The goal of water treatment is to reduce or remove all contaminants that are present in the water. No water, irrespective of the original source, should be assumed to be completely free of contaminants. The most common process used for treatment of surface water and ground water consists of sedimentation, coagulation, filtration, disinfection, conditioning, softening, fluoridation, removal of tastes and odors, corrosion control, algae control, and aeration.
Sedimentation allows any coarse particles to settle out. Coagulation consists of forming flocculent particles in a liquid by adding a chemical such as alum; these particles then settle to the bottom. Filtration, as the name implies, is the passing of the water through a porous media; the amount of removal is a function of the filtering media. Disinfection kills most harmful organisms and pathogenic bacteria—chlorine is the most commonly used disinfecting agent. Softening means removal of materials that cause "hardness," such as calcium and magnesium. Corrosion is an electrochemical reaction in which metal deteriorates when it comes in contact with air, water, or soil.
In a typical municipal water treatment process, water flows through pumps to a rapid mix basin, then to a flocculation basin, to a settling basin, through filters to a clear well, then after disinfection, to storage tanks, and finally to the end users.
In areas that derive their water from rivers, pumps must be used since rivers are usually in low areas. Water enters the treatment plant at what is called the rapid-mix basin, where aluminum sulfate, polyelectrolytes, polymers, or lime and furic chloride are added as coagulants. The water flows next to the flocculation basins, where the coagulant mixes with the suspended solids. The coagulant is used to form suspended solids into clumps, or floc, which then settle out of the water. Floc forms when the particles from small solids gather to form larger particles. The water then slowly flows through settling basins where the floc settles from the water. Activated carbon is then added to the water to remove color, radioactivity, taste, and odor. Filtration then removes bacteria and turbidity from the water as it removes any remaining suspended solids and the activated carbon.
The water then enters a clear well, where additional chlorine is added to kill any pathogens which may be present. A minimum free-chlorine residual of at least 0.2 ppm is recommended in plants requiring sanitary protection through the whole water distribution system. In water supplies that are fluoridated, 1 milligram per liter of fluoride is added.
At this stage in the process, the water is potable, palatable, and ready for consumption. The water is moved into elevated tanks for storage through pumps. The water flows down from these tanks into the community.
Raw water and post-treatment water are tested for bacterial, physical, and chemical standards, particularly pH, color, and turbidity. The Safe Drinking Water Act of 1974 established maximum contaminant levels, which are the national drinking water standards. These apply to any water distribution system that serves at least twenty-five units daily. Standards may vary from state to state, but they cannot be lower than those prescribed by the federal government.
Mark G. Robson
(see also: Ambient Water Quality; Clean Water Act; Dissolved Solids; Drinking Water; Groundwater; Sanitation; Wastewater Treatment; Water Quality )
Koren, H., and Bisesi, M. (1997). Handbook of Environmental Health and Safety, Vol. II. Boca Raton, FL: Lewis Publishers.
Morgan, M. (1993). Environmental Health. Madison, WI: Brown & Benchmark.