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What is the drinking water treatment process and how do water treatment plants make water safer? Public drinking water systems go through a rigorous process of cleaning and disinfecting water so communities consume safe drinking water. Public drinking water must meet EPA drinking water standards under the Safe Drinking Water Act (SDWA).
Most urban communities collect water from a natural water body in the catchment, whether a stream, river, or underground aquifer. The water collected may then be stored in a reservoir for some time. Unless it is already of very high quality, it then undergoes various water treatment processes.
These water treatment processes include removing any chemicals, organic substances or organisms that could be harmful to human health. The water is then delivered to the community through a network of mains and pipes called a distribution system. This article describes a brief history and the steps taken to disinfect water making it safe to consume.
The earliest water treatments were widely adopted by Europe in the eighteenth century. These treatments were based on filtering driven by the desire to remove the taste. While the importance of good drinking water in maintaining human health was recognized early in history, it took centuries before people understood that their senses alone were not adequate judges of water quality. In the second half of the nineteenth century, the nature of infectious disease was first recognized. Additionally, the ability of water supplies to transmit diseases, such as cholera and typhoid, was first demonstrated. After this, concerns about the quality of drinking water focused on disease-causing microorganisms (pathogens) in public water supplies.
Scientists discovered two things. First, visible cloudiness, or turbidity, made the water look unappealing. The turbidity was caused by particles in water that could harbor pathogens. Secondly, the water cloudiness could also indicate potential health risk. As a result, drinking water treatment systems were designed to reduce turbidity. Thus, removing pathogens that were causing typhoid, dysentery and cholera.
By the early twentieth century, better protection of water supplies from sewage pollution and simple but effective methods of water treatment (chlorination, sand filtration) had greatly reduced rates of waterborne disease in developed nations. Since then, scientists and engineers have been developing ways of processing water more quickly and effectively in a more controlled way, and at lower cost.
The processes and technologies used to remove contaminants from water and to improve and protect water quality are similar all around the world. Ozone application in drinking water treatment has been in use for over 100 years (see ozone use timeline). The choice of which water treatment to use depends on the characteristics of the water, the types of water quality problems likely to be present, and the costs of different treatments. The most widely applied water treatment technology, a combination of some or all of coagulation, flocculation and sedimentation, plus filtration, has been used routinely for water treatment since the early part of the twentieth century.
Coagulation is often the first step in a water treatment process. When particles are slow to settle or are non settling, chemicals (coagulants) with a positive charge such as aluminum are added to the water. These react with the unwanted particles to form larger particles, called floc. The larger size and weight of the flocs then causes them to settle rapidly.
Flocculation follows coagulation. When chemicals (coagulants) are added to the water, gentle mixing occurs to form larger particles called flocs. Additional chemicals may be added during this step to aid in forming the flocs. Both coagulation and flocculation are effective at removing fine, suspended particles that attract and hold bacteria and viruses to their surface. These first two steps can remove up to 99.9% of the bacteria and 99% of the viruses from water supplies. They also remove some of the organic matter that gathers as water travels across the landscape, from raindrop to river. However, certain taste and odor problems may remain.
As flocs are formed, the larger size and weight of the flocs cause the large particles to settle rapidly. These particles settle because they are heavier than the water. And some particles will spontaneously settle out from standing water. This is the sedimentation process.
Filtration occurs as the water passes through filters that help remove even smaller particles. One of the oldest and simplest processes used to treat water is to pass it through a bed of fine particles, generally sand. This process of sand filtration usually removes fine suspended solid matter as well as some other particles, such as larger microorganisms. Filters can also be made of layers of sand, gravel and charcoal. The development of new synthetic materials has led to a new range of filter materials and methods. Processes based on these new materials are used increasingly to treat water for urban and industrial purposes. In membrane filtration, water is filtered through tiny holes (pores) in a membrane wall rather than a bed of sand. The smaller the pore size, the more material is held by the membrane as the water passes through.
Of the different kinds of membrane filtration processes, micro filtration is the most widely used in water treatment, becoming increasingly popular for small-scale water treatment plants supplying smaller communities. This is because it is an effective treatment, is simpler to operate, and requires less constant operational control. Two other types of membrane filtration, with even smaller pores – ultrafiltration and nanofiltration – are not as widely used.
While coagulation, often combined with filtration, will remove most of the troublesome contaminants from water, these processes do not usually remove all the material dissolved in the water. If the water contains undesirable impurities, additional treatment such as adsorption and oxidation may be required.
Adsorption is a form of chemical filtration. It involves removing dissolved substances by chemically or physically binding them to the filter material. It’s quite different from the similarly sounding process of absorption. In water treatment, specialized adsorbent materials such as activated carbon and ion exchange resins are used to remove certain soluble contaminants from water.
One way of using activated carbon is to percolate water through a bed of carbon granules. Once the carbon is saturated with the contaminants, it needs to be replaced or regenerated by heating it to a high temperature. If water contamination occurs only occasionally, but can be detected by a regular monitoring program, a better approach is to add powdered activated carbon to a conventional coagulation/flocculation process when a problem arises. The saturated carbon is collected in the filters and then discarded with the normal sludge from the water treatment plant. This form of intermittent dosing is widely used where there are occasional problems with blue-green algal blooms, which can cause taste and odor problems, and can also be toxic.
Oxidation with chemicals such as ozone or chlorine dioxide, a common treatment technology in Europe, has appeared in Australia only recently. Strongly reactive chemicals such as ozone are used to disinfect water and to destroy soluble contaminants such as algal toxins, taste and odor compounds and, particularly in Europe, traces of pesticides.
Some water supplies can become acidic or alkaline by dissolving or reacting with the material they are in contact with. This can cause piping systems and hot water services to corrode and cause dissolved metals to appear in the water. For example, a common sign of copper corrosion is a bluish stain where a tap drips onto a surface. To prevent corrosion, many waters are chemically stabilized to a particular pH before distribution. This is done by adding lime and sometimes carbon dioxide.
Water is disinfected to kill any pathogens present in the water supply and to prevent them from regrowing in the distribution systems. Without disinfection, the risk from waterborne disease is increased. The two most common methods to kill microorganisms in the water supply are oxidation with chemicals such as chlorine or ozone or irradiation with ultra-violet (UV) radiation.
Spartan Environmental Technologies, LLC
PO Box 1270
Mentor, OH 44061-1270