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Water treatment: How does it work?

Bruce Donners

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Logo 1Water treatment or purification and decontamination of water varies as to the source and types of water. Municipal water, such as consist of surface and groundwater, and their treatment is different from industrial water. Municipal water is treated by public or private water utilities to make the water potable (safe to drink) and palatable (aesthetically pleasing) and to assure an adequate supply of water to meet society's needs at a reasonable cost. Except in extremely rare cases, the whole supply is treated to drinking water quality for three reasons: It is generally not possible to deliver water to more than one quality, it is difficult to control public access to water is not treated to drinking water quality, and a significant part of the treatment may be required even if the water is not intended for human consumption.

Raw (untreated) water drawn from either a surface water (such as a lake or stream) or from an underground aquifer (through wells). The water flows or is pumped to a central treatment facility. Large municipalities may use 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 usually consists of a network of pipes (water pipes) associated with the ground or elevated storage (reservoirs). Since it is withdrawn from surface water source is usually shielded by steel, usually about 1 in (2.54 cm) thick and about 2 in (5.08 cm) apart, to prevent large objects such as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 plant. Finer screens are sometimes used to remove leaves. If the water is very turbid (cloudy or muddy), it can be prepared in a large basin known as a pre-sedimentation basin to allow time for sand and larger particles of sludge to settle out.

All surface water has the potential to carry pathogenic (disease causing) microorganisms and must be disinfected before food. Since adequate disinfection can not be guaranteed in the presence of turbidity, it is first necessary to remove suspended particles causing the water to be turbid. This is achieved through a sequence of treatment processes which normally 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 surface of the particles (suspended solids) is in the water, thereby eliminating the repulsive forces between the particles and allows them to aggregate. Coagulation is usually dispersed in the water by rapid mixing.

Other chemicals can be added at the same time, even powdered activated charcoal (to absorb the taste and odor-causing chemicals or remove synthetic chemicals), oxidants such as chlorine, ozone, chlorine dioxide, or potassium permanganate (to initiate disinfection, to oxidize organic pollutants, to control taste and odor, or to oxidize inorganic impurities such as iron, manganese and sulfide), and acid or base (to control pH). Coagulated particles together to form large and rapidly solve “Floc" particles through flocculation, is achieved by gently stirring the water with paddles, turbines or impellers. This process usually takes 20 to 30 minutes. The flocculated water is then gently introduced into a sedimentation basin, where Floc particles are about two to four hours to solve. After sedimentation, the water is filtered, usually through 24-30 in (61-76 cm) of sand or anthracite, which has an effective diameter of about 0.02 in (0.5 mm).

When raw water is low in turbidity can be coagulated or flocculated water is taken directly to the filter, past sedimentation, this practice is referred to as direct filtration. When the water has been filtered, it can be satisfactorily disinfected. Disinfection is the elimination of pathogenic microorganisms from the water. It does not make the water completely sterile, but makes it safe to drink from a microbial point of view. Most treatment plants in the United States relies primarily on chlorine for disinfection. Some tools use ozone, chlorine dioxide, chloramines (formed by the chlorine and ammonia), or a combination of chemicals that are added at various points during treatment. There are important advantages and disadvantages of each of these chemicals, and the optimal choice for a particular water requires careful study and expert advice.

Chemical disinfectants react not only to microorganisms but also naturally occurring organic matter in the water, producing trace amounts of pollutants collectively as disinfection by-products (DBPs). The best known DBPs are trihalomethanes. Although DBPs are not known to be toxic at the concentrations found in drinking water, some known to be toxic at very high concentrations. Therefore, caution dictates that reasonable efforts be made to minimize their presence in drinking water. The most effective strategy for minimizing DBP formation is to avoid putting chemical disinfectant until the water is filtered and only add the amount necessary to achieve adequate disinfection. Some DBPs may be minimized by switching to a different disinfectants, but all chemical disinfectants form DBPs. Regardless of the chemical disinfectant is used, extreme caution must be exercised to ensure adequate disinfection, because the health risks associated with pathogenic microorganisms greatly outweigh those associated with DBPs.

There are a number of other processes that can be used to treat water, depending on the quality of source water and the desired quality of the treated water. Processes that can be used to treat either surface or groundwater are:

lime softening, which involves the addition of lime during rapid mixing to precipitate calcium and magnesium ions;

stabilization, to prevent corrosion and scale formation, usually by adjusting the pH or alkalinity of the water or by adding scale inhibitor;

activated carbon adsorption, to remove the taste and foul-smelling chemicals or synthetic organic contaminants, and

fluoride, increasing the concentration of fluoride to the optimal level for preventing tooth decay.
Compared to surface water, groundwater is relatively free of turbidity and pathogenic microorganisms, but they are more likely to contain unacceptable levels of dissolved gases (carbon dioxide, methane and hydrogen sulphide), hardness, iron and manganese, volatile organic compounds (VOCs) from chemical spills or improper waste disposal practices, and dissolved solids (salinity). High quality groundwater does not require filtration, but they are usually disinfected to protect against pollution 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 number of strict criteria. Ground water from shallow wells or along the river bank can be considered “under the influence of surface water", as they normally are required by law to be filtered and disinfected.

Hard ground can be treated with lime softening, as well as many hard surface or by ion exchange softening, where calcium and magnesium ions are exchanged for sodium ions as the water passes through a bed of ion-exchange resin. Ground water with high concentrations of dissolved gases or volatile is usually treated by air stripping, which is achieved by passing air over small drops of water so that the gases leaving the water and into the air. Many ground-about a quarter of those used for public water supply in the U. S. is contaminated with naturally occurring iron and manganese, which tend to dissolve into the groundwater in their chemically reduced forms in the absence of oxygen.

Iron and manganese are commonly removed by oxidation (achieved 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 lime softening. Groundwater high dissolved solids can be treated using reverse osmosis, where water is forced through a membrane under high pressure, leaving the salt behind. Membrane processes are changing rapidly and membranes that are suitable for removing hardness, dissolved organic matter and turbidity from both ground and surface water have been developed recently.

Further Reading: Water treatment


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