Unit 13Text: Water Treatment ProcessesOne of the great achievements of modern technology has been to drastically reduce the incidence of waterborne of diseases such as cholera and typhoid fever. These diseases are no longer the great risks to pubic health that they once were. The key to this advance was the recognition that contamination of pubic water supplies by human wastes was the main source ci infection, and that it could be eliminated by more effective water treatment and better waste disposal.Tod ay’s water treatment plants are designed to provide water continuously that meets drinking water standards at the tap. There are four main considerations involved in accomplishing this1 source selection, protection of water quality, treatment methods to be used, and prevention of recontamination. Common precautions to prevent groundwater and surface water pollution include prohibiting the discharge of sanitary and storm sewers close to the water reservoir, installing fences to prevent pollution from recreational uses of water, and restrictions on the application of fertilizers and pesticides in areas that drain to the reservoir.Screening, coagulation/flocculation, sedimentation, filtration. and disinfection are the main unit operations involved in the treatment of surface water. Water treatment operationa fulfill one or more of three key tasks1 removal of particulate substances such aa sand and clay, organic matter, bacteria, and algae; removal of dissolved substances such as those causing color and hardness; and removal or destruction of pathogenic bacteria and viruses. The actual selection of treatment processes depends on the type of water source and the desired water quality.Occasionally, raw water with low turbidity can be treated by plain sedimentation (no chemicals) to remove larger particles and then filtration to remove the few particles that failed to settle out. Usually, however, particles in the raw water are too small to be removed in a reasonably short time through sedimentation and simple filtration alone. To remedy this, a chemical is added to coagulate/flocculate the small particles, called colloids, into large ones, which can then be settled out in sedimentation tanks or removed directly in filters.Removal of Particulate MatterThe unit operations employed for the removal of particulate matter from water include screening, sedimentation, coagulation/flocculation, and filtration.Screening to remove large solids such as logs, branches, rags, and small fish is the first stage in the treatment of water. Allowing such debris into the treatment plant could damage pumps and clog pipes and channels. For the same reasons, water intakes are located below the surface of the lake or river in order to exclude floating objects and minimize physical damage from ice.Sedimentation, the oldest and moat widely used form of water and wastewater treatment, uses gravity settling to remove particles from water. It is relatively simple and inexpensive and can be implemented in basins that are round, square, or rectangular. As noted earlier, sedimentation may follow coagulation and flocculation (for highly turbid water) or be omitted entirely (with moderately turbid water) .Particulates suspended in surface water can range in size from 10-1 to 10-7mm in diameter, the size of fine sand and small clay particles, respectively. Turbidity or cloudiness in water is caused by those particles larger than 10 mm, while particles smaller than 10 mm contribute to the color and taste.Coagulation/flocculation is a chemical-physical procedure whereby particles too small for practical removal by plain sedimentation are destabilized sod clustered together for faster settling©. A significant percentage of particulates suspended in water are so small that settling to the bottom of a tank would take days or weeks. These colloidal particles would never settle by plain sedimentation.Coagulation is s chemical process used to destabilize colloidal particles. The exact mechanism is not well understood, but the general ides is to add a chemical which has positively charged colloids to water containing negatively charged colloids. This will neutralize the negative change on the colloids and thus reduce the tendency for the colloids to repel each other. Rapid mixing for a few seconds is required to disperse the coagulant. Gentle mixing, called flocculation, of the suspension is then undertaken to promote particle contact. This is achieved by mechanical mixing through the use of slowly rotating paddles inside the coagulation/flocculation tank, or by hydraulic mixing which occurs when flow is directed over and around baffles in the tank. Detention time in the coagulation/flocculation tank is usually between 20—40 minutes in tanks 3—4m deep. Through the combined chemical/physical process of coagulation/flocculation, the colloidal particles which would not settle out by plain sedimentation are agglomerated to form larger solids called floe. These appear as fluffy growths of irregular shape that are able to entrap small noncoagulaed particles when settling downward. Aluminum sulfate is the most common coagulant but organic polymers may also be used alone or in combination with alum to improve flocculation. The floe suspension is gently transferred from the coagulation/flocculation tanks to settling tanks, or directly to filters where the floes are removed.DisinfectionTo ensure that water is free of harmful bacteria it is necessary to disinfect it. Chlorination® is the most common method of disinfecting public water supplies. Sufficient quantities of chlorine from chlorine gas or hypochlorites are added to treated water to kill pathogenic bacteria. Chlorination is a reliable, relatively inexpensive, and easy disinfection method to use. Other disinfectants include chloramines, chlorine dioxide, other halogens, ozone, ultraviolet light, and high temperature. Ozonation, which has been used extensively in France, is now gaining acceptance in North America, especially as an alternative to prechlorination where natural organics are present. Although effective, ozone does not leave a lasting residual for long-term disinfection.Ozoanation is the disinfection of water by adding ozone, which is a powerful oxidant of inorganic and organic impurities. Its advantages over chlorine are that it leaves no tastes or odors, and unlike chlorine, it apparently does not react with natural organics to form compounds hazardous to humans.Removal of Dissolved SubstancesAeration is used to remove excessive amounts of iron and manganese from groundwater. These substances cause taste and color problems, interfere with laundering, stain plumbing fixtures, and promote the growth of iron bacteria in water mains. By bubbling air through water, or by creating contact between air and water by spraying, dissolved iron or manganese (Fe2+ , Mn2+) is oxidized to a less soluble form (Fe3+ , Mn4+), which precipitates out and can be removed in s settling tank or filter. Aeration also removes odors caused by hydrogen sulfide (H2S) gas.Softening of water is a process that removes hardness, caused by the presence of divalent metallic ions, principally Ca2+ and Mg2+. Hardness in water is the result of contact with soil and rock, particularly limestone, in the presence of CO2.Activated Carbon is an extremely adsorbent material used in water treatment to remove organic contaminants. Activated carbon is produced in a two-stage process. First, a suitable base material such as wood, pest, vegetable matter, or bone is carbonized by heating the material in the absence of air. Then the carbonized material is activated by heating it in the presence of air, CO2 , or steam to burn off any tars it has and to increase its pore size. Adsorption of gases, liquids, and solids by activated carbon is influenced by the temperature and pH of the water as well the complexity of the organics being removed.In reverse osmosis (RO), fresh water is forced through a semipermeable membrane in the direction opposite to that occurring in natural osmosis. Because the membrane removes dissolved salts, the main application for RO has been in desalination. However, the process also removes organic materials, bacteria, and viruses, and its application in water treatment is growing.。
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