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There are several methods that are used to treat contaminated water. These methods and a brief discussion of each are as follows:
Retention Natural microbial and physical degradation are ongoing processes acting to reduce pathogens. In still or slow moving water, plant pathogen propagules tend to settle to the bottom where continuous degradation takes place. If water is collected in large basins, longer retention of captured water before reuse promotes settling and degradation.Filtration Natural purification of water occurs as it seeps through the soil on its way to the groundwater table. Many factors, such as the soil's composition and pH, its permeability, its ability to bind with pollutants, as well as its distance to the water table, contribute to the effectiveness of this purification process. See the City of Berlin's Senate Department of Urban Development for additional information on this topic.Filtration of recycled water is an important component of all decontamination methods. Filtration is used to remove particulate contaminants prior to further microbial decontamination attempts. Difficulties arise quickly, however, if the physical filter pores are small enough to remove all pathogens as these pores are readily clogged. Suspended solids remaining in the water will reduce the efficacy of treatments such as chlorine, ozone or UV light. Reverse flushing and chemical cleaning techniques are required to achieve effectiveness in the filtration process. Activated Carbon is a highly adsorbent material used in the remediation of contaminated water and groundwater. Its extensive porosity and large available surface are the primary factors that make this relatively inexpensive and versatile compound so useful. In its granular form, activated carbon is used as a filter medium through which contaminated water is passed. Activated carbon is used in water and wastewater treatment primarily as an adsorbent for the removal of relatively low levels of organic and inorganic contaminants via transfer from the dissolved phase to the solid carbon surface. Its non-polar nature makes activated carbon effective in the removal of a variety of organic contaminants, including trihalomethanes, pesticides and herbicides, and polyaromatic hydrocarbons. However, activated carbon may also be used for the removal of trace metals such as cadmium and lead, and it has also been effective in removing some polar organics as well. Activated carbons do not effectively remove contaminants of high solubility or inorganic salts like nitrates. For more information on this filtration method, refer to the discussion on Activated Carbon and Some Applications for the Remediation of Soil and Groundwater Pollution Sand filtration can be used to remove some of the larger particulate matter from water to reduce the pathogen load, however, the pore size between the grains is too large to remove matter such as fungal spores or bacterial cells. Dissolved organic materials also pass through. Microfiltration filters provide a broad specturm of pore diameters in their membranes, which still allows for the passing of certain pathogen materials. Ultrafiltration, which uses snythetic fibers embedded in an epoxy resin, has a much lower mean pore diameter. Reverse osmosis is a water purification technology that utilizes household water pressure to force water through a selective semi-permeable membrane that separates contaminants from the water. Reverse osmosis and activated carbon filtration are often used in a complimentary fashion. Combining these processes results in a more effective treatment against a range of water impurities and contaminants. Treated water emerges from the other side of the membrane, and the accumulated impurities left behind are washed away. Eventually sediment builds up it necessitates membrane replacement. Reverse osmosis is effective in removing total dissolved solids, turbidity, asbestos, lead and other heavy metals, radium, and many dissolved organics. Reverse osmosis is less effective against other substances. The process will remove some pesticides, but not others, and most heavier-weight contaminants. Reverse osmosis is not effective at removing lighter-weight contaminants, such as THMs (the chlorine by-product) and TCE (trichloroethylene),and certain pesticides. These compounds are either too small, too light, or of the wrong chemical structure to be screened out by a reverse osmosis membrane. For a home based, point-of-use product that uses reverse osmosis, see Reverse Osmosis Water Filtration All filtration procedures increase the effectiveness of subsequent disinfection by improving the pre-treatment water purity while having no compounding negative effects on materials already in the water. However, some pathogens may still pass through, depending upon the pore size of the filters. Ultrafiltration is effective at removing pathogen spores but clogging is a problem and the filtration equipment is costly. The material produced from a backwashing of the filters may require separate disposal, depending on the filtration system. For an example of the products offered by one company that deals with the removal of iron, manganese and arsenic, see USFilter Water Technologies For photographs of a filtration plant, as well as some charcoal and green sand treatment tanks, see a project undertaken in Cape Cod. Evaluation procedures are slowly being developed by various groups that will help to determine the effectiveness of various filtration methods. For an brief introduction to this topic, see the web site of Wilkes University, Center for Environmental Quality and GeoEnvironmental Sciences For an experimental persective on how Cryptosporidium is trapped during sand filtration, see The Fate of Cryptosporidium parvum in Porous Media Chlorination At room temperature, chlorine is a yellow-green gas that is heavier than air and has a strong irritating odor. It can be converted to a liquid under pressure or cold temperatures. Chlorine is mainly used as a bleach in the manufacture of paper and cloth and to make a wide variety of products. When released to air, chlorine will react with water to form hypochlorous acid and hydrochloric acid, which are removed from the atmosphere by rainfall. Chorine is also used to treat our drinking water. Factors affecting activity the killing effect of chlorine depends on concentration, time, water quality, especially organic matter content, temperature and pH. In the past 25 years, the Safe Drinking Water Act (SDWA) in the U.S. has been used to protect public health from water-borne bacteria and viruses. Disinfection of drinking water is considered to be one of the major public health advances in the 20th century. One hundred years ago, typhoid and cholera epidemics were apparently common in many cities; disinfection was a major factor in reducing these epidemics; however, the disinfectants themselves can react with naturally-occurring materials in the water to form unintended byproducts that are now known to pose health risks. In addition in the past ten years, we have learned that there are specific microbial pathogens, such as Cryptosporidium, which causes illness and which are resistant to traditional disinfection practices.Chlorination of water is generally achieved by adding metered amounts of sodium hypochlorite solution, calcium hypochlorite solution, or chlorine gas. Chlorine that is present in solution as chlorine, hypochlorite or hypochlorous acid is known as free or available chlorine. These molecules are very reactive and will readily combine with organic matter, ammonia or nitrogen in oxidation reactions. Chlorine, once combined chemically with other substances, becomes unavailable for further action. The amount of chlorine that is inactivated by chemical reaction depends on the impurities, particularly organic matter, in a water supply. When source water has not been sufficiently protected and water treatment is required, chlorine is often used as the first line of defense. However, some organizations warn us of the dangers of resorting to chlorine without proper knowledge of its risks. These organizations alert us to the harmful effects that we need to be seriously concerned about when treating our drinking water. They note that "Health concerns of chlorine exposure include, but are not limited to: possible increased risk of miscarriage, birth defects, rectal and bladder cancer, respiratory complaints, corrosion of the teeth, inflammation of the mucous membranes of the nose, and increased susceptibility to tuberculosis. There is an alarming lack of comprehensive test data." For more information on these concerns see the web site of Zero Waste America. See also the web site of the Chlorine Chemistry Council. Ozonation Ozone is a powerful oxidizing and disinfectant agent now commonly used as a component of systems for the purification of drinking water, swimming pool water, and municipal and industrial waste water. It has a wide spectrum of biocidal activity. The disinfective capacity of ozone is affected by organic matter, pH, conductivity, and the amount and type of iron-chelate present. If ozone reacts with organic matter, the amount of ozone remaining in solution and available for killing microorganisms is reduced. More recently ozone has been used for treating recycled irrigation water. Treatment with ozone involves bubbling the ozone gas through water, using fine bubbles to ensure a good contact with the solution. Excess ozone must be deactivated (usually by venting through an activated charcoal filter) before release to the atmosphere, as it is a severe nasal and throat irritant, and poses health risks to workers. Ozone is commonly produced by passing a high voltage electrical discharge across a dry, oxygen rich gas. About ten percent of the energy supplied is used to make ozone and the remainder is lost, primarily as heat. Use of a hundred percent oxygen stream, rather than air, results in production of approximately twenty times as much ozone. The disadvantages of ozone include potential harm to human health (e.g., eye burns, and skin and nasal irritation, breathing problems, headaches, and fatigue). It can also affect crop heath. It has a sharp, irritating odor that is readily detectable at low concentrations (0.01 to 0.05 ppm). Ozone can explode on contact with organic substances or strong reducing agents. Ozone attacks most metals. For more information on ozonation, please see Ozone Solutions. Also see the American Water Works Association's Fact Sheets. UV Light Ultra-violet (UV) treatment is also being used to disinfect contaminated water. It involves the process of passing water by a special light source thats emits radiation in the 240-280 nm range. In this range, light waves can inactivate harmful biological substances. This water treatment method can sometimes be used without the addition of chemicals, an important fact that enhances its appeal as a treatment alternative. UV light is effective against bacteria, viruses and microscopic pathogens such as Cryptosporidium and Giardia. The ultra-violet rays are similar to the sun’s UV rays but stronger. They have the effect of altering the nucleic acid (DNA) of viruses, bacteria, molds or parasites, so that they cannot reproduce and are thereby rendered inactive. UV treatment does not alter water chemically as nothing is added except energy. UV treatment does not remove dirt and other particles, metals such as lead or iron, or hard minerals such as calcium. Only a few seconds of exposure to UV light is required if the water is sufficiently clear to allow the light to pass. UV light is selectively absorbed by nucleic acids (DNA, RNA) in organisms, resulting in genetic and physiological damage. Absorption is maximal at approximately 254 nm. In humans excess exposure can lead to skin cancers. Sufficiently high exposure doses to UV light will exceed the self-repair capacity of cells, leading to physiological cell disruption and death. For more information on the use of UV light to treat water, see: |
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