The processes employed to provide municipal water treatment solutions vary somewhat according to the proposed end use of the treated product. All such uses share a common need for the removal of various contaminants. However, the precise nature of the impurities in question and the extent to which they will need to be removed is dependent both upon the raw source and whether it is intended to provide a product suitable for drinking, for irrigation, for use in various industrial processes, or for maintenance of river flow, to name some of the more common applications.
The processes used in municipal water treatment solutions fall into three categories, and either depend upon physical, chemical, or biological actions to achieve their objectives, while the overall operation may be divided into primary, secondary, and tertiary stages. During the first two of these stages, the process employed are primarily physical in nature. The primary stage includes pumping from source into pipes or storage tanks, and is accompanied by the use of screens to remove large items of debris, such as branches and leaves, or trash that could otherwise interfere with the processing procedures that follow.
During this stage, in cases where the source is unduly hard due to the presence of high concentrations of soluble calcium salts, municipal water treatment solutions normally include a preconditioning step, in which soda ash is added to produce a precipitate of calcium carbonate, which can then be filtered out.
During the secondary stage, a variety of techniques may be employed to remove both suspended material, such as fine solids and bacteria, as well as certain dissolved organic and inorganic components. The addition of chemical flocculants and coagulants helps to increase particle density, enabling them to sediment out and leave a clarified effluent that will then undergo filtration, usually through sand. Where necessary, municipal water treatment solutions may also include a top layer of carbon above their sand filters. This acts to absorb any last vestige of organic compounds still present that might otherwise result in an unpleasant taste or odour.
Because, despite all the previous processing, there is still a small risk that some pathogens could remain and so the tertiary stage is one of disinfection. For this purpose, chlorine gas, chloramines, and ozone may all be used, but the chlorine derivatives leave a disinfectant residue to provide long-term protection.
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