AWWA M30:1995 pdf download.Precoat Filtration
Engineers should evaluate the efficiency and economics of pretreating source waters with turbidity levels higher than 10 ntu to obtain an influent with less than 10 ntu. Because turbidity is an indirect measure of the number and nature of the particulates in a source water, identification of the specific particulates present in a source water is recommended. Figure 1-1 portrays the size spectrum of waterborne particles of interest in contrast with the pore sizes of various filter media.
Both the number and physical nature of the solids present in source water affect the ability of a filter process to remove those solids. Some particulates, such as sand grains, some clays, and protozoan cysts, are nondeformable, discrete particles and do not pose problems. Deformable particles, however, tend to clog the media. In addition, some bacteria and viruses may pass through the filter. Turbidity readings, weight measurements of suspended solids, and particle counts and size distribution analyses are helpful techniques for determining precoat filtration's feasibility. However, those techniques alone may not be sufficient indicators of whether the process can be applied successfully and economically. Study of the types and number of algae present throughout the year and detection of the presence of organic colloids or fine clays in a water source proposed for precoat filtration are important in the evaluation process and in identifying the need for pretreatment.
Surface water sources must be evaluated on the basis of normal conditions that may prevail for most of the year. High-flow and runoff periods should also be examined for increased suspended material and its effect on the filtration process. Facilities for source water or finished water storage to provide adequate potable water supply during periods of adverse source water quality may be needed. Presettling of source water at the plant site may diminish some of the benefits of precoat filtration because of the increased capital and operating costs of the presettling facilities.
Many surface water supplies, such as lakes or ponds, may have algae, color, or taste and odor problems that require additional treatment in conjunction with precoat filtration. A microstrainer used before the precoat filter has been effective in removing microscopic material, including planktonic organisms and amorphous matter. This form of pretreatment has resulted in increased filtration run length.AWWA M30 pdf download.