Nematodes are one of those biological organisms that because of their abundance in surface water and their resistance to disinfection are of great importance in control of water treatment processes. Because of lack of information about efficiency of nematodes removal in direct filtration process, this subject has been examined under various conditions in this research with regard to size of filter media, filtration rate, injection rate of coagulant and activity or inactivity of nematodes, by using a pilot plant consisting of raw water unit, coagulation and flocculation units as well as two parallel single and triple media filter. Based on results obtained, the average efficiency for removal of active nematodes by single and triple media Filters has been 68.8 and 73.9 percent respectively. Based on microscopic survey, the mobility of nematodes has been the main reason for low rate efficiency. By making nematodes inactive, prior to entering the filter ,the average efficiency for removal of nematodes in single and triple media filters increased by 93.7 and 95.8 percent respectively. Hence, this is strongly recommended.
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