When the turbidity of raw water can not meet the requirements of the subsequent system, the pretreatment system needs to set up a multi-media filter to improve the effluent quality. The filtration principle of multi-media filter is to fill quartz sand and bituminous coal with different sizes inside the filter, and use the functions of membrane filtration, osmotic filtration and contact filtration to effectively remove large particle impurities, suspended solids, colloids and other substances in the water. The SDI value of the produced water is not more than 5.
The multi-media filter has three operation modes: forward washing, backwashing and water production. It has the advantages of low operation cost and simple daily maintenance, so this process is widely used in China. The treatment effect of the multi-media filter is related to the flow rate. The smaller the flow rate is, the better the water treatment effect is. Routine maintenance is also relatively simple, just need to set up the automatic control program of the multi-media filter for regular backwashing, so as to discharge some impurities trapped in the filter material gap, and its processing function can be restored. Whether backwashing is needed or not can be evaluated by turbidimeter, inlet and outlet pressure difference or setting backwashing time interval. In order to avoid the influence of influent water quality fluctuation on the operation state of multi-media filter, the manual start backwashing function is usually set in the system design.
In general, the backwash pump is mostly vertical multi-stage pump. The clean raw water can be used as the backwash liquid of the multi-media filter. When the multi-media filter enters the backwash mode, it is usually flushed at 2 ~ 3 times of the design flow rate for about 10 ~ 15 minutes. After the reverse flushing, the medium bed can be reset by a short forward flushing in the direction of operation flow. In order to ensure the good operation effect of the multi-media filter, it is necessary to replace the packing medium in the device regularly. The replacement period is usually about 2 ~ 3 years, depending on the raw water quality.
Multi media filter is an important part in the pretreatment of purified water system. In the system design, the selection of multi-media filter needs to focus on the analysis of turbidity and silicide content in raw water. When the turbidity of raw water is high and the content of silicide is high, it is necessary to put an appropriate amount of flocculant into the front end of the multi-media filter to reduce the turbidity of the water and remove the silicide by coagulation, so as to further improve the treatment effect of the multi-media filter.
However, it is worth noting that this coagulant material may leak to subsequent treatment units, which will affect the quality of purified water. Therefore, once the flocculant is used, the residue must be strictly detected and verified to reduce the quality risk of produced water. At the same time, it is suggested that enterprises should provide raw water with better quality as far as possible in combination with their own reality, so as to reduce the quality risk and operation cost of the whole system.
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