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.

多介質過濾器有正洗、反洗、產水三種運行模式，其具有運行成本低、日常維護簡便的優點，因此這種工藝在國內得到廣泛應用。多介質過濾器的處理效果跟流速有關，流速越小，淨水處理效果越好。日常維護也相對簡單，隻需要在多介質過濾器的自控程序設置進行定期反洗，將截留在濾料空隙中的一些雜質排出，它的處理功能即可恢複。而是否需要反洗，可以通過濁度儀、進出口壓差或者設定反洗的時間間隔來評估。為了避免因進水水質波動對多介質過濾器運行狀態造成較大的影響，通常在係統設會設置手動啟動反洗功能。

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 evalsuated 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.

通常情況下，反洗泵大多采用立式多級泵。可把清潔的原水作為多介質過濾器的反向衝洗液。當多介質過濾器進入反洗模式時，通常以2~3倍的設計流速進行衝洗，持續約10~15分鍾。待反向衝洗結束後，再以操作流方向進行短暫的正向衝洗，使介質床得以複位。為了保證多介質過濾器的良好運行效果，需要定期更換裝置內的填料介質，通常更換周期約2~3年，視原水水質情況而定。

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.

多介質過濾器是純化水係統預處理中一個重要環節。在係統設，選擇多介質過濾器需要分析原水中的濁度和矽化物含量。當原水濁度較高、矽化物含量較多，需要在多介質過濾器前端投入適量絮凝劑，利用絮凝作用來降低水的濁度，以及混凝脫矽，進一步提高多介質過濾器的處理效果。

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.

但值得注意的是，這種混凝劑物質有可能泄露後續的處理單元，會影響純化水的水質。因此，一旦有使用到絮凝劑，必須嚴格檢測殘留量並驗證，降低對產水水質的質量風險。同時，建議企業結合自身實際，盡可能提供水質較好的原水，從而降低整個係統的質量風險和運行成本。

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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