Recirculation backwash water

Typically, for cooling water makeup, the filtration rates are 4 to 6 gpm/sq ft of media surface area, with backwash rates of 10 to 12 gpm/sq ft for a period of 5 to 8 minutes. When the filter is used for removing suspended solids from recirculating cooling water, the filtration rate may be as high as 10 to 15 gpm/sq ft and the backwash rate 15 to 20 gpm/sq ft. 

A.2 Cross-Flow, Dead-End Configurations Microfiltration and UF systems are operated in two possible filtration modes. Figure 6.10 shows the cross-flow configuration in which the feed water is pumped tangential to the membrane. Clean water passes the membrane while the water that does not permeate is recirculated as concentrate and combined with additional feed water. To control the concentration of the sohds in the recirculation loop, a portion of the concentrate is discharged at a specific rate. In dead-end or direct filtration, all the feed water passes through the membrane. Therefore, the recovery is 100%, and a small fraction is used periodically for backwash in the system (5-15%). 

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