Page 175 - ro membanes

158 8. GRANULAR MEDIA FILTRATION
expansion of only 10%e15%. Filter media with size of 0.8 mm needs an expansion of 20%e25%. For sand filter media of 0.4e0.6mm the backwash rate should provide 30%e50% of media bed expansion for optimal filtration performance.
The number of filter cells and the individual production capacity of each cell are typically selected to allow full flow operation with one filter cell out of service in backwash and one out of service for maintenance. Additional information on the design of granular media filtration systems is provided elsewhere (Wilf et al., 2007; AWWA, 2007).
8.3 KEY FILTRATION SYSTEM COMPONENTS 8.3.1 Filter Cells
As indicated previously, typical granular media filtration system consists of number of individual units (cells or vessels), which operate in parallel. The number of filter cells is mainly dependent on the total flow these filters are designed to process. The construction cost of the filtration system is usually reduced when fewer individual cells are used. However, the minimum number of filters is limited by the following key factors: (1) the practical maximum size of individual filter bed (100e150 m2/1080e1610 ft2)dlarger area beds are likely to result in nonuniform backwash of the filter bed; (2) the increase of the filtration rate of the filters remaining in operation, when one or two filters are in a backwash mode; (3) the configuration of the RO system, i.e., the number of individual trains and the planned mode of operation of the desalination plant.
To maintain a consistent, high-quality filter performance, the number of filter cells should be selected in such that when one cell is out of service, the hydraulic loading rate of the filters remaining in operation does not exceed 20% of the average loading rate with all units in service, and when two units are out of service, this rate is less than 30% of the average loading rate.
In general, even for very small desalination plants, the minimum number of individual pretreatment filters is recommended to be at least four. For plants of capacity higher than 5000 m3/day (1.3 MGD) 6e8 filter units are preferable (Kawamura, 2000).
For desalination plants larger than 10,000m3/day (2.6MGD), filter cells are usually divided into two groups that can be operated independently and paired with one-half of the desalination plant RO trains. In plants larger than 200,000 m3/day (53 MGD), the desalination plant is typically divided into at least two sets of two filter groups, each with 8e32 individual filter cells.
8.3.2 Filter Media
Filter media type, uniformity, size, and depth are of key importance for the performance of the pretreatment filters. Dual-media filters have two layers of filtration mediadtypical design includes 0.6e1.0 m (2.0e3.3 ft) of anthracite or pumice over 0.4e1.6 m (1.3e5.2 ft) to of sand. Usually, filters with a total depth of the media of 1.8 m (6 ft) or more are referred to as deep media filters. The bottom 25%e30% of the media of these filters develops and sustains biofilm of microorganisms, which are capable of biodegrading a portion of the dissolved organics contained in the source water. In this case, the depth of the anthracite level is enhanced to between 1.2 and 1.8 m (4.9e5.9 ft).