Page 184 - ro membanes

8.4 FILTER TYPES AND CONFIGURATIONS 167
FIGURE 8.6 Vertical pressure filters.
plants with relatively poor water-quality use two-stage pressure filtration systems. Pressure filters are available in two vessel configurationsdvertical and horizontal.
Vertical pressure filters (see Fig. 8.6) are customarily used in smaller plants and individual vessels have maximum diameter of 3 m. Horizontal pressure filters (Fig. 8.5) are used more frequently in desalination plants and are more popular for medium- and large-size facilities. Horizontal filters allow larger filtration area per filter vessel as compared to vertical units. However, usually vertical vessels can be designed with deeper filter media, if deep filters are needed to handle spikes of source seawater turbidity.
Compared to gravity media filters that operate under a maximum water level over the filter bed of up to 2.5 m (8.2 ft), pressure filters typically run at feed pressure equivalent to 15e30 m of water column (49e98 ft). The magnitude of the feed pressure is often driven by the suction pressure requirements of the high-pressure feed pumps of the downstream RO system. One key advantage of the use of pressure filters is that they could allow avoiding intermittent pumping of the pretreated source water.
A typical RO system with gravity pretreatment filters requires installation of filter-effluent transfer pumps to convey the filtrate from the filter effluent well to the high-pressure RO-feed pumps. Use of pressure filters could eliminate the need for such interim filter-effluent transfer pumps because the filtrate is already pressure driven by the intake pumps and the pretreatment filters do not break the hydraulic grade line.
8.4.5.4 Pressure FiltersdKey Advantages 8.4.5.4.1 LOWER CONSTRUCTION COSTS
Pressure filters are prefabricated steel structures and their production costs per unit filtration capacity are lower than these of concrete gravity filters. Since pressure filters are designed at approximately two to three times higher surface loading rates than gravity filters [25e45 vs. 8e15 m3/m2 h (10e18 gpm/ft2 vs. 3e6 gpm/ft2), their volume and size are smaller and, therefore, they usually are less costly to build and install.