Page 185 - ro membanes

168 8. GRANULAR MEDIA FILTRATION 8.4.5.4.2 SMALLER FOOTPRINT
Because of their smaller volume and filtration area, pressure filters occupy smaller footprint. If the available site is of limited footprint, this is an important factor to consider, when selecting granular media filtration technology.
8.4.5.4.3 SIMPLER INSTALLATION
Since pressure-filter vessels are prefabricated, the installation time of this pretreatment system is approximately 20%e30% shorter than that of gravity filters with concrete structures.
8.4.5.4.4 NO EFFECT OF SUNLIGHT ON ALGAL GROWTH ON FILTER WEIRS
Pressure filters are completely enclosed and sunlight cannot reach the filter weirs, distribution system, and media, and to induce an algal growth that would have negative impact on filter performance. For comparison, gravity filters (if they are not located in a building or covered with nontranslucent panels) would grow algae on all components of the filters exposed to direct sunlight.
8.5 FILTER PERFORMANCE 8.5.1 Removal of Solids
Typically, saline source water collected by open intakes has high content of both fine silt and suspended solids. The purpose of the pretreatment filters for RO plants is to remove both of these particulate foulants. Because removing fine silt from saline-source water to levels below SDI15 of 4 by granular media filtration is usually much more challenging than the reduction of source-water turbidity by 99%, the design of these pretreatment facilities is typi- cally governed by the filter effluent SDI target level rather than by the target turbidity or pathogen removal rates.
Filter solids-removal efficiency (reduction of turbidity/total suspended solids) is not directly related to its silt and fine-colloid removal efficiency (SDI reduction capability). Dissolved organics and coagulant (iron salts) can absorb on/in the SDI-filter test pad and result in increased SDI values. Full-scale experience at many granular media pretreatment filter installations indicates that filters can consistently reduce source-water turbidity to less than 0.1 NTU, while at the same time filter effluent could have SDI15 frequently exceeding 4. In many cases, granular media filters at RO-desalination plants need to be designed more conservatively than similar filters in conventional surface water treatment plants to capture fine solids, silt, and colloidal organics contained in the saline source water.
8.5.2 Removal of Organics
Typical gravity and pressure dual-media filters with filter-media bed depth of 1.0e1.4 m (3.3e5.3 ft) have relatively low organics removal rate, i.e.,15%e20%. This removal rate, however, increases significantly with depth and could reach 25%e35% for filters of total filter depth of 1.8 m (6.6 ft) or more. If a carbon cap is installed on the top of the filter media (above the layer of anthracite), filter TOC removal rate could be increased to 40%e50%.