Page 183 - ro membanes

166 8. GRANULAR MEDIA FILTRATION 8.4.5.2.5 SIMPLER INSPECTION AND MAINTENANCE
Gravity filters are typically covered with light plastic covers that protect the filter cells from direct sunlight or are installed in buildings to prevent excessive algal growth on the filter troughs and media. If covers are used, they can be easily removed and the filter cells can be inspected visually for irregularitiesdmalfunctioning of filter backwash nozzles, weir corrosion, poorly backwashed areas of filter media, formation of “mud-balls,” etc.
Pressure filters are completely enclosed and very difficult to inspect for the condition of the media, and integrity of the internal equipment and underdrain. As a result, these filters have to be designed with higher contingency factor (reserve capacity). A 15%e20% reserve capacity is recommended if pressure filters are used to accommodate for potential flow-distribution problems and uneven backwash air and water distribution.
8.4.5.2.6 EASIER TO ACCOMMODATE MEMBRANE PRETREATMENT IN THE FUTURE
As membrane pretreatment technology evolves and new membrane systems available on the market are designed to better-handle algal bloom challenges, for some existing plants, it may be advantageous to modify the exiting conventional granular media filters into submersible mem- brane pretreatment filters. This upgrade would be possible as long as gravity filter cells are designed with adequate depth and configuration to accommodate submersible UF/MF mem- branes. Pressure filter vessels would not be possible to modify into membrane holding tanks.
8.4.5.3 Pressure FiltersdDescription
Pressure filters have filter bed configuration similar to that of gravity filters, except that the filter media is contained in steel or plastic pressure vessel. They have found application mainly for small- and medium-size desalination plantsdusually with production capacity of less than 20,000 m3/day (5 MGD). However, there are a number of installations worldwide where pres- sure filters are used for pretreatment of significantly larger volumes of water. An example is one of the largest SWRO desalination plants in the Middle Eastdthe 190,000 m3/day Al-Ghu- brah desalination plant in Oman. The pressure filters for this plant are shown on Fig. 8.5.
In most cases for good source water quality (SDI < 5 and turbidity less than 5 NTU) the pressure filters are designed as single-stage, dual-media (anthracite and sand) units. Some
FIGURE 8.5 Horizontal pressure filtersdAl-Ghubrah desalination plant, Oman.