Page 182 - ro membanes

8.4 FILTER TYPES AND CONFIGURATIONS 165
threshold, pressure filters would have the disadvantage to cause an accelerated biofouling when filtering source water of very high algal content. This effect is likely to manifest itself mainly in the summer and during algal blooms when the level of TOC in the source water exceeds 2 mg/L.
Pressure filters are used in medium- and large-size desalination plants in Spain, Algeria, and Australia. However, in most successful applications the source water quality is very good (TOC < 1 mg/L, SDI < 4 and turbidity < 4 NTU). Most of the Spanish desalination plant intakes are relatively deep and the algal content in the source water is fairly low. At depth of 10e20 m (33e66 ft), the concentration of algae is significantly lower than that at the water surface and, therefore, as long as desalination plant intake is deep (e.g., 10 m or more), biofouling caused by breakage and decay of algal biomass may not be as significant problem as it would be for shallow intakes or intakes located at the surface of the water body (i.e., near-shore open intakes).
8.4.5.2.2 LONGER USEFUL LIVE OF THE FILTER STRUCTURE
Typically, gravity filters are concrete structures that have useful life of 50e100 years. Pressure filters are steel structures with a lifespan of 25 years or less. The internal surface of the pressure filters used for seawater desalination is typically lined up with rubber or polyurethane coating that needs to be replaced every 5e10 years and inspected occasionally. In recent years, several manufacturers began to offer plastic pressure filters, which are expected to address the issues associated with the limited longevity of steel filter vessels. However, at present these filters have limited track record to establish their actual useful life.
8.4.5.2.3 LOWER POWER USE
Gravity filters operate without the need to pressurize the feed water through the filtration media. Therefore, they usually consume several times less energy than pressure-driven filters of the same volume of pretreated water.
8.4.5.2.4 HIGHER SOLIDS RETENTION CAPACITY AND BETTER HANDLING OF TURBIDITY SPIKES
Gravity media filters have approximately two to three times larger volume of filtration media and retention time than pressure filters for the same water production capacity. Therefore, this type of filters can retain proportionally more solids and as a result, pretreatment filter performance is less sensitive to occasional spikes in source water turbidity. If the source water consistently has low content of particulate solids (e.g., <2 NTU), the gravity and pressure filters perform comparably.
Pressure filters usually do not handle solids/turbidity spikes as well because of their smaller solids retention capacity (i.e., smaller volume of media pores that can store solids before the filter needs to be backwashed). If the source water is likely to experience occasional spikes of high turbidity (20 NTU or higher) due to rain events, algal blooms, ship traffic, ocean bottom dredging operations, strong seasonal winds, seasonal change in underwater current direction, or spring upwelling of water from the bottom to the surface, than pressure filters will produce effluent with inferior effluent quality (SDI and turbidity) during such events and therefore, their use would likely result in a more frequent RO cleaning.