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8.4 FILTER TYPES AND CONFIGURATIONS 161 8.4 FILTER TYPES AND CONFIGURATIONS
8.4.1 Single-, Dual- and Trimedia Filters
Single-media filters are not very commonly used for saline water pretreatment because of their limited ability to perform under varying source water conditions. Typically, such filters could be applied for desalination plants with subsurface intakes, producing turbidity of <2 NTU, TSS of <3 mg/L, and SDI15 < 6. The majority of the desalination plants worldwide use dual-media filters with the top layer of pumice or anthracite and the bottom layer of sand.
Trimedia filters are used for pretreatment of saline waters only when the source water quality varies significantly and the intake area experiences frequent algal blooms dominated by small-size algae (average algal cell size < 20 mm) and/or very fine silt. Such filters are suitable for capturing small and diverse size particles, which cannot be well retained by the top two layersdanthracite and sand. Since the cost of filter cells increases with depth, often instead of a deep single trimedia gravity filter, a combination of first-stage coarser media (anthracite-sand) gravity filter followed by a second-stage pressure filter containing finer (sand and garnet) media is used.
8.4.2 Single- and Two-Stage Filters
Two-stage filtration is typically applied when the source water contains high levels of turbidity (usually above 20 NTU) and organics (TOC > 6 mg/L) for long periods of time (i.e., weeks/month). Such conditions occur in: desalination plant intake areas exposed to prolonged red-tide events (which sometimes could last for several months); river estuaries that are exposed to elevated turbidity levels occurring during the wet season of the year; or intakes influenced by seasonal winds or currents, which for prolonged periods of time collect source water of elevated content of particulate or organic foulants.
Two-stage filtration systems typically consist of coarse (roughing) filters and fine (polishing) filters operated in series. Usually, the first-stage filter is a mono-media type (i.e., coarse sand or anthracite) or dual media (e.g., anthracite and sand) while the second-stage filter is configured as a dual-media filter with design criteria described in the previous section. The first (coarse- media) filter typically removes 60%e80% of the total amount of solids contained in the source water and is designed to retain all large debris and floating algal biomass. The second-stage filter removes over 99% of the remaining solids and fine silt as well as microalgae contained in the source seawater, typically producing effluent turbidity of less than 0.05 NTU.
Two-stage filters have several advantages. The filtration process through the coarse media filters not only removes large particulate foulants but also enhances coagulation of the fine particulates contained in the source water, which makes their removal in the second-stage filters less difficult and allows the second-stage filters to be designed as shallow-bed- rather than deep-bed filters and to operate at higher surface loading rates. This benefit results in reduced size of the dual-media filters and in lower total amount of coagulant (ferric salt) needed to achieve the same final filter effluent water quality, as compared to single-stage dual-media filters.
Two other benefits of the two-stage filters are that: (1) they can handle larger fluctuations of intake source water turbidity because of the larger total filter media volume/solids