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7.1 Particulate6 Cartridge microfilters are designed to remove at least 90 percent of particles larger than their rating size. Most are depth filters, which trap particles within tortuous passages as the water runs through. They are dead end systems that process all the water passing through after an initial wetting period. When the back pressure reaches a specified level (65 to 240 kPa), the filters are replaced. Particulates are disposed with the filter as solid waste. Cartridge life expectancy depends on the particulate loading and the flow rate. Costs for cartridge filtration depend on the micrometer rating, size, replacement rate, and type of cartridge housing. For the purity level required by membrane processes, the cost is $0.01 to $0.08 per cubic meter using twenty-five 5-pm filters (Parekh, 1991). 7.2 Colloidal Fouling Colloids are metal oxides, soaps, detergents, proteins, organic matter, silicates, and clay usually found in surface water. These substances generally have a negative surface charge. Water molecules and positively charged ions form a double charge layer around colloidal particles (see fig. 7.1). Positive molecular poles and cations are attracted to the surface of the particle. The negative poles of water molecules around the particle cause a repulsive force between particles. Conditions of high pressure and ionic concentrations at the membrane surface cause these repulsive forces to be overcome and attractive van der Waals forces then cause the particles to coalesce. The result is the formation of a colloidal slime on the membrane that is difficult to remove. For this reason, the SD1 (silt density index), a measurement of colloid concentration, should be no higher than 5.0 for spiral wound membranes and no greater than 3.0 for hollow tine fiber membranes (C’Melia, 1972). 7.!2.1 Avoidance. - Since colloids occur naturally in surface water, one way to avoid the problem of colloidal fouling is to locate intake pipes in wells adjacent to the surface water source. In this way, the soil and rock can be used as a natural filter. This method is used at some seawater RO plants. By using the sand as a filter, they reduce the SDI, the turbidity, and the salinity of the intake water. 7.2.2 ZGccu.!otion, Settling, and Filtration. - Colloids can be removed by co-precipitation with AKIII), FeCII), or SiCV) hydroxides. The negatively charged colloids are surrounded by the metal cations and thus form a nucleus for their precipitation (see figure 7.1). The same thing happens when lime is used for softening. When polymer is added for coagulation the long chains can act as bridges linking colloidal particles and aiding in floe formation (O’Melia, 1972).PDF Image | DESALTING AND WATER TREATMENT MEMBRANE
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