Page 291 - ro membanes

274 12. REVERSE OSMOSIS SYSTEM DESIGN AND PRETREATMENT 12.5.1.3 Adsorption of Organics by Activated Carbon
Activated carbon adsorption could yield 10%e30% of TOC removal from the saline source water, and when combined with granular media filtration, it could enhance removal of organics in the pretreatment system to over 40%. The addition of a layer of 0.3e0.5 m (1.0e1.5 ft) of granular activated carbon (GAC) on the top of the anthracite in gravity media filters (often referenced in practice as installation of “carbon cap”) usually is an effective measure for reduction of dissolved organics during moderate- and high-intensity algal blooms to levels that prevent the occurrence of excessive biofouling of the downstream RO membranes.
It should be pointed out, however, that the organics absorption capacity of typical carbon cap is limited to 3 to 4 months only. After this period, the GAC layer have to be removed from the surface of the gravity media filters and sent for regeneration and replaced with a new GAC. Because the installation, replacement, and regeneration of GAC are relatively costly, the use of GAC-carbon caps has found limited application in desalination plants, to date.
If membrane pretreatment is used, then enhanced removal of dissolved organics in the source water could be achieved by introduction of powdered activated carbon (PAC) to the feed-water upstream of the desalination plant’s microscreens. The size of the PAC should be such that it can successfully pass through the microscreens. Practical experience at the Adelaide desalination plant shows that using the microscreens for mixing of the saline source water and the PAC is very successful and is preferable to feeding the PAC directly upstream of the MF/UF pretreatment system. In both cases, the PAC with the dissolved organics absorbed by it will be retained on the pretreatment membranes and removed with the mem- brane backwash water.
The most suitable dosage for PAC addition is between 0.5 and 1.0 mg/L of PAC per each mg/L of TOC contained in the source water. Because this dosage is relatively high and the PAC cannot be recovered, regenerated, and reused in the treatment process, the use of this biofouling control method has found very limited application in full-scale desalination plants.
12.5.1.4 Gentle Removal of Algal Biomass Form Source Water
Since algal biomass caries significant amount of biodegradable organics, its gentle removal by DAF or slow downflow gravity-driven granular media filtration could be a very efficient measure for controlling the release of organics in the source water and subsequent SWRO biofouling. This approach is very cost-effective when the source seawater intake is exposed to frequent and extensive algal blooms. Algal biomass removal could reduce the content of source water organics with over 50%, by preventing the release of the organics contained in the algal cells into the seawater fed to the SWRO system.
12.5.2 Reduction of the Bacterial Content of Source Water
The concentration of the microorganisms in the source water can be effectively reduced by: (1) their exposure to strong oxidants (e.g., disinfectants) or UV light; or (2) by depriving the