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10.8 FILTRATION MEDIA REPLACEMENT COSTS 229
some immersed membrane system manufacturers offer membrane modules of up to 20,000 m3/day (5.3 MGD) of production capacity. In comparison, the maximum size of the individual granular media filter cells can reach 32,000 m3/day (8.5 MGD) or more, thereby allowing the higher overall construction cost reduction due to the fewer filter cells, less service equipment, and piping.
One of the current source water pretreatment trends worldwide is to use membrane tech- nologies for large-plant applications. As the number and type of large plant membrane appli- cation opportunities increase in the future, it is likely that the membrane manufacturers will develop larger-size individual membrane modules, which in turn would improve membrane pretreatment system economy of scale and competitiveness.
10.8 FILTRATION MEDIA REPLACEMENT COSTS
Well-operating granular media filters lose 5%e10% of filter media every 5 years, which has to be replaced to maintain consistent performance. The costs of granular media replace- ment are usually well predictable and relatively low. At present, membrane element useful life typically varies in a range of 5e7 years. Assuming 5 years of average pretreatment mem- brane useful life, approximately 20% of the membrane elements would need to be replaced per year to maintain pretreatment system production capacity and performance. Taking under consideration that the annual costs for replacement of MF/UF membranes are compa- rable to the annual costs for replacement of RO elements, the use of membrane pretreatment would result in an order-of-magnitude higher initial and annual expenditures for media replacement than granular media filtration.
Additional factor that may contribute to the need for more frequent replacement of membrane elements is the failure of membrane element integrity. Typically, the main reason triggering the need for early membrane element replacement is the loss of integrity rather than the loss of production capacity. The limited track record of long-term use of membrane systems and the uncertainty related to the factors triggering the need for their replacement have to be taken under consideration when selecting between granular media and membrane pretreatment for RO desalination plants. The risk of loss of membrane integrity should be handled accordingly in the membrane element useful life warranty provided by the mem- brane manufacturer/supplier. Upon request, most MF or UF membrane suppliers are willing to guarantee maximum percent of fiber breakage per year, which should be below 0.1% of the total number of membrane fibers.
As indicated previously, in most cases, the use of membrane pretreatment would result in filtered source water of lower particular and colloidal fouling capacity. As a result theoretically, the use of membrane pretreatment instead of granular filtration should reduce the frequency of RO membrane cleaning and replacement. This observation holds true especially for source seawaters of low-microbial fouling potential. However, because of the limited full-scale performance track record to verify this assumption and the fact that for seawaters with high-microbial fouling potential membrane pretreatment would make very little difference in terms of frequency of RO cleaning, at present most RO membrane suppliers are reluctant to provide warrantees for longer useful life or lower cleaning frequency of the RO membranes downstream of the membrane pretreatment system. As a result, this potential benefit of