Figure 1 "Conventional" RO Pretreatment
Microfiltration Figure 2 Continuous Microfiltration RO Pretreatment
Backwash to secondary pre-treatment
The following sections review three case studies.
3 WEST BASIN WATER DISTRICT, CALIFORNIA
The confidence in CMF/RO technology developed through the work done at Water Factory 21, [l] led to the two West Basin wastewater reuse projects.
Over-abstraction has depleted the aquifer, causing saline ingress. In addition, industrial activity has caused further pollution making the aquifer unusable. There is complete dependence on expensive imported water.
Secondary sewage from the Hyperion works is now used in a variety of ways to alleviate this. In one application, commissioned in 1997, Memcor processes are used in 15 Mlitre/day CMF/RO system to supply the nearby Mobil and Chevron refineries with feed for their boiler water plants. In an earlier project Memcor supplied an 11.5 Mlitre/day CMF plant as pretreatment for RO to provide water used for injection in a barrier scheme to keep out further sea water.
CMF has simplified the wastewater reclamation process by eliminating the lime handling, addition and recovery systems and the flocculation, clarification, recarbonation and filtration processes, resulting in lower chemical requirements and eliminating the production and disposal of solid waste sludge.
The frequency and cost of replacing the membranes is of key importance to the viability of a project, especially when membrane cost is typically 15% to 25% of capital cost.
The CMF system at West Basin, which contains 5 units, automatically checks the integrity of 9 million membrane fibres as part of the operating procedure. Two sub-modules have been repaired in the last two years of operation.
The following capital and operating costs comparison for the conventional and microfiltration pretreatment to RO were presented in March 1999 at the AWWA membrane Conference. 
The operation costs for the two pretreatment systems were calculated from July 1997 to October 1998.
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