Challenge

The Department of Water Quality, an agency of the county of Maui, Hawaii, needed to provide drinking water to approximately 34,000 service connections.  Known in the state for having some of the most difficult waters to treat, the county of Maui sought a treatment technology that would produce reliable water while minimizing land requirements.  The raw water at each of the county’s four plants has distinct characteristics in terms of solids and minerals.  At Kamole and Iao, the raw water contains high algae, and turbidity spikes can be as high as 600 NTU.  At Lahaina, there is high algae and the color is greater than 25 true color units.  Olinda has elevated iron, color between 180 and 200 and low alkalinity.

Solution

Ten years ago, the county became a pioneer in the use of low-pressure membrane filtration for drinking water.  After conducting pilot studies, they chose Memcor pressure-driven membrane modules for Lahaina, to treat 2.5 MGD. Following completion and successful startup of this plant, Maui’s three subsequent facilities were also specified with the same low-pressure polypropylene membrane systems for treating 6.75 MGD at Kamole, 2.5 MGD at Iao and 1.8 MGD at Olinda. 

Pretreatment at each facility varies according to source water characteristics and site considerations.  The facilities are equipped with programmable logic controllers and SCADA workstations, allowing operators precise control over the membrane systems.  In-situ integrity testing is automatically conducted every 24 hours. Maintaining optimum filer performance and achieving long membrane life are major objectives for the Department of Water Supply.  For this reason, Siemens conducts on-site process audits annually to analyze performance and recommends any operating changes that might be required. 

Results

Early identification of potential problems and taking corrective measures has kept these plants operating reliably and at peak performance.  In fact, two of the systems are still operating successfully with the original membranes after nearly nine and 10 years of continuous operation.  This has provided the Department of Water Quality with many essential lessons on day-to-day operation and maintenance of drinking water membrane plants for maintaining long, useful membrane life.  The Department plans to replace the original polypropylene membranes with new PVDF membranes.  The cost of operation will be considerably less, the membranes can be cleaned with chlorine, and the liquid, rather than air, backwash will consume less energy, reducing the Department’s power costs.

The addition of new membrane filtration technology, combined with the Department’s long-term experience with operating and maintaining low-pressure membranes for drinking water filtration, will help the county to cost-effectively achieve its drinking water quality and supply goals well into the future.