Submerged Membrane System Allows for Larger Flows

On the weekend before the American Water Works Association annual conference, Kenosha, Wis., is holding its ribbon cutting ceremony for the largest operating drinking water microfiltration system in the United States, a Memcor CMF system. At the same time, USFilter is unveiling its newest product, the Memcor CMF-S at the AWWA Exhibition in McCormick Place in Chicago.

On the weekend before the American Water Works Association annual conference, Kenosha, Wis., is holding its ribbon cutting ceremony for the largest operating drinking water microfiltration system in the United States, a Memcor CMF system. At the same time, USFilter is unveiling its newest product, the Memcor CMF-S at the AWWA Exhibition in McCormick Place in Chicago.

As engineers and owners gain microfiltration experience, and the cost of membranes drop, the trend is to design bigger and bigger microfiltration plants. But there is a point above 15 MGD where supplying multiple skidded systems is not economical. The Memcor CMF-S (Submerged) system addresses this concern with a pre-engineered modular membrane system designed to be submerged into built-on-site rectangular concrete tanks.

History

Continuous Microfiltration (CMF) is an established and proven technology. USFilters Memcor Products group has installed more than 700 CMF systems around the world. During this decade, CMF capital and operating costs have dropped by over 50 percent. This trend continues with the introduction of the CMF-S.

History

USF Memcor researched and developed the submerged membrane system over the past two years. The aim was to reduce capital costs and simplify and scale up the microfiltration process. After running more than a half dozen small scale trials on a range of feed waters, the design was validated. The first commercial plant was commissioned in August 1998 at Dalesford, Victoria, Australia.

Performance

The Memcor CMF-S uses a membrane with a 0.2-micron nominal pore size and demonstrates up to 6 log removal of Giardia and Cryptosporidium. A vacuum pump draws water through the membrane fibers of sub-modules submerged in the open top filter tanks. The fibers are the same polypropylene material used in the conventional CMF process, but the Memcor CMF-S operates under vacuum, so maximum driving pressure is only 85-100 kPa. This lower pressure limit is not the disadvantage it first appears because filter cake characteristics improve at lower pressures. Compared against earlier versions of CMF, operation of the Memcor CMF-S system shows: 1.) Fluxes 80-95 percent of CMF 2.) The same backwash efficiency and backwash intervals 3.) The same cleaning efficiency and CIP intervals 4.) The same membrane integrity

Integrity Testing

The Memcor CMF-S uses the same membrane integrity testing system as USFilter uses for conventional CMF. Membrane integrity testing gives automatic feedback of membrane condition. Plant operators and service technicians use membrane integrity in the field to ensure consistent filtrate quality.

Integrity Testing

The integrity test involves the filtrate side of the sub-modules being pressurized with air to 15 psi. At this pressure, air cannot overcome the membranes bubble point (except for a small amount of air that flows through diffusion). The rate of pressure decay is measured over a period of time. If this rate is low, the cell is proven integral. If it is over a threshold value, it indicates that membrane integrity is suspect. Damaged fibers (which can be repaired through pinning) or leaking O-rings are the most common causes of integrity problems.

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