The Kenosha Water Utility's (KWU) 22-mgd microfiltration membrane plant broke ground a decade ago as the world's largest low-pressure membrane system producing potable water, the first in Wisconsin and the first direct-membrane filtration process on Lake Michigan. Since the O. Fred Nelson Water Treatment Plant went on line, hundreds of people from countries such as Canada, France, Germany, Australia, New Zealand, China, Japan, Korea, and England have toured it to examine the leading-edge technology.
KWU is well satisfied with the decision to build a membrane plant. However, since the December 1998 start-up, the utility's demand for water has evolved. Operations and maintenance methods have improved, as have membrane quality and replacement options. Lessons learned since then include the importance of regular membrane integrity testing, regularly scheduled chemical cleaning, preventive maintenance, staff training, financial planning, and a positive relationship with regulatory agencies.
KWU built the membrane plant to update a 40-mgd conventional treatment facility, which consisted of two plants. The original 20-mgd “west plant” started as a steam-powered pumping station in 1895 and was upgraded to a conventional filtration plant in 1917. A 20-mgd “east plant” was added in 1964. After evaluating several options, and in the wake of the 1993 cryptosporidiosis epidemic in nearby Milwaukee, KWU decided to upgrade and continue operating the east plant and replace the west plant with a new membrane filtration system.
While the installed cost of the membrane plant turned out to be less than the projected cost of a conventional process, the primary motivation behind KWU's decision to become a membrane pioneer was the commitment to provide the highest possible water quality for its customers. KWU's vision has been confirmed every day since the membrane plant's start-up by producing high-quality potable water that meets all U.S. EPA and Wisconsin Department of Natural Resources (DNR) drinking water requirements.
TESTING MEMBRANE INTEGRITY
In a direct filtration process of Lake Michigan water, Wisconsin DNR requires testing membrane integrity three times per day, at 8-hour intervals, along with continuous on-line turbidity measurements. Results from a series of fiber cut tests conducted during the plant's start-up convinced the DNR that a pressure decay test (PDT) would provide more sensitive measurement of unit integrity and reflect actual performance better than that obtained from turbidity or particle count measurements alone. Membrane-filtered water turbidity measurements are at the lowest end of the on-line turbidity instruments' range, always <0.02 nephelometric turbidity units (NTU), regardless of raw water turbidity. Turbidity can exceed 100 NTU, and in some years has exceeded 200 NTU, from storms on Lake Michigan.
The PDT used at KWU is a fully automated sequence conducted with a challenge test pressure of 15 psi on each membrane skid, and can detect a breach in membrane integrity as small as 3.0 microns. Wisconsin DNR requires operating staff to respond to the smallest change in PDT value by finding the individual membrane module(s) that triggered the change, isolating the leak, and scheduling a repair.
With more than 10 million fibers in each skid, looking for a leak may seem like trying to find a needle in a haystack. However, using an air hold test when responding to a change in PDT allows air bubbles to be heard, pinpointing a specific membrane module leaking air. This enables plant staff to identify even the smallest breach in integrity, isolate the leak, and fix it as part of scheduled maintenance.
The Wisconsin DNR's “zero tolerance” to changes in PDT created more fiber repair work than KWU initially anticipated, but never required additional staff. A set of replacement membranes installed in 2004 has proven to be more durable than the original supply, reducing the fiber repair work.
Maintaining membrane integrity is essential to providing the best possible treatment. Regular, effective membrane cleaning is the most important way to extend membrane life and ensure that the filtration capacity will consistently meet needs. Unlike high-pressure membranes that can weaken with repeated chemical cleaning, low-pressure membranes will decline in capacity if the membranes are not washed or are cleaned ineffectively. Low-pressure membranes are washed with chemicals using temperatures that do not degrade performance.
Transmembrane pressure (TMP) is the driving force that moves water through the membranes to the filtrate side of the system. TMP increases as membranes get dirty (foul) when filtering the same amount of water. Chemical cleaning removes foulant from the membrane surface and lowers TMP. New membranes can be operated for 2,000 to 3,000 hours before the TMP rises to the point where a chemical clean-in-place (CIP) is required.