Filtering through microfiltration By Ray Barrington News-Chronicle One of the keys to a suburban deal with Manitowoc for water is that both use, or want to use, a system called membrane filtration or microfiltration. The system removes impurities from the Lake Michigan water through fibers. By contrast, Green Bay, which also has made an offer to supply water to the suburbs, uses a sand filtration system. Manitowoc had used sand filtration but switched to the new system last year. The Central Brown County Water Authority has planned for a long time to use microfiltration. The membrane filtration process takes lake water through several steps. First, the water is screened for large solids (driftwood and vegetation), then pumped through a smaller screen to remove smaller particles (sand and silt). It then goes through microfiltration. Pipes are filled with hundreds of thousands of tiny, hollow filters (rather like tiny straws) that remove bacteria-sized particles, about one-hundredth the diameter of a human hair. The particulate matter accumulates on the outside of the straws while the water flows through into the distribution and storage system. Manitowoc then adds fluoride for tooth decay prevention; polyphosphate, for homeowners with lead piping; and a small amount of chlorine. A brochure issued by Manitowoc Public Utilities said the process eliminates the use of most chemicals and decreases the use of chlorine. The residue is blasted off the straws by air and sent back to the lake. The sand filtration system used by Green Bay uses chemicals and fine sand to filter the water. One major difference comes when the plants need expansion. The membrane filtration process is designed for modular expansion, allowing new modules to be brought into the system to add a million gallons of capacity at a time. Sand filtration is not as easily expandable. Manitowoc also has an advantage because it uses water it takes from the lake as coolant in its municipal power plant before sending it to treatment. This warms the water which, during cold months, makes the filtering more effective.