Filtration
Our source of raw Lake Michigan water is delivered to the filtration plant pre-filtered using submerged intakes as described in "Plant History." The average turbidity level for our raw water is 0.09 NTU's. At the filtration plant, the treatment technique is "Direct Filtration"; there are no sedimentation basins after flocculation.
The water is filtered through eight General Filter cells, each with a filtration area of 304.5 square feet and designed for a filtration rate of 4.5 gallons per minute per square foot. Each filter cell contains 30 inches of dual media, 12 inches of sand, and 18 inches of anthracite. Pre-treatment at the plant consists of chlorination, an aluminum sulfate (alum) precipitation. Suspended particles and alum (precipitate) are trapped on the filter bed. After approximately 100 hours, the filter is removed from service and then backwashed. Backwashing is completed in 10 minutes, which is enough time to break up and remove all impurities that were trapped on top of the filter. Our filters have the capability of using water and air (multi-wash) during backwashing; although, we have found that water alone is sufficient to remove particles.
Turbidity for filtered water remains below 0.10 NTU (nephelometric turbidity unit) during normal filtration conditions. As with any filter that has been backwashed and put back into service, an increase in turbidity usually follows (ripening period). During this period, an individual filter cell's turbidity may increase to a level between 0.20 and 0.30 NTU’s for 10 to 15 minutes before settling back to normal conditions. Filters are also affected by raw water turbidity and temperature variations throughout the year. Our operators consistently provide clean, clear water to our customers 24 hours a day by monitoring turbidity levels, temperatures, and other chemical and physical parameters. Our turbidity levels, on a monthly average, never exceed 0.10 NTU.
With the completion of the deep-bed high-rate, mono-media direct filtration study, we now have the Department of Environmental Quality’s approval to provide high quality drinking water at a faster rate. Deep-bed high-rate, mono-media direct filtration consists of 52 inches of anthracite, eliminating the 12 inches of sand that was used in our existing dual-media filters. We’re anticipating the completed filter and facility expansion by 2011; this will provide the surrounding communities with safe and reliable drinking water over the next 10 to 15 years.