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<br />Similar Project Experience 6-4 <br />M&E conducted pilot testing to determine the most cost-effective means for reducing short- <br />i circuiting in the settling basins. Modifications resulted in a 25 percent improvement in settling <br />basin performance. Problems in the gravity filters included air binding, inadequate filter <br />bottoms, media condition, and filter run length. Metcalf & Eddy performed field measurements, <br />pilot testing, and theoretical calculations. The resulting recommendations for operational <br />changes have enabled a 25 percent increase in filtration rates. <br />M&E recommended modifications to the existing clearwell and provisions for a new clearwell <br />structure that would operate in parallel with the existing structure. Other recommendations <br />included a system for recycling filter backwash water, and upgrading operation of the existing <br />sludge handling system to meet the increasing flow rates through the plant. <br />M&E prepared a conceptual design for expansion of the plant, if economically feasible. The <br />proposed increase in plant capacity had implications for plant operations and sludge handling, <br />which involved washing down each sedimentation basin every four to six months with flow to a <br />holding lagoon. The sludge was then supposed to be dredged, thickened, and centrifuged. The <br />centrifuge was never used after the first six months of operation. As the sludge built up in the <br />lagoons, rainfall caused washout to a creek, requiring the plant to have a NPDES permit. M&E <br />developed recommendations that would enable the county to close two lagoons. <br />Instead of periodic washdowns of the sedimentation tanks, which would have become more <br />frequent as solids deposition increased under greater capacity, Metcalf & Eddy proposed <br />continuous sludge removal using a bottom-suction device. The team also recommended <br />installation of more and larger gravity thickeners, more centrifuges, and continuation of the <br />recirculation of filter backwash water to the head of the plant. <br />M&E evaluated sludge disposal options. Beneficial reuse was not suitable for local clay soils <br />since the alum sludge would increase water retention of the soil. The county operates the local <br />landfill, making landfilling the most economical disposal solution. For acceptance at the landfill, <br />however, thickened sludge had to contain at least 18 percent solids. Plate and frame filter <br />presses were judged too expensive, and belt filter presses produced cake with only 12 to 15 <br />percent solids. M&E conducted on-site pilot testing with a manufacturer of a centrifuge process <br />involving 3 percent solids feed and polymer treatment that proved to produce cake in excess of <br />25 percent solids. <br />Massachusetts-American Water Company, Hingham, Massachusetts <br />Metcalf & Eddy piloted, designed, and provided construction services and resident inspection for <br />the 7-mgd George W. Johnstone water treatment facility operated by the Massachusetts- <br />American Water Company in Hingham, Massachusetts. The state-of-the-art facility serves the <br />towns of Hull and Hingham, treating water from the company's wells, Accord Pond, Accord <br />Brook, and the Fulling Mill infiltration system. <br />Process requirements for the facility were based on three seasons of pilot testing conducted by <br />M&E on the combined source water. The pilot tests were conducted to evaluate the performance <br />a.A.?.