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??<... ' Similar Project Experience 6-6 responsibility for coordinating this program, including preparing, directing, and managing the design of primary, secondary, and residuals treatment facilities plus tunnels and the outfall; establishing design standards; and managing and coordinating the activities and interfaces between the project design engineering firms under contract to MWRA. Residuals Treatment Metcalf & Eddy reviewed and coordinated the design of the Deer Island sludge processing facility, one of the largest sludge treatment facilities in the world. The facility includes six 2.31-mgd center-feed gravity thickeners (70-foot diameters), twelve 900-gpm waste sludge thickening centrifuges, twelve 3-million-gallon anaerobic egg-shaped digesters, four 900-gpm digested sludge thickening centrifuges, and two 3-million-gallon digested sludge/gas storage tanks. From the 48 stacked primary clarifiers, capable of treating up to 1.2 billion gallons of wastewater each day, primary sludge is pumped with chlorinated dilution water to the gravity thickener distribution box. Dilution was an M&E innovation to "freshen" the sludge and maintain a constant hydraulic loading rate to improve the efficiency of the thickeners. With one unit out of service, design year average loading to the gravity thickeners is 332,000 pounds per day or 17 pounds per day per square foot. At a diluted sludge concentration of 0.5 percent and a hydraulic loading rate of 600 gpd per square foot, the thickened sludge concentration averages 6 percent with 95 percent solids capture. The most challenging aspect of designing these thickeners was their physical and process placement within the Deer Island secondary treatment facility. M&E developed a gravity thickener process flow schematic showing the interconnection of these thickeners within the larger facility. Key M&E roles leading to the successful thickener operation at Deer Island include: ? Layout, placement, and coordination within the remainder of the residuals facilities and the entire treatment plant • Hydraulic and pumping coordination between upstream process components such as the primary sludge pumps and downstream components such as the sludge digesters and centrifuges ? Dynamic process aspects based on hydraulic and solids loading and solids balancing for each start-up phase ? Operation and control including interface with the plant-wide distributed control system ? Development of process control set points for testing and start-up ? Development of a work-around to allow start-up of the new primary treatment facilities to meet a court-ordered schedule, eight months before completion of the new residuals handling facilities