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7.10.2 <br />7.10.3 <br />To allow for more than one filter to be backwashed in one day, multiple backwash tanks are <br />required. To provide for efficient operation, three backwash tanks are recommended. <br />Backwash Alternative 2 — Lamella Plate Settlers <br />Lamella settling is a process that receives continuous flow from the backwash tank, and provides <br />high rate thickening of the sludge, and reclaims decanted clear water to the beginning of the <br />treatment train. High rate inclined plate settlers typically thicken backwash sludge to <br />approximately 0.5 to 1.5% solids prior to discharging to either a sludge storage tank for further <br />thickening, or directly to the sanitary sewer. <br />Backwash waste water is pumped from the <br />backwash tank directly into the gravity <br />settler, alleviating settling time. A coagulant <br />is added immediately, as the water flows <br />through a flash mixer and into a flocculation <br />tank. The coagulated sludge then flows into <br />a series of inclined plates, the surfaces <br />which collect the sludge and direct it to a <br />sludge hopper. The clear effluent flows out <br />the overflow and is recycled to the raw <br />water. The inclined orientation of the plates <br />allows for more surface area for the solids to <br />settle upon, while limiting the total space <br />taken up by the equipment. <br />As with treatment process equipment in <br />general, redundancy is recommended so <br />that in the event that one lamella settler is <br />down, settling operations can continue <br />seamlessly. Therefore, one single lamella <br />settler is not recommended. <br />Flow distribution uriarices <br />a i.e-liirge Aimee <br />I1 uI.11on tank <br />Oluctea <br />Backwash Alternative Comparison <br />The advantages of lamella plate settlers is that they do not require settling time prior to recycling <br />the backwash water. This eliminates the need for batch processing of backwash water from <br />backwash tanks and provide significant operational flexibility. Backwash tanks can dictate when <br />and if a filter can be backwashed. <br />Lamella plate settlers waste between 60 and 900 gallons per million gallons of water treated, <br />depending upon whether a sludge holding tank is utilized. Backwash tanks waste between 750 <br />and 5,000 gallons of water per million gallons of water treated. <br />Because lamella plate settlers provide significantly more operational flexibility and waste less <br />water, lamella plate settlers are recommended for Ramsey. <br />FEASIBILITY STUDY RAMSY 154354 <br />Page 34 <br />