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7.10.1 <br />7.10.2 <br />7.10.3 <br />70,000 gallons &water. After a backwash, the solids are allowed to settle and the clear water is <br />recycled back to the filters. This can be done with backwash tanks or lamella plate settlers as <br />discussed below. <br />Backwash Alternative 1 — Backwash Tanks <br />Backwash tanks simply involve discharging the backwash water to a tank where the water is <br />allowed to settle for a period of time (typically 8 hours). Clear water is decanted from the <br />backwash tank and recycled to the beginning of the treatment process. A backwash polymer <br />may be utilized to increase settling efficiency. <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 tiiitrAmew, wefie. <br />D.c.heiv <br />Pow. b.im <br />Urcleribror <br />REM, mipi <br />Como u 6.61 <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 />FEASIBILITY STUDY -DRAFT <br />Page 3 2 <br />RAMSY 154354 <br />