17 DEC 2003
Polymer will be added to enhance floc formation in the to reject water to aid
settling in the thickening tank. Jar settling tests conducted during the pilot test deter-
mined the settling rate of the solids in the reject flow, and that a settling time of 12 hours
provided good results. Size the thickening tank to provide 1 day of storage to allow sol-
ids settling and sufficient detention time so that the thickener tank requires decanting
once each day. Thickened solids will be pumped to a drying bed as required.
Size Thickening Tank for One-Day Storage
0.32 L/s x 3600 s/hr x 24 hr/day = 27,648 L
(5 gpm x 60 min./hr x 24 hr/day = 7200 gal.)
Assume that it takes 2 hours to empty the tank of sludge and water. Additional volume
required for 2 hours is
0.32 L/s x 3600 s/hr x 2 hr = 2304 L
(5 gpm x 60 min./hr x 2 hr = 601 gal.)
For freeboard and additional capacity, use a 37,850-L (10,000-gal.) thickening tank.
Flow Equalization Tank. Flow from five wells into the equalization tank is
1.90 L/s (30 gpm) total. Assume that all the reject flow from the upflow filter is returned
to the equalization tank--0.32 L/s (5 gpm). The flow from the equalization tank is equal
to the flow rate from the well field plus the reject rate, which is recycled to the head of
1.90 L/s + 0.32 L/s = 2.2 L/s
(30 gpm + 5 gpm = 35 gpm)
C-1.9.1 Allow 2 hours to empty the sludge thickening tank to the equalization tank.
Flow from the sludge thickening tank to the equalization tank is:
(27,648 L / 2 hr) / 3600 s/hr = 3.84 L/s
([7,200 gal. / 2 hr] / 60 min./hr = 60 gpm).
Total flow into the equalization tank in 2 hours is:
(1.90 L/s + 3.84 L/s) x 3600 s/hr x 2 hr = 41,328 L
([30 gpm + 60 gpm] x 60 min./hr x 2 hr = 10,900 gal.)
Flow out of the equalization tank in 2 hours:
(1.90 L/s + 0.32 L/s) x 3600 s/hr x 2 hr = 15,984 L
([30 gpm + 5 gpm] x 60 min./hr x 2 hr = 4200 gal.)