Page 444 - Civil Engineering Formulas

370 CHAPTER THIRTEEN

TABLE 13.1 Facility Capacity and Number of Centrifuges
Facility size, Dewatering Centrifuges operating
3
Mgd (m /d) operation, h/d Spare @ gal/min (L/s)
2 (7570) 7 1 1 @ 25 (1.58)
5 (18,930) 7.5 1 1 @ 50 (3.16)
20 (75,700) 15 2 1 @ 50 (3.16)
50 (189,250) 22 2 1 @ 75 (4.73)
100 (378,500) 22 3 2 @ 100 (6.31)
250 (946,250) 22 4 2 @ 200 (12.62)
(Design Manual for Dewatering Municipal Wastewater Sludges, U.S. EPA)
The sludge feed rate for the dewatering facility is
gal/day of digested sludge
Sludge feed rate (13.35)
operating h/day, 7 days/week (60 min/h)
And the sludge weight flow rate is

(V)(p)(s.g.)(% solids)(60 min/h)
W s (13.36)
7.48 gal/ft 3
W weight flow rate of sludge feed, lb/h (kg/h)
s
V volume flow rate of sludge feed, gal/min (L/s)
s.g. specific gravity of sludge
% solids percent solids expressed as a decimal
3
3
density of water, 62.4 lb/ft (994.6 kg/m )
Since the solids exiting the centrifuge are split between the centrate and the
cake, it is necessary to use a recovery formula to determine solids capture.
Recovery is the mass of solids in the cake divided by the mass of solids in the
feed. If the solids content of the feed, centrate, and cake are measured, it is pos-
sible to calculate percent recovery without determining total mass of any of the
streams. The equation for percent solids recovery is
F C c
C s
R 100 F C s C c (13.37)
where R recovery, percent solids
C cake solids, percent solids (25%)
s
F feed solids, percent solids (5%)
C centrate solids, percent solids (0.3%)
c
The dewatered sludge (cake) discharge rate is
Cake discharge rate (lb/h) dry solids (sludge feed rate, lb/h)(solids recovery)
(13.38)