DESIGN OF A SOLID-BOWL CENTRIFUGE
FOR SLUDGE DEWATERING
A 4.0 Mgd (15,140 m
3
/d) municipal wastewater treatment facility produces 6,230 gal/day
(23.6 m
3
/d) of aerobically digested sludge at 5.0 percent solids. Determine design param-
eters for the specification of a solid bowl centrifuge for dewatering the sludge including:
number of centrifuges, solids feed rate, percent solids recovery, dewatered sludge (cake)
discharge rate, centrifugal force, polymer dosage, and polymer feed rate. Assume the fol-
lowing apply:
- Feed sludge is aerobically digested at 5.0 percent solids.
- Dewatered sludge (cake) is to be 25 percent solids.
- Centrate assumed to be 0.3 percent solids.
- Polymer solution concentration is 25 percent.
Calculation Procedure:- Select the number of centrifuges
The separation of a liquid-solid sludge during centrifugal thickening is analogous to the
separation process in a gravity thickener. In a centrifuge, however, the applied force is
centrifugal rather than gravitational and usually exerts 1,500 to 3,500 times the force of
gravity. Separation results from the centrifugal force-driven migration of the suspended
solids through the suspending liquid, away from the axis of rotation. The increased set-
tling velocity imparted by the centrifugal force as well as the short settling distance of the
particles accounts for the comparatively high capacity of centrifugal equipment.
Centrifuges are commonly used for thickening or dewatering Waste Activated Sludge
(WAS) and other biological sludges from secondary wastewater treatment. In the process,
centrifuges reduce the volume of stabilized (digested) sludges to minimize the cost of ul-
timate disposal. Because centrifuge equipment is costly and sophisticated, centrifuges are
most commonly found in medium to large wastewater treatment facilities.
The capacity of sludge dewatering to be installed at a given facility is a function of the
size of a facility, capability to repair machinery on-site, and the availability of an alterna-
tive disposal means. Some general guidelines relating the minimal capacity requirements
are listed in Table 1. This table is based on the assumption that there is no alternative
mode of sludge disposal and that the capacity to store solids is limited.
TABLE 1. Facility Capacity & Number of Centrifuges
Dewatering Centrifuges operating +
Facility size, Mgd (m^3 /d) operation, h/d spare @ gal/min (L/s)
2(7,570) 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)