TABLE 3. Viscosity of Sewage Sludge vs.
Percent Solids
Percent solids Viscosity in centipoise (cp)
1 1 0
2 8 0
3 25 0
4 56 0
5 1,05 0
6 1,76 0
7 2,75 0
8 4,00 0
9 5,50 0
10 7,50 0(Courtesy Alfa Laval Pumps, Inc.)- Find the installed horsepower of the pump
Actual installed horsepower will need to be greater than this calculated horsepower to al-
low for the random torque increases due to large solids and to provide a prudent safety
factor. Therefore, the minimum installed motor horsepower is calculated as follows:
(1.2)(Calculatedhp)
Installed Motor hpF = —— -——^-- x 100
Motor Drive Efficiency (%)Assuming a motor efficiency of 90 percent, the installed motor horsepower is
Installed Motor hp =
(L2)(1
Q^hp)
x 100 = 1.73 hp (1.29 kW)
90/oUse the next largest motor size available, which is 2.0 hp (1.49 kW). Therefore, the rotary
lobe sludge pump will have a 2.0 hp (1.49 kW) motor.
To summarize, the requirements for a rotary lobe pump for this application are: design
condition of 26 gal/min (1.64 L/s) at 26 lb/in^2 (179.1 kPa), operating at 175 rpm with a
2.0 hp (1.49 kW) motor. Often a variable speed drive is provided so that the pump rpm
(hence flow rate) may be adjusted to suit varying sludge flow rate demands at the down-
stream dewatering facility. Also, a second pump is generally provided as a spare.
DESIGN OF AN ANAEROBIC DIGESTOR
A high rate anaerobic digestor is to be designed to treat a mixture of primary and waste
activated sludge produced by a wastewater treatment facility. The input sludge to the di-
gester is 60,000 gal/day (227.1 m^3 /d) of primary and waste activated sludge with an aver-
age loading of 25,000 Ib/d (11,350 kg/d) of ultimate BOD (BODL). Assume the yield co-
efficient (Y) is 0.06 Ib VSS/lb BODL (kg/kg), and the endogenous coefficient (kd) is 0.03
d"^1 at 350 C (95^0 F). Also assume that the efficiency of waste utilization in the digester is
60 percent. Compute the digester volume required, the volume of methane gas produced,
the total volume of digester gas produced, and the percent stabilization of the sludge.