Handbook of Civil Engineering Calculations

R = 100(25/5) ["(5 _°Q3) 1 = 95.14%

4. Determine the Dewatered Sludge Cake Discharge Rate
   The dewatered sludge (cake) discharge rate is calculated using the following:

Cake discharge rate (Ib/h) dry solids = (sludge feed rate, lb/h)(solids recovery)

= (670 lb/h)(0.95 14) = 637.5 Ib/h (289.4 kg/h) dry cake

The wet cake discharge in Ib/h is calculated using the following:

,. , ^n Dry cake rate, Ib/h

Wet cake discharge 5 v (Ib/h) = ———————

' Cake % solids

Wet cake discharge (Ib/h) = :

= 2550 Ib/h wet cake (1157.7 kg/h)

The volume of wet cake, assuming a cake density of 60 Ib/ft^3 is calculated as follows:

A7Volume , r of , wet cake ,-, (ft 3 „, /h) Wet = ——— cake rate, Ib/h^2550 Ib/h

v :—' ,„, = ^ t1 ,„.

' Cake density, Ib/ft^3      60 Ib/ft^3

= 42.5 ft^3 /h (1.2 m^3 /h) wet cake

For a dewatering facility operation of 4 h/d, the volume of dewatered sludge cake to be

disposed of per day is:

(42.5 ft^3 /h)(4 h/d) = 170 fWd = 1272 gal/day (4.81 L/d)

5. Find the percent reduction in sludge volume
   The percent reduction in sludge volume is then calculated using the following:

Sludge volume in - Sludge volume out

% Volume Reduction = —— :

Sludge volume in

x 100%

= 6,230 gal/day - 1,272 gal/day

6,230 gal/day

x 100% = 79.6%

Centrifuges operate at speed ranges which develop centrifugal forces from 1,500 to
3,500 times the force of gravity. In practice, it has been found that higher rotational
speeds usually provide significant improvements in terms of performance, particularly on
wastewater sludges.
In most cases, a compromise is made between the process requirement and O&M