Environmental Engineering Reference
In-Depth Information
Example 10.27
Problem: Thickened thermally conditioned sludge is pumped to a vac-
uum filter at a rate of 50 gpm. The vacuum area of the filter is 12 ft wide
with a drum diameter of 9.8 ft. If the sludge concentration is 12%, what
is the filter yield in lb/hr/ft 2 ? Assume the sludge weighs 8.34 lb/gal.
Solution: First calculate the filter surface area:
Area of a cylinderside .14DiameterLengt
=
×
×
h
=
3.14
×
9.8ft12ft
×
369.3ft 2
=
Next, calculate the pounds of solids per hour:
50 gpm
1min
60 min
1hr
8.34 lb
1 gal
12%
100
×
×
×
=
3002.4lb/hr
%
Dividing the two:
3002.4lb/hr
369.3ft
2
=
8.13 lb/hr/ft
2
10.7.3 Pressure filtration
Pressure filtration differs from vacuum filtration in that the liquid
is forced through the filter media by a positive pressure instead of a vac-
uum. Several types of filter presses are available, but the most commonly
used types are plate-and-frame presses and belt presses. The belt filter
includes two or more porous belts, rollers, and related handling systems
for chemical makeup and feed, as well as supernatant and solids collec-
tion and transport.
The plate-and-frame filter consists of a support frame, filter plates
covered with porous material, a hydraulic or mechanical mechanism for
pressing the plates together, and related handling systems for chemi-
cal makeup and feed, as well as supernatant and solids collection and
transport. In the plate-and-frame filter, solids are pumped (sandwiched)
between plates. Pressure (200 to 250 psi) is applied to the plates and
water is squeezed from the solids. At the end of the cycle, the pressure
is released; as the plates separate, the solids drop out onto a conveyor
belt for transport to storage or disposal. Performance factors for plate-
and-frame presses include feed sludge characteristics, type and amount
of chemical conditioning, operating pressures, and the type and amount
of precoat.
The belt filter uses a coagulant (polymer) mixed with the influent
solids. The chemically treated solids are discharged between two mov-
ing belts. First water drains from the solids by gravity. Then, as the
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