Residuals Management - Public Water Systems - page 786

Filter Cloths

Feed Slurry

Filtering Chamber

Diaphragm Diaphragm

High Pressure Water

Filtrate Filtrate

Step 1
Low Pressure Filtration

Step 2
Compression of Sludge by the Diaphragm

Cake Wash Water

Step 3
Cake Discharge

Step 4
Filter Cloth Washing

Fig. 23-21. Operational cycle for a variable volume diaphragm filter press

complete separation of the solids from the media of the filter press. The assistance
with cake removal typically consists of an operator with a ''rowboat'' oar scraping,
helping pry the cake off the filter cloth (see Fig. 23-22). Process yields can typically
be increased 10 to 30 percent by carefully controlling the optimum cycle times with
a microcontroller.
Filter presses are normally installed well above floor level so that the cakes can
drop out into trailers positioned underneath the presses. Alternatively, conveyors can
be installed under the presses to transport the cakes to a dewatered cake storage area.
In order for pressure filtration to be economical, alum sludges must be conditioned
to achieve a lower resistance to filtration. Lime is an effective conditioner, and fly ash
can be equally effective. 9 The choice of conditioning agent is an economic one that
should be investigated for each application.
Lime is typically added to alum sludge until the pH of the slurry is raised to about
11. There must be sufficient residence time of the lime-treated sludge to allow complete
reaction of the lime with the sludge. A minimum time requirement is typically 30