PRESSURE FILTRATION OF WASTE INDUSTRIAL SLUDGES-
MODELING, DESIGN AND OPERATIONAL PARAMETERS
James L. Martin, Sanitary Engineer
Phillip L. Hayden, President
Pollution Control Science, Inc.
Miamisburg, Ohio 45342
INTRODUCTION
One of the most expensive unit operations of biological and certain other water and
wastewater treatment processes is handling and dewatering the large quantities of sludges
produced.   If the ultimate disposal method is landfill or incineration, economic considerations as related to transportation costs or fuel costs necessitate a high level of solids
concentration.   More stringent landfill requirements may also dictate higher sludge solids
concentrations.
Several dewatering techniques have been employed with varying degrees of success, such
as horizontal-belt vacuum or pressure filters, rotary vacuum filters, centrifuges and vertical
plate and frame or recessed-plate filter presses.   Overall economics as related to capital,
transportation, chemical, manpower and maintenance costs or perhaps specific solids level
requirements will determine what type of system is employed.
Pressure filtration dewatering produces well-formed compact sludge cakes which are
easily handled.   The cakes have a density of approximately 65 to 80 lb/ft3 with a solids
concentration of between 20 to 50%, depending upon sludge characteristics and operational parameters.   Sludge at concentrations of between 2 to 11% solids is pumped into
the filter at pressures which may ultimately be in excess of 100 psig.
The recessed-plate filter press consists of several separable plates, each side of which
forms one half of a cake chamber.   Each face of the plate is corrugated in some fashion
to provide support for the filter cloth and channels for the filtrate to drain to outlet
ports.   Sludge is pumped into the cavity between the filter cloths, where the solids are
retained.   The effluent, which is essentially free of suspended particles, passes through.
The filter media actually becomes the initial layer of sludge solids deposited on the cloth,
thus the rate of filtration decreases as the sludge layer thickens.   Upon completion of a
filter run, dictated by the cake solids concentration desired or the rate of filtrate discharge, the plates are separated sequentially to permit the sludge cakes to fall to a conveyor or hopper for removal from the filter room.
A filter precoat of fly ash, diatomaceous earth, or paper fiber is often used to prevent
cloth blinding and to provide a shear plane to permit the cakes to readily separate from
the filter cloth.   Overall dewatering rates may be enhanced considerably by using chemical coagulants such as lime, alum, and ferric chloride and/or sludge admixtures (also
termed body feed or filter aid) such as fly ash, diatomaceous earth and paper fiber.
The design parameters, i.e., cycle length, cake solids, cake density, or mass loading per
unit area as a function of time, may be obtained by the following methods:
1. Pilot-Scale Tests.  Several vendors supply pilot-scale filter units consisting of three
chambers, 4 in.,6 in. or 12 in. in size.  The filter unit is equipped with an air operated feed pump and is rented on a monthly basis plus shipping charges.  Approximately 2 to 20 gal of sludge are required per test.
2. Bench-Scale Tests.     Bench-scale equipment which may be quite simple or elaborate
and expensive is also available from some vendors.  Although some vendors offer
to conduct tests in their laboratories, dewatering tests performed onsite with
fresh sludges will provide more realistic and reliable results.
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