23 JULY 2003
Stay bosses: Raised surfaces on the interior main surface of the plate used to minimize
plate deflection under operating conditions. When the filter is closed, the faces of
bosses of adjacent plates contact one another, in effect forming solid columns from one
end of the filter to the other.
Thickening: A sludge pretreatment process used to increase the solid content or
decrease the moisture content in sludge prior to the primary dewatering process. The
solids concentration of the resultant sludge is typically 3 to 12%.
Thixotropic: A characteristic of certain materials, often associated with sludge, that
refers to a time-dependent change of decreasing viscosity and the resultant fluid-type
characteristic that occurs because of applied agitation or shearing force, followed by a
gradual recovery or "setting up" when an agitation or shearing force is stopped. An
example of this characteristic is an ice cream milkshake, which "sets up" in its container
and will only flow out when the container is rapped or jarred several times. Other
examples include drilling muds, mayonnaise, and paints.
Variable-volume press: A plate and frame press that forms a sludge cake in chambers,
formed by filter plates equipped with membranes or diaphragms that are expanded with
water or air pressure to provide the primary "squeezing" force at the end of the filtration
Objectives. The overall objective of this chapter is to provide engineering and
design details for the plate and frame filter press technology, equipment, and ancillary
technologies and equipment. This chapter also includes a discussion of the differences
between the two plate and frame filter press systems (i.e., recessed fixed-volume and
variable-volume [diaphragm] recessed plate and frame filter presses).
PRINCIPLES OF OPERATION. The recessed plate and frame press consists
of a series of plates, supported in a frame, that contain adjacent (facing) recessed
sections that form a volume into which liquid sludge can be transferred for dewatering.
The plates that form the recessed chambers are lined with filter media to retain sludge
solids while permitting passage of the filtrate. The plates are also designed to facilitate
filtrate drainage while holding the filter media in place.
During the filtration cycle, sludge is pumped under varying pressures and flow
rates into the volume formed between the plates. As the filtration process continues, the
filtrate passes through the solid cake and filter media. This process continues until a
terminal pressure or minimum flow rate is achieved.
The two types of plate and frame filter presses typically used in dewatering
sludge are fixed-volume and variable-volume presses. The fixed-volume system is the
more commonly used press. However, the variable-volume press, otherwise called the
diaphragm or membrane press, has become more popular in recent years. Following is
a brief overview of both presses.