23 JULY 2003
achieved, the system switches to a constant pressure until the flow rate diminishes to a
predefined low level.
The plate and frame filter press process typically operates in a batch filtration cy-
cle that involves the following steps: initial fill, increasing cake formation, approaching
constant pressure filtration, and cycle termination. Figure 2-1 is a schematic of the
filtration cycle, showing pressures, flow rates, and cycle times. During the initial fill
period, sludge is fed into the press at a relatively high and constant rate and at a
relatively low pressure. As the press fills and solids accumulate on the filter media, cake
formation increases, flow rate decreases, and pressure increases. As the filter cake
continues to form, filtration flow is severely restricted by a change in the porosity of the
cake, and the pressure increases to a near constant rate. At a set pressure point, the
constant pressure will be maintained while the solids continue to accumulate. As this
step continues, the flow rate decreases and the filter cycle is terminated.
2-1.6 Definitions. The following provides definitions for terms used throughout this
Blinding: Adverse particle accumulation or clogging of filter cloth or media.
Cake solids: The amount of solids in the sludge cake after it has been dewatered. The
term is typically expressed in percent solids, where 1% is approximately 10,000 mg/L
Cloth dog: A protrusion from the rim of a non-gasketed plate over which grommets of
the filter media are hooked.
Coagulation: Floc formation as the result of adding coagulating chemicals. Coagulants
destabilize (reduce repulsive forces) suspended particles, allowing them to agglomer-
Conditioning: The act of pretreating sludge (before dewatering) to enhance water
removal or solids capacity by the addition of inorganic and organic chemicals, solids
washing (elutriation), or thermal treatment.
Core blowing: The act of removing liquid sludge from the sludge feed port with com-
pressed air before sludge cake discharge.
Cycle time: The time, typically defined in minutes or hours, that is required to filter one
batch of material. This time includes the filtration period, core and air blowdown period,
and sludge cake discharge time.
Dewatering: Reduction of moisture content in sludge, which usually results in solids
concentrations of 12 to over 50%.