Belt Press Introduction
Belt presses are designed for use in municipal, industrial or specialized applications where removal of a liquid (usually water) from a solid-liquid suspension, slurry, is desirable. A Belt Filter Press does this through the use of two or three porous belts, which allow both gravity and pressurized dewatering. In the gravity stage, the sludge is allowed to thicken and most of the filtrate, water, is removed. During the pressure filtration stage, the sludge is also exposed to shear forces that greatly increase filtration. The filtrate removed is captured and piped to the base of the machine. This water which has a very low solids is often drained back to the head of the plant. In some instances, the filtrate may be recycled as belt wash water. The discharge of the machine is a friable cake with greatly increased solids content. When equipped with three belts they can be operated as a gravity belt thickener or belt filter press.
Prior to filtration, the sludge must be conditioned to cause solid particle agglomeration or clumping. This process, called flocculation is accomplished by injecting a polyelectrolyte, or polymer, into the slurry. Since most slurries are charged, it is important to select the proper type of polymer to neutralize the charge. As the nature of flocculated sludge controls to some extent the dewatering characteristics of that slurry, polymers of varying charge, strength, and molecular weight should be tested. Aspen Rentals can assist by analyzing individual slurries and recommending the optimum polymer and dosage. Along with the type of polymer, mixing energy and retention time play a major role in the flocculation process. Ashbrook typically supplies one in-line variable orifice mixer and polymer injection ring per machine. We recommend that three polymer injection locations in the sludge feed line be established. The locations should be selected to allow 15, 30 and 45 seconds retention time before the polymer sludge mixture enters the press. These locations allow the mixer and polymer injection ring to be relocated easily should more or less retention time be required for optimum dewatering. To remix flocculated sludge is counter-productive, so the unused locations should be fitted with spool pieces of nominal size.
The flocculated sludge is discharged from the feed line onto the press through a sludge chute. The sludge first hits a hinged baffle. The baffle dissipates the energy of incoming sludge. The opening of the baffle varies automatically with sludge flow. If an impurity stays between the baffle and the upper belt, the upcoming sludge will push the baffle up around the hinge and let the impurity pass through. This baffle is a non-plugging energy-dissipating device. The chute expands to the whole width of the press at the beginning of the gravity separation zone. When equipped with the 3-belt option the flocculated sludge is fed into a feed tank spreader assembly at the beginning of the gravity deck. The sludge distributes evenly to the whole width of the press and the gravity separation starts. The dewatering process sludge is conveyed on a porous weave belt. The belt allows the free water to pass through where it is captured by drain pans and piped to the machine base. The sludge is prevented from running off of the sides of the press by sludge restrainers and rubber seals. As the sludge is moved through the gravity section, it is turned over by chicanes. These plow-like devices greatly increase the gravity dewatering by clearing places for the water to drain as they turn the sludge mass. At the end of the gravity section, the sludge is loosely structured cake ready for the pressure dewatering. With the Aquabelt. conversion or three-belt option the thickened sludge can be discharged and not be passed through the pressure zone of the belt filter press.
The horizontal wedge section uses the natural effects of gradually increasing pressure imposed by an adjustable wedge plate. After the wedge the two belts further converge at the radius grid. This zone further reduces the volume of sludge in preparation of pressing. Then the sludge enters the roller compression zone. The first roller encountered in this stage is a 16" diameter perforated roller. This perforated roller has axial vanes to channel captured filtrate out the ends of the roller and away from the sludge. As the belts travel past the perforated roller, they take an S-shaped path around various diameter rollers, decreasing in diameter toward the discharge. This increases both the pressure and the shear-action to the sludge. The sets of rollers are sized to give optimum performance in the high- pressure section of the dewatering press. The shearing action in the high-pressure section turns the sludge mass between the belts exposing the wet inner cake to the belts and expelling the free water. At the end of these rollers the belts separate and the sludge cake is discharged from the machine.
The belts continue past scrapers that remove any residual cake and prepare the belts for cleaning. The belts are washed in separate wash stations that remove particles that may have embedded in the porous belt. The belts must be continually cleaned to prevent belt blinding and a loss of dewatering ability. The clean belts exit the wash stations to begin the process again.