Many industrial processes use substances that are in an aqueous or “muddy” form, and the liquid portion must be removed. This may take place during production processes—for example, in the filtration of edible oils or yeast solutions—but more frequently it happens at the end of a process with waste sludge from various industries, including wastewater treatment.
Because the disposal cost of such sludge is calculated using weight and volume, thickening and drying are particularly lucrative for the producer. During the process, sludge is treated using chemicals and/or physical processes so the substances in it form flake-like solids.
After pH adjustment to neutral or alkaline using milk of lime, the dewatering process separates out water for disposal and significantly reduces the remaining sludge volume.
Chamber Filter Presses
The simplest technology for this process involves sludge collection and thickening via gravity. It is more effective to use technical drying methods with processes like centrifuges and evaporators, or more commonly, chamber filter presses (see Image 1).
A chamber filter press includes plastic frames that are pressed together under high pressure. Inside the frames are hollow chambers that are surrounded by filter cloth.
When pressure feeds the sludge into the chambers, a “filter cake” forms inside the chambers, and the filtrate flows through the filter cloths into drainage channels. When all the chambers are completely filled, the sludge feed stops. The press can be opened and the solid filter cake removed. After closing, the press is ready for a new pressing process.
To fill these presses, filter material and pressure are required. The pressure—usually between 8 and 15 bar at its peak—should be even to avoid destroying the flocculated particles during feeding. The flocs should also have enough free space in the feed area. In addition to the constantly increasing counterpressure that occurs until the end of the pressing, another constraint is that an empty-running sludge tank can lead to dry-running of the pump used to generate pressure.
To build pressure, variations of displacement pumps are frequently used. They include piston diaphragm pumps, which are frequently applied for large presses.
In these large units, one or two diaphragms are hydraulically actuated and feed the sludge into the press with valves. These pumps incur large overhead costs, such as an air-pressure vessel to equalize the feed rate and a maximum pressure monitor or bypass.
Eccentric screw pumps are also used, either as self-regulating pumps (with motors that are electronically controlled via a frequency converter) or as cyclical systems (where an air-pressure vessel is “charged” by the pump). This valve-free procedure is advantageous when processing large sludge quantities and when long fibers prevent the use of valves. There are constraints in small- and medium-sized plants because of their sensitivity to abrasion and dry-running. The space required to use this system is also considerable.
Other utilized pumps include hose-diaphragm piston pumps, which function in a similar way to piston diaphragm pumps but with crimped hoses rather than diaphragms. Another is the piston pump, which typically generates strong pulsation and requires constant lubrication. Both of these pumps are characterized by their simple electrical operation and fairly high installation and maintenance costs.
By comparison, an air-operated double-diaphragm (AODD) pump can be easier to use in these applications. These pumps are low-maintenance, self-priming, self-regulating, highly compact, and resistant to dry-running. Without the control or intervention of an operator or electronic system, the counterpressure of the chamber filter press regulates the feed rate automatically.
The feed rate decreases continuously as counterpressure increases simultaneously to the degree of filling. This effect can be used to detect when the chamber filter press is full. When this is reached, the pump virtually stops—zero feed rate—or only occasionally makes a delivery stroke.
In addition, the use of compressed air as drive power to move the diaphragms results in a highly efficient, regular and gentle cyclic drive that allows the medium to be fed smoothly.
A standard AODD pump is typically limited to the pressure of the supplied air, which is often insufficient to fill the press. It is often necessary to increase the pressure, for which there are three very different technical solutions.
The first variant uses one of the diaphragms on a standard pump to generate additional pressure. The force of this diaphragm, which is surrounded only by compressed air, is transferred to the feed diaphragm via the internal diaphragm connection, enabling the feed diaphragm to work with double the pressure.
This method is rarely used because it leads to high pulsation, low feed rates and high air requirements. It also commands high service costs because the diaphragm on the air side is very sensitive and breaks quickly.
Another variant is to operate a standard pump with an air pressure amplifier, which drives the pump with increased air pressure. This process is limited by the fact that a standard pump is often used. Although these pumps are equipped with external reinforcements, from a technical perspective, the standard pumps in question are designed and built for significantly lower pressures and have limited resistance to the increased strain.
Additionally, the increased pressure resulting from these air pressure amplifiers, or “boosters,” pulsates strongly and can influence the product’s flow. Boosters also seem to reach their limits in maintaining pressure (for example, during repressing) because the devices are almost always too small. They yield the required end pressure but may require a longer filling time.
The third variant is a pump with internal pressure conversion. Figure 1 shows how this technical solution applies compressed air to a differential piston along with the diaphragms. The increased surface area—typically twice as large or more—causes the compressed air to generate a corresponding amount of increased force. This converted force acts on the feed diaphragms with increased (double) pressure.
The entire construction is designed for the high strain caused by the maximum amount of pressure, as well as the strain caused by the typically abrasive sludge. For this reason, the pump housing is built from materials such as stainless steel or polyethylene (PE UHMW). This tough material is a decisive factor in the durability of the pump.
Using compressed air to power a pump is effective because of constructive measures. The pump operates with minimal dead space—the space inside the pump that must be filled without serving the actual feed process. As a result, the pump always has sufficient power reserves to handle large volumes of wastewater.
A New Generation of Pumps
With the introduction of a new generation of high-pressure AODD pumps, there is an additional variant that combines the highly robust housing of the pressure-converting pump with an air section where no conversion takes place. This version is suitable for all applications under heavy load conditions, ranging from low-feed pressures to high-pressure applications of up to 15 bar (218 pounds per square inch gauge). If users operate a pump at such high air pressure—whether it is from an external booster or directly from the compressor—they can be assured that the pump is structurally designed for such pressure ranges and need not be held together by external reinforcements.
On these new pumps, the diaphragms are equipped with specially developed heavy-duty diaphragms with an integrated metal core that provides a long service life and the ability to handle heavy loads. The diaphragm’s vulcanized core supports extremely thick layers of elastomer. To transfer the suction forces, the core is reinforced with a special textile that is barely flexible in any direction.
In addition, these pumps can be combined with a sensor that responds to the diaphragm’s movement and allows the cycle to easily be monitored. The slow stroke frequency that accompanies a full press rarely triggers a signal. If a programmable logic controller is used to program a time window within which a stroke should take place, a full chamber filter press is indicated by the fact that these rare signals no longer occur within this time window. The compressed air can be switched off and a signal can be set for the operator to empty the press. This method functions purely by physical means and is independent of sensitive pressure gauges and contaminating sensors in the wastewater current.
When selecting pumps for filter press operations, AODD pumps are a solid solution that incorporates a number of characteristic advantages. Conventional displacement pumps with electric drive and control elements do not have these properties that are specific to the design of these pumps, which include run-dry capability, good controllability and a gasketless mechanical design.