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.