In many industrial processes that feature the handling of liquids, the substances being transferred may occur in an aqueous or "muddy" state with the liquid portion needing to be removed. This liquid-removal stage may take place during the production process &#; for example, in the filtration of edible oils or yeast solutions &#; but it more frequently occurs at the end of a process such as wastewater treatment when the waste sludge that has formed must be withdrawn.
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Since the disposal of such sludge is calculated based on weight and volume, its thickening and drying is particularly lucrative for the processor. During the thickening and drying process, the sludge is treated using chemicals and/or physical processes so that the waste sludge forms into flake-like solids. After adjusting the pH level to neutral or alkaline via the use of milk of lime, the subsequent dewatering process separates the water for disposal, and the remaining volume of sludge is significantly reduced.
The simplest technology for this process involves the collection and thickening of the sludge through the use of gravity. However, it is significantly more effective to use technical drying methods via processes such as centrifuges and evaporators or, more commonly, chamber filter presses (see Figure 1).
The operational principle of the chamber filter press relies on the use of plastic frames pressed together under high pressure. Inside the frames are hollow chambers (from which the press gets its name) surrounded by filter cloth. When the sludge is fed into the chambers using pressure, 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 is then opened and the solid filter cakes can be 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 (116 and 218 psig) at its peak &#; should be even so the sludge flocs are not destroyed 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, a further constraint is that an empty-running sludge tank can lead to dry-running of the pump used to generate pressure.
To build pressure, different systems of displacement pumps are frequently used, including piston diaphragm pumps, applied most often for large presses. In these large and costly units, one or two diaphragms are hydraulically actuated and feed the sludge into the press through a series of valves. These pumps may require an air-pressure vessel to equalize the feed rate and a maximum pressure monitor or bypass, resulting in large overhead costs even for small plants. 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. However, operational constraints need to be considered in small and medium-sized plants due to their sensitivity to abrasion and dry-running. The space required to use this system is also considerable.
The list of filter press pumps also includes hose diaphragm piston pumps, which function similarly to piston diaphragm pumps but with crimped hoses rather than diaphragms. Piston pumps typically generate strong pulsation and require constant lubrication. Both of these pumps are characterized by their simple electrical operation and fairly high installation and maintenance costs.
By comparison, it is easier to use air-operated double-diaphragm (AODD) pumps; they are resistant to dry-running, virtually maintenance-free, self-priming, self-regulating and highly compact. Without the need 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 point 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.
of filter press pump
A standard AODD pump is typically limited to the pressure of the supplied air, which may be insufficient to fill the press. For this reason, it is often necessary to increase the pressure, for which there are three different technical solutions, outlined here.
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 air and 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 anymore 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 sensitive and breaks quickly.
&#;When selecting a filter press pump, AODD pumps offer several advantages.&#;
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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 are designed and built for significantly lower pressures and, as a result, have limited resistance to the increased strain. Also, the increased pressure resulting from these air-pressure amplifiers, or "boosters," pulsates strongly, which can influence the flow of the product. Boosters also may reach their limits in maintaining pressure, e.g., during repressing, because the devices used are almost always too small. These devices yield the required end pressure but may require a longer filling time.
Belt filter presses and chamber filter presses are the special equipment for separating solids and liquids through filter cloth. Sludge, as the the main material treated, which is coproduct of sewage treatment. The sewage is cleared after treatment and the pollutants are transferred to the sludge.
How to choose the proper model?
Generally, belt sludge dewatering machine is composed of filter belt, roller cylinder, filter belt tensioning system, filter belt deflection system, filter belt washing system and filter belt drive system. When making model selection, the following aspects should be considered:
1. Filter belt. The high tensile strength, tortuosity, acid and alkali resistance, temperature resistance and other characteristics are required. At the same time, the appropriate woven texture should be selected according to the specific properties of sludge, to make filter belt have good gas permeability and interception performance of mud particles.
2. Deflection system of roller cylinder. It is usually done by pneumatic means.
3. Tensioning system of filter belt. It is also generally controlled by pneumatic system. The filter belt tension is generally controlled at -0.3-0.7 MPa, and the usual value is 0.5 MPa.
4. Belt speed control. Different types of sludge have different requirements on belt speed, that is, there is a range of belt speed control for any specific sludge. Within this range, the dewatering system can guarantee a certain processing capacity, and can get high quality mud cake.
Chamber filter press is extruded through the plate frame to make the water in sludge discharged through filter cloth to achieve the purpose of dehydration. It is mainly composed of concave man filter plate, frame, automatic-pneumatic closed system measuring plate suspension system, filter plate vibration system, air compression device, filter cloth high-pressure flushing device and body side photoelectric protection device. When selecting, the following aspects should be considered:
1. The requirement for solid content of mud cake. Compared with other types of dewatering machines, the chamber filter press has the highest solid content. If considering the factors of reducing the land occupation of sludge, the plate and frame filter press can be selected.
2. Frame material.
3. The material of filter plate and filter cloth. Corrosion resistance is required, and the filter cloth must have a certain tensile strength.
4. How the filter plate moves. The requirements can be fully or semi-automatically completed by hydraulic-pneumatic devices to reduce the labor intensity of operator.
5. The filter cloth oscillates device to make filter cake easy to fall off. Compared with other types of dewatering machines, the biggest disadvantage of filter presses is that they have a large footprint.
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