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1. Belt filter press A belt filter press uses two tensioned filter belts that hold the sludge layer, which then passes through a series of regularly arranged rolling cylinders in an S-shaped path. The tension of the filter belts themselves generates compression and shear forces on the sludge layer, thereby squeezing out the capillary water present in it and producing a sludge cake with a higher solid content, thus achieving sludge dewatering. Band-type dewaterers are less affected by fluctuations in sludge load. They also feature a low moisture content in the discharged sludge, stable operation with low energy consumption, relatively simple management and control, and lower requirements regarding the qualifications of the operating staff. However, blockages can occur easily. To prevent these blockages, it is necessary to use large amounts of water for flushing, which not only leads to waste of water resources but also increases the burden on the internal recycling system for wastewater treatment. Moreover, once it gets blocked, the machine must be shut down for maintenance, which prevents the dehydrator from operating continuously and affects the normal production activities of enterprises or society ； Moreover, the operating costs are extremely high; when including the electricity and water bills, labor costs, and maintenance expenses over two years, the total cost is even higher than the price of the equipment itself. However, since belt filter presses were introduced to China early on, most newly built wastewater treatment plants now use belt filters for dewatering. 2. Centrifugal dewaterer The centrifugal dewaterer is mainly composed of a transfer device and a screw conveyor with a hollow shaft; the sludge is fed into the drum through this hollow shaft, and under the centrifugal force generated by high-speed rotation, it is immediately thrown into the drum cavity. Sludge particles have a high specific gravity, resulting in a strong centrifugal force that causes them to be flung against the inner wall of the rotor, forming a solid layer ； Water has a low density, so the centrifugal force is also low; only a liquid layer is formed on the inner side of the solid layer. The sludge in the solid layer is slowly pushed by the screw conveyor to the cone end of the transfer device, from where it is continuously discharged through the outlets surrounding the transfer device. The liquid, on the other hand, overflows over the weir and is discharged outside the transfer device, where it is collected before being sent to the dewatering machine. Disadvantages of centrifugal dehydrators: 1. High noise level, which can cause an increased heart rate at the installation site ； 2. Changes in material concentration require timely adjustment of speed and speed difference ； 3. In high-speed operating environments, the site is dirty, resulting in mist containing mud ； 4. Improper operation can lead to clogging of the sludge discharge port, and backwashing may cause the sludge to become slurry again. The advantage is continuous operation, with the slurry not requiring concentration. In China, only a few manufacturers are capable of producing small centrifugal dewatering machines. If large centrifugal dewatering machines are chosen, imports are the only option, which increases the cost of the project. Moreover, these machines are highly sensitive to fluctuations in sludge load, and they require skilled operators; as a result, centrifugal dewatering processes are not generally used in sewage treatment plants. With technological progress, centrifugal dewatering technology has made significant advances abroad. For example, the spiral centrifugal dewaterers produced by the Swedish company AlfaLayal can achieve a solid content in the sludge cake of over 30 percent. Moreover, operations take place in a completely enclosed environment; there is no sludge or wastewater around the dewaterer, nor any unpleasant odors. This **improves the working conditions for operators, which is why such devices are favored by industry professionals. 3. Plate and frame filter press: A plate and frame filter press achieves dewatering by using the compression of plates and frames to force the water in the sludge to pass through the filter cloth. It is mainly composed of a concave filter plate, a frame, an automatic-pneumatic closing system, a plate suspension system, a filter plate vibration system, an air compression unit, a high-pressure flushing device for the filter cloth, and a photoelectric protection device on one side of the machine body. Its advantages are: low cost and high solid content in the mud cake ； The disadvantages are: intermittent operation, large floor space, and high maintenance frequency. Therefore, most wastewater treatment plants no longer use this equipment. The following aspects should be considered when making a selection: 1. The requirement for the solid content of the sludge cake. Generally, compared to other types of dewatering machines, plate and frame filter presses achieve the highest solid content in the sludge cake, which can reach 35 percent. If reducing the space required for storing sludge is a concern, then plate and frame filter presses should be the preferred choice. 2. Material of the frame. 3. Material of the filter plate and filter cloth. Corrosion resistance is required, and the filter cloth must have a certain tensile strength. 4. Movement method of the filter plate. The requirement can be met through hydraulic-pneumatic devices for fully or semi-automated operation, in order to reduce the workload on operators. 5. Filter cloth oscillation device to facilitate the shedding of the filter cake. 4. Screw press sludge dewaterer – The screw press sludge dewaterer is a new type of equipment that has only appeared on the domestic market in the past two years. Its principle of operation is as follows: 1. The spiral wound body of the dehydrator is a filtering device formed by stacking fixed rings and moving rings together, with a spiral shaft running through it. The front section is the concentration unit, and the rear section is the dehydration unit. 2. Filter gaps are formed between the fixed ring and the moving ring, and the pitch of the spiral shaft gradually decreases from the concentration section to the dewatering section. 3. The rotation of the spiral shaft not only pushes the sludge from the concentration section to the dewatering section but also continuously moves the floating ring to clean the filter gaps, preventing blockages. The principle of dehydration is as follows: After being concentrated by gravity in the concentration section, the sludge is transported to the dehydration section. As it moves forward, the filter gaps and screw pitches gradually become smaller, and together with the effect of the back pressure plate, a high internal pressure is generated; this causes the volume of the sludge to decrease continuously, thereby achieving thorough dehydration. The disadvantages of the spiral press sludge drier are its high cost and low processing capacity. Its advantages are mainly reflected in four aspects: 1. It is not prone to clogging, as there is no filter cloth; instead, a multi-layered sheet spiral pressure filtration method is used ； 2. It saves water and electricity, requires no cleaning to prevent clogging of the filter gaps, operates at low speed resulting in minimal noise and vibration, and is safe to use. 3. Compact design: it is tightly structured, integrating concentration and dehydration processes, thus requiring less space and reducing design and construction costs. 4. Durable, with a stainless steel body; few replacement parts are required, resulting in a long service life. However, since the spiral press sludge dewaterer was introduced to China relatively late and has a low market share, few sewage treatment plants choose it