What is a Screw Press (II)

2. Differences Between Screw Press and Other Dehydration Machines

Screw Press

The screw press occupies a small area, requires less flushing water, produces low noise, and maintains a good workshop environment. However, each unit has a relatively small processing capacity, and the solid content in the separated liquid is relatively high. Therefore, its application in China is not extensive. It is mainly used for direct dewatering of activated sludge in small sewage stations. It allows for the elimination of sludge concentration tank design in sewage station construction, saving costs.

When the sludge processing volume is the same, the screw press can save 85% more energy compared to the centrifugal dehydration machine and occupies only half of the space. In terms of surface cleaning, it can save 95% more water compared to belt filter presses.

The screw press is particularly suitable for treating oily sludge, such as fibrous materials from food processing plants that have undergone pressurized treatment. The moisture content requirement for screw press dewatering is generally between 95% and 99.5%, and the moisture content after dewatering is generally between 75% and 80%.

Plate and Frame Filter Press

The plate and frame filter press achieve dewatering by squeezing through the pressing of plates and frames, allowing water within the sludge to pass through the filter cloth and achieve dehydration.

The plate and frame filter press can filter suspensions with solid particle diameters above 5μm and solid concentrations ranging from 0.1% to 60%. It is suitable for filtering difficult-to-filter materials with high viscosity or colloidal properties, as well as substances with high quality requirements for filter cake.

The advantages and disadvantages of the plate and frame filter press are evident. It has strong adaptability to sludge concentration, high solid content in the dewatered sludge, and requires minimal coagulant dosage (or none at all). However, the equipment has a large footprint, operates intermittently, cannot achieve continuous sludge treatment, has low efficiency, and operates in an open environment. Additionally, the plate and frame filter press has more ancillary equipment, leading to a relatively higher failure rate during post-dewatering processing.

However, in recent years, with development, plate and frame filter presses have adapted to the requirements of modern sewage treatment plants. For instance, with PLC system control, the system can operate fully automatically, reducing labor intensity.

Belt Filter Press

The belt filter press is a machine where two tight filter belts carry the sludge layer through a series of regularly arranged roller press cylinders in an S-shaped path. It relies on the tension of the filter belts to exert squeezing and shearing forces on the sludge layer, extruding the capillary water from the sludge layer to obtain a cake with a high solid content, achieving sludge dewatering.

The belt filter press was introduced to China early on and has been accepted by most sewage and water treatment plants. It has the advantages of low noise, low electricity consumption, and its dewatering effect is not significantly affected by the influent load. The benefit lies in its low dewatered sludge moisture content, relatively simple management and control, and lower requirements for operator qualifications. However, the drawbacks include a large footprint, high flushing water consumption, and a propensity for issues like filter cloth clogging and belt misalignment.

Centrifugal Dehydrator

The centrifugal dehydrator consists mainly of a rotating drum and a helical conveyor with a hollow shaft. It features a compact layout, a small footprint, and can significantly reduce infrastructure investments.

The centrifugal dehydrator integrates sludge concentration and dehydration functions and can perform continuous dewatering with minimal susceptibility to fluctuations in sludge concentration. It yields sludge with a high solid content and large dewatered sludge output. Compared to belt and plate-and-frame filter presses, it consumes less coagulant and cleaning water, resulting in lower operating costs.

Drawbacks include high electricity consumption and substantial noise. The outer edge of the centrifugal dehydrator's rotor or screw is prone to wear. Currently, most new centrifugal dehydrators design the outer edge of the screw as an assembly block for easier replacement, typically made of tungsten carbide, which can be costly.

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