Selection of sludge dewatering equipment such as filter press

     The type selection of sludge dewatering equipment such as filter press will produce a large amount of sludge after sedimentation treatment. Even after concentration and digestion treatment, the water content is still as high as 96%, and the volume is very large, so it is difficult to absorb and dispose. Therefore, dewatering treatment must be carried out to improve the solid content of sludge cake, so as to reduce the land occupation of sludge stacking.

     Generally, mechanical dewatering is used in large and medium-sized sewage treatment plants. There are many kinds of dehydrators, which can be divided into vacuum filtration dehydration, pressure filtration dehydration and centrifugal dehydration according to the dehydration principle. This paper introduces the working principle of filter press (including belt filter press and plate and frame filter press) and centrifugal dehydrator commonly used in domestic sewage treatment plants, the key problems to be considered in equipment selection and the maintenance and operation cost.

 

1. Belt filter press dehydrator belt filter press dehydrator is composed of two tensioned filter belts carrying the sludge layer, passing through a series of regularly arranged roller pressing cylinders in S shape. Depending on the tension of the filter belt, the pressure and shear force on the sludge layer are formed, and the capillary water in the sludge layer is squeezed out to obtain the sludge cake with high solid content, so as to realize sludge dewatering. The general belt filter press dehydrator is composed of filter belt, roller pressure cylinder, belt tension system, belt deviation adjustment system, belt washing system and belt drive system. The following aspects should be considered in the selection of aircraft type:

(l) Filter belt. It is required to have high tensile strength, resistance to twists and turns, acid and alkali resistance, temperature change resistance, etc. at the same time, the specific properties of sludge should be considered, and the suitable woven texture should be selected to make the filter belt have good air permeability and interception performance for sludge particles.

(2) The deviation adjustment system of roller press cylinder. It is usually accomplished by pneumatic device.

(3) Belt tension system. It is also controlled by pneumatic system. The tension of filter belt is generally controlled at 0.3-0.7mpa, and the commonly used value is 0.5MPa.

(4) Belt speed control. The different properties of sludge have different requirements on the belt speed, that is, there is a better control range of belt speed for any specific sludge. Within this range, the dewatering system can not only ensure certain treatment capacity, but also obtain high-quality sludge cake. The belt filter press dehydrator is less affected by the fluctuation of sludge load. It also has the characteristics of low moisture content of sludge, stable operation, less start-up consumption, relatively simple management and control, and low quality requirements for operators. At the same time, as the belt filter press dehydrator entered China earlier, a considerable number of manufacturers can produce this equipment. In the decision-making of sewage treatment project construction, belt filter press can be selected to reduce project investment. At present, most of the newly-built domestic sewage treatment plants use belt filter press dehydrators. For example, all the five dehydrators in Beijing Gaobeidian Sewage Treatment Plant Phase I project are all belt filter press dehydrators, which have been in good condition since they were put into operation. Therefore, this type of filter press is still used in phase II equipment selection.

 

2. Centrifugal dehydrator centrifugal dehydrator is mainly composed of transfer and screw conveyor with hollow shaft. After the sludge is sent into the drum by the hollow shaft, it is immediately thrown into the hub cavity under the action of centrifugal force generated by high-speed rotation. Due to the large proportion of sludge particles, the centrifugal force generated is also large, which is thrown onto the inner wall of the hub to form a solid layer; the water density is small, and the centrifugal force is small, and only a liquid layer is generated inside the solid layer. The sludge in the solid layer is slowly pushed by the screw conveyor, and is transported to the cone end of the transfer, and is continuously discharged through the outlet around the transfer. The liquid is discharged from the overflow of weir No.4 to the outside of the transfer, and is collected and discharged into the dehydrator. The key component of centrifugal dehydrator is the hub. The larger the diameter of the hub, the greater the dehydration capacity. However, the manufacturing and operating costs are quite high, which is not economic. The longer the transshipment length is, the higher the solid content of sludge is, but the longer the transshipment length is, the lower the performance price ratio will be. In the process of using, the speed of transfer is an important control parameter. The key to the operation of centrifugal dehydrator is to control the rotating speed of the hub, which can not only obtain higher solid content but also reduce energy consumption. At present, low speed centrifugal dehydrator is widely used. In the selection of centrifugal dehydrator, the outer edge of runner or screw is easy to wear, so there are special requirements for its material. The screw outer edge of new type centrifugal dehydrator is mostly made into assembly block for easy replacement. The material of assembly block is usually tungsten carbide, which is expensive. Centrifugal dehydrator has the disadvantages of high noise, high energy consumption and low processing capacity. Only a few domestic manufacturers can produce small centrifugal dehydrators. If large-scale centrifugal dehydrators are selected, they can only rely on imports, which will increase project investment. At the same time, centrifugal dehydrators are greatly affected by the fluctuation of sludge load, which requires high quality of operators. Therefore, centrifugal dewatering process is not used in general sewage treatment plants. But in recent years, with the progress of science and technology, the dewatering technology of centrifugal dehydrator has made great progress in foreign countries. For example, the screw centrifugal dehydrator produced by Alfa layal company in Sweden has achieved a solid content of mud cake More than 30%, and the operation is carried out in a fully enclosed environment. There is no sludge and sewage around the dehydrator, and there is no odor. It can greatly improve the working environment of operators, so it is favored by the industry.

3. Plate and frame filter press and dehydrator the plate and frame filter press makes the water in the sludge discharged through the filter cloth through the extrusion of the plate and frame, so as to achieve the purpose of dehydration. It is mainly composed of concave type filter plate, frame, automatic pneumatic closing system, measuring plate suspension system, filter plate vibration system, air compression device, filter cloth high-pressure washing device and photoelectric protection device on one side of the fuselage. The following aspects should be considered in equipment selection:

(1) The requirement of solid content of mud cake. Compared with other types of dehydrators, the plate and frame filter press has a higher solid content of sludge cake, up to 35%. If considering the factors of reducing the land occupation of sludge stacking, the plate and frame filter press should be the first recommended scheme.

(2) The material of the frame.

(3) Material of filter plate and filter cloth. It is required to be corrosion-resistant, and the filter cloth should have a certain tensile strength.

(4) How to move the filter plate. It is required that the hydraulic pneumatic device can be fully automatic or semi-automatic to reduce the labor intensity of operators.

(5) Filter cloth oscillation device to make filter cake easy to fall off. Compared with other types of dehydrators, the larger disadvantage of plate and frame filter press is that it covers a larger area. Taking the belt filter press used in the first stage project of a sewage treatment plant in Beijing and the plate and frame filter press used in a sewage treatment plant in Liaoning Province as an example, the treatment capacity of Gaobeidian sewage treatment plant is 500000 T / D, the sludge production is 1852.5m3/d, and the dry matter is 92.63t/d. Five imported klein-ks30 belt filter presses are used, and the foundation area of each filter press is only 2750 * 3500mm, A wastewater treatment plant in Liaoning Province has a treatment capacity of 22T / D and dry matter of 275t / d. six plate and frame filter presses are used. The foundation of each filter press covers an area of 2400 * 12000mm. At the same time, due to the intermittent operation of plate and frame filter press, low efficiency, poor operating room environment and secondary pollution, large domestic wastewater treatment plants have rarely used it. However, a large number of research and development work has made it adapt to the requirements of modern sewage treatment plant. For example, PLC system control can realize the full automatic operation mode of the system. The operation of filter press, filter plate movement, filter cloth oscillation, compressed air supply, filter cloth washing, feeding and other operations can be completed through PLC remote control, greatly reducing the labor intensity of workers.

4. Summary

The reasonable choice of sludge treatment plant should be made according to the characteristics and requirements of sludge treatment plant and the cost of sludge treatment plant.

We supply slurry & tailings dewatering equipments for mining industry, including belt press, filter press, disc filter etc. If anyone who is interested in, Welcome to joyce@doyenchina.com or visit https://www.doyenchina.com.