After sedimentation treatment, sewage will produce a large amount of sludge. Even after concentration and digestion treatment, the moisture content is still as high as 96%, which is very large and difficult to absorb and dispose. Dehydration treatment must be carried out to improve the solid content of mud cake, so as to reduce the floor area of sludge stacking. Generally, large and medium-sized sewage treatment plants adopt mechanical dehydration. There are many kinds of dehydrators. According to the dehydration principle, they can be divided into three categories: vacuum filtration dehydration, pressure filtration dehydration and centrifugal dehydration. This paper introduces the working principle of filter press (including belt filter press and plate frame filter press) and centrifugal dehydrator often used in domestic sewage treatment plants, the key problems to be considered in equipment selection and maintenance and operation cost.
1. Belt filter press dehydrator: the belt filter press dehydrator is composed of two upper and lower tensioned filter belts carrying the sludge layer, passing through a series of regularly arranged roller cylinders in an S-shape, relying on the tension of the filter belt itself to form the pressing and shear force on the sludge layer, extrude the capillary water in the sludge layer, and obtain the mud cake with high solid content, so as to realize sludge dehydration. The general belt filter press dehydrator is composed of filter belt, roller, filter belt tensioning system, filter belt deviation adjustment system, filter belt flushing system and filter belt drive system. When making model selection, the following aspects should be considered:
(l) Filter belt. It is required to have high tensile strength, twists and turns resistance, acid and alkali resistance, temperature change resistance and other characteristics. At the same time, the specific properties of sludge should be considered and suitable woven texture should be selected to make the filter belt have good air permeability and interception performance to sludge particles.
(2) Deviation adjustment system of roller. Generally, it is completed by pneumatic device.
(3) Tensioning system of filter belt. Generally, it is also controlled by pneumatic system. The tension of filter belt is generally controlled at 0.3-0.7mpa, and the common value is 0.5MPa.
(4) Belt speed control. Sludge with different properties has different requirements for belt speed, that is, there is an optimal belt speed control range for any specific sludge. Within this range, the dehydration system can not only ensure a certain treatment capacity, but also obtain high-quality mud cake.
The belt filter press dehydrator is less affected by the fluctuation of sludge load. It also has the characteristics of low sludge moisture content, stable operation, less start-up consumption, relatively simple management and control, and low requirements for the quality of operators. At the same time, because 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 the project investment. At present, most of the newly-built sewage treatment plants in China use belt filter press dehydrators. For example, the five dehydrators in the phase I project of Beijing Gaobeidian sewage treatment plant are all belt filter press dehydrators. They have been in good condition since they were put into operation, so they are still used in the selection of phase II equipment.
2. Centrifugal dehydrator: the centrifugal dehydrator is mainly composed of a transfer and a screw conveyor with a hollow rotating shaft. After the sludge is sent into the drum by the hollow rotating shaft, it will be immediately thrown into the hub cavity under the centrifugal force generated by high-speed rotation. The specific gravity of sludge particles is large, so the centrifugal force is also large, which is thrown on the inner wall of the hub to form a solid layer; The water density is small, and the centrifugal force is small. Only the liquid layer is generated inside the solid layer. The sludge in the solid layer is slowly pushed by the screw conveyor to the cone end of the transfer, which is continuously discharged through the outlet around the transfer, and the liquid is discharged from the overflow of weir 4 to the outside of the transfer, which 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 treatment capacity, but the manufacturing and operation costs are quite high and uneconomical. The longer the length of reprint, the higher the solid content of sludge, but too long reprint will reduce the performance price ratio. In the process of use, the rotating speed of the transfer is an important control parameter. Controlling the rotating speed of the hub to obtain high solid content and reduce energy consumption is the key to the operation of the centrifugal dehydrator. At present, low-speed centrifugal dehydrators are mostly used. In the selection of centrifugal dehydrator, because the outer edge of runner or screw is very easy to wear, there should be special requirements for its material. The outer edge of the screw of the new centrifugal dehydrator is mostly made into assembly blocks for easy replacement. The material of assembly block is generally tungsten carbide, which is expensive. Centrifugal dehydrator has the disadvantages of high noise, high energy consumption and low treatment capacity. Only a few domestic manufacturers can produce small centrifugal dehydrators. If large centrifugal dehydrators are selected, they can only be imported. The price is very high, which will increase the project investment. At the same time, centrifugal dehydrators are greatly affected by the fluctuation of sludge load and have high requirements for the quality of operators. Therefore, centrifugal dehydration process is not adopted in general sewage treatment plants.
3. Super sludge dryer: the super sludge dryer is developed by Zhengzhou Dingsheng company in combination with advanced technical achievements at home and abroad. As long as it is aimed at the critical high-pressure sludge dryer of difficult dewatering media such as municipal sludge, it has made a lot of research and improvement in the process configuration of sludge dewatering and the performance of conditioner. Now we have successfully developed a series of super sludge dryers with a filtration area of 30-200 m2.
This product is energy-saving and efficient. It can reduce the municipal sludge with moisture content of more than 90% to 40-60% at one time without adding lime. This series of equipment is widely used in municipal sludge, pharmaceutical, electroplating, papermaking, petroleum, printing and dyeing and other sludge treatment and environmental protection fields. Make outstanding contributions to the efficient solution of sludge and sludge resource utilization.
The super sludge dryer adopts 2.0MPa high-pressure feed and 8-10mpa high-pressure press dehydration to realize rapid sludge dehydration.
4. Plate and frame filter press dehydrator: the plate and frame filter press is used to discharge the water in the sludge 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 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 shall be considered when selecting equipment
(1) Requirements for solid content of mud cake. Generally, compared with other types of dehydrators, the plate and frame filter press has the highest solid content of mud cake. If considering the factors of reducing the land occupation of sludge stacking, the plate and frame filter press can be selected.
(2) The material of the frame.
(3) Material of filter plate and filter cloth. Corrosion resistance is required, and the filter cloth must have a certain tensile strength.
(4) Movement mode of filter plate. It is required to be fully automatic or semi-automatic through hydraulic pneumatic device, so as to reduce the labor intensity of operators.
(5) Filter cloth oscillation device to make the filter cake easy to fall off. Compared with other types of dehydrators, the biggest disadvantage of plate and frame filter press is that it covers a large area.
According to the comparative analysis of the performance of different types of dehydrators, it can be seen that when choosing sludge dewatering methods, different sewage treatment plants should comprehensively consider the sludge characteristics, operation conditions, personnel quality, requirements for sludge cake, capital and cost, so as to make a relatively reasonable choice.