Factors Affecting Dewatering Effect of Horizontal Decanter Centrifuge

In recent years, with the rapid development of sewage treatment in China, horizontal decanter centrifuge has been more widely used in sewage treatment, because of its advantages of automatic operation, low water content of mud cake after dewatering, clean operating environment, etc. According to the experience of commissioning and operation, the factors affecting dewatering effect of horizontal decanter centrifuge are discussed as follows:

1. Structure and Principle of Horizontal Decanter Centrifuge

Horizontal decanter centrifuge is mainly composed of drum, screw, differential system, liquid level baffle, drive system and control system.

Due to there is a density difference between solid phase and liquid phase, the centrifuge realizes the solid-liquid separation by accelerating the settling speed of solid particles under the action of centrifugal force. The sludge and flocculant solution are fed into the mixing chamber in the drum through the inlet pipe to be mixed and flocculated. Because of the high-speed rotation and frictional resistance of the rotor (screw and drum), the sludge is accelerated inside the rotor and forms a cylindrical liquid layer. Under the action of centrifugal force, the solid particles with higher density will settle to the inner wall of the drum, and be pushed to the cone section of the drum by the screw. After dewatering, the sludge will be discharged from the slag outlet, and the liquid will be discharged from the liquid outlet.

2. Non-adjustable Mechanical Factors

2.1 Drum Diameter and Length

The larger the drum diameter and the longer the drum length, the larger the settlement area, the greater the processing capacity, the longer the retention time of the material in the drum. At the same speed, the larger the separation factor, the better the separation effect. Due to the limitation of the drum material, the diameter of the drum can not be increased without limitation. Generally, the diameter of the drum is 200-1000mm, and the length diameter ratio is 3-4. In addition, under the same processing capacity, the centrifuge with large drum diameter will operate at a lower speed difference.

2.2 Half Cone Angle of Drum

When the sediment settled in the inner side of the drum is pushed to the outlet along the drum cone section, due to the centrifugal force, it will be affected by the downward sliding reflux force. The larger the half cone angle is, the greater the torque oft the screw thrust is, the greater the wear of the blades is. If the half cone angle is small, its effective settlement area will be greatly reduced, and the separation effect will be reduced. When the depth of the liquid pool is fixed, the cone angle is large, the length of the drying zone is short, the dewatering time will be short, and the water content of the sediment will be high, the quality of the filtrate will be relatively good. On the contrary, if the cone angle is small, the length of drying zone is long, the dewatering time will be long, and the water content of the sediment will be low, the quality of the filtrate will be poor.

According to the above analysis, in terms of clarification effect, the cone angle is required to be as large as possible, while in terms of slag transportation and dewatering effect, the cone angle is required to be as small as possible. For materials that are difficult to separate, such as activated sludge, the half cone angle is generally within 6°in order to reduce the reflux rate of sediment. For ordinary materials, the half cone angle is within 10°to ensure the smooth transportation of sediment.

2.3 Pitch

Pitch, that is, the space between two adjacent screw blades, is a very important structural parameter, which directly affects slag transportation. When the diameter of the screw is fixed, the larger the pitch is, the greater the helix angle is, the easier the materials will be blocked between the screw blades. At the same time, the large pitch will reduce the number of the screw blades, which will cause the uneven distribution of the materials at the cone end of the drum, and increase the vibration of the machine. Therefore, it is difficult to transport the slag for the materials which are difficult to separate, such as activated sludge. When the pitch is small, generally 1/6-1/5 of the drum diameter, it is conducive to slag transportation. For the materials which are easy to separate, the pitch is generally 1/5-1/2 of the drum diameter, so as to improve the transport capacity of sediment.

2.4 Screw

Screw is one of the main components of the centrifuge. Its function is to transport the sediment settled in the drum and discharge the sediment smoothly. It determines the production capacity, service life and separation effect of the centrifuge.

According to the relative movement of liquid and solid in the drum, the screw can be divided into counter flow type and parallel flow type. The feeding chamber of the counter flow centrifuge is located in the middle of the screw, so as to ensure that the liquid phase has enough settling distance. When the materials are fed into the drum from the feeding chamber, the settled solid particles will be disturbed and float again, turbulence and additional eddies will also appear, then the separation effect will be reduced.

Because the feed inlet of the parallel flow centrifuge is at the end of the drum, the turbulence will be avoided, and the sediment will not be disturbed. The settling area will be enlarged, and the retention time of the suspension in the centrifuge will be increased, so that the separation effect will be improved. Due to the extension of settling time and no interference, the use of flocculants can be effectively reduced, the flow state of fluid in the machine can be greatly improved. The parallel flow centrifuge is especially suitable for the materials with small solid-liquid density difference, poor solid-phase settling performance and low solid content.

3. Adjustable Mechanical Factors

3.1 Drum Speed

The adjustment of drum speed is usually realized by frequency conversion motor or hydraulic motor. The higher the rotating speed is, the larger the centrifugal force is, which is conductive to improving the solid content of sludge. However, when the rotating speed is too high, the flocculation will be destroyed and the dewatering effect will be reduced. At the same time, if the rotating speed is too high, the wear of the machine will increase, the power consumption, vibration and noise level will increase accordingly.

3.2 Speed Difference

Speed difference has a direct impact on the ability of slag discharge, sludge dryness and filtrate quality. Increasing the speed difference is conducive to improving the slag discharge capacity, but the dewatering time will be shortened, the water content of mud cake after dewatering will be increased, the filtrate quality will be relatively poor. Reduce the speed difference, the dewatering time will be extended, the water content of mud cake after dewatering will be reduced, the filtrate quality will be relatively good, but the load of the screw will be increased, the slag discharge capacity will be reduced, the sediment in the centrifuge can not be discharged in time, which will lead to blockage. Therefore, the speed difference should be determined according to the material properties, processing capacity, processing requirements and the parameters of the centrifuge. It is generally believed that the dewatering time should be controlled between 4-6s.

3.3 Thickness of Liquid Layer

Thickness of liquid layer is an important parameter in the process optimization, which directly affects the effective settling volume and the length of drying zone, thus affects the treatment effect of the centrifuge. Generally, the thickness of liquid layer is realized by manually adjusting the height of the liquid level baffle when the centrifuge was shutdown. During the adjustment, it must be ensured that the height of each liquid level baffle is the same, otherwise it will cause violent vibration during the operation of the centrifuge.  

With the increase of the thickness of the liquid layer, the settlement area will be increased, and the residence time of the materials in the drum will also be increased correspondingly. The result is that the quality of the filtrate will be improved, but at the same time, the length of the drying zone will be shortened, which leads to the reduce of the dryness of the mud cake. On the contrary, a higher solid content of the mud cake can be obtained by reducing the thickness of liquid layer at the expense of filtrate quality. Therefore, the height of the liquid level baffle should be adjusted reasonably to make the best combination of sludge dryness and filtrate quality.

4. Technological Factors

In order to improve the dewatering effect of sludge, an appropriate amount of organic polymer flocculant, such as polyacrylamide (PAM), should be added evenly before mechanical dewatering to reduce the specific resistance of sludge.

According to the actual operation, the dosage of flocculant has little effect on the solid content of the mud cake, but has a great effect on the quality of the filtrate. Therefore, centrifugal dewatering can be used without flocculant or adding less flocculant, so as to reduce the operating cost. However, adding a proper amount of flocculant can significantly improve the solid recovery rate. In municipal sludge treatment, the proportion of organic polymer flocculant solution is generally 1‰-5‰, and the dosage of flocculant is generally 3-5kg/TDS.

5. Conclusion

There are many factors affecting the dewatering effect, and each factor affects each other, so the treatment effect is the result of the comprehensive action of each factor. The selection of centrifuge should be base on the actual situation of the project, and the adjustment of operation parameters should be considered comprehensively from the water content, solid recovery rate and the investment required by the final disposal method.