A larger length-diameter ratio can prolong the settlement and dewatering time of sludge in the decanter centrifuge, thereby improving its separation performance. However, when the length-diameter ratio increases to a certain extent, the dewatering process requires high energy consumption and larger floor space, which also increases the bending and work instability of the drum in the longitudinal direction. Based on the above factors, suitable length-diameter ratios should be selected according to the characteristics of sludge and the rotational speed of the decanter centrifuge during the mechanical design of equipment.
A smaller half cone angle is favorable to the settlement and retention of sludge, but unfavorable to its extrusion and dewatering in the conical section of the drum. A larger half cone angle is conducive to solid-liquid separation, but leads to increased axial thrust, slag conveying power consumption, and wear of the screw blades, as well as lower slag conveying efficiency. Although a larger half cone angle can increase the centrifugal pressure of the suspension to facilitate dewatering, the torque of the screw pusher needs to be increased, while blade wear can reduce dewatering effectiveness and impair the normal operation of the decanter centrifuge.
Different inlet injection angles directly affect the velocity of sludge entering the helical flow passage of the decanter centrifuge and its disturbance state in the passage. In order to demonstrate the effect of inlet injection angle on the separation performance of the decanter centrifuge, some researchers have simulated and tested the scenarios with inlet injection angles of 30°, 45°, 60°, and 90°, respectively. The results show that the turbulent energy of fluid in the flow passage of the decanter centrifuge is smaller when the inlet injection angle is 30°, indicating that the effect of disturbing sludge in the radial direction is smaller, the axial velocity of sludge is more suitable for entering the flow passage, and solid-liquid separation of sludge is promoted, thus accelerating the squeezing and dewatering of sludge in the conical section. To achieve an inlet injection angle of 30° on the screw conveyer tube, an inclined baffle can be set along the helical flow passage according to the fluid streamline design to avoid dead zones when the inlet injection angle is 90°.
The differential speed reflects the settlement time of sludge in the sedimentation section of the drum. A small differential speed indicates longer sludge settling time, longer centrifugation time, longer squeezing time, more liquid phase extrusion, lower water content of sediment, and lower sludge treatment capacity but poorer economic benefits. A large differential speed indicates shorter sediment settling time, shorter solid-liquid separation time, greater sludge disturbance, and higher sludge treatment capacity. However, it should be noted that longer sediment settling time is not always better, as excessive settlement time can lead to sludge accumulation and blockage in the helical channel, resulting in slag conveying failure from the discharge port.
The liquid ring layer thickness is an important parameter for process optimization of the decanter centrifuge, and it can be changed by adjusting the height of the liquid level baffle manually in the shutdown state. Changing the thickness of the liquid ring layer can alter the drying zone length inside the drum. A thinner liquid ring layer increases the solid content of the filtrate and reduces the water content of the mud cake, while a thicker liquid ring layer increases the quality of the filtrate but raises the water content of the mud cake. Therefore, it is necessary to balance the water content of the mud cake and the concentration of the filtrate when adjusting the thickness of the liquid ring layer.
Practice has shown that sludge properties, such as the size and distribution of sludge particles, surface charge, inlet sludge moisture content, sugar content, protein content, sludge age, and filamentous bacterial length, can affect the separation performance of the decanter centrifuge.
In the decades of its development history, the decanter centrifuge has undergone significant changes in its structural parameters and performance, and has been widely used in sewage treatment plants. To improve its separation performance, multiple factors, such as structural parameters, differential speed, liquid ring layer, and sludge properties, should be considered.