For sludge drying, the best choice is a automatic PLC membrane filter press, namely, a programmable membrane filter press. Since the sludge generally has a certain viscosity, the conventional chamber plate filter press is easy to block filter clothes, so the secondary squeezing method can achieve the expected dewatering effect. In addition, such sludge generally has a relatively big capacity, and need a relatively longer filtration cycle, so there are more actions of the machine, for example, feeding, pressing, and other auxiliary equipment controls. Then the completion of these logic needs to be realized by PLC (Programmable Logic Controller). In the field of filter press application, it is mainly used to control the sequence of various actions, so that can ensure stable operation and greatly reduce the equipment failure rate. Comparing with the early relay control, it has following advantages: effectively improve the efficiency; simple electrical circuits and easy to maintain; greatly saves the cabinet space. Therefore, the adoption of PLC promotes the automation of the filter presses.
In China, for plate filter presses, Siemens PLC is popular used. Most filter press manufacturers use S7 series, such as: S7-200, S7-300. Of course, if you have higher requirements, it can use S7-1200 or S7-1500. The basic PLC logic of our diaphragm filter press are as following: First, the filter plate are pressed tightly to complete automatic pressure keeping. Then, turn on the feed pump to pump the sludge into the filter chamber. When the pressure reaches the set value, then turn on the squeezing pump for secondary squeezing, and control the squeezing time through the time relay. Finally, the filtration is complete, and the filter plates are is opened to unload the filter cakes. If there have drip tray or belt conveyor, before opening the filter plate, they will be opened or run in advance. After discharging the filter cake, the equipment enter into the next cycle. Through membrane squeezing, the residual cake moisture can reach about 55% (for municipal sludge), which can achieve dry stack.