| With the rapid development of China's economic and the speeded-up of urbanization process, every day a large number of sewage generate, and discharge directly and untreated. This has resulted in serious water pollution and waste of water. China is also a country facing serious water shortages, and the lagging water pollution control exacerbated the shortage of water. This vicious circle increased the contradiction between water supply and demand. Therefore, people pay more attention to treating urban sewage, and using the treated water. In particular, from "Ninth Five-Year", both national and local governments give priority support on urban sewage treatment facilities. So China's sewage treatment has rapid development.However, China's sewage treatment business started late, so many problems are still wide spread, such as inappropriate technology,outdated facilities, aging control system, low degree of automation etc.. Therefore, sewage treatment plant compatible with China's national conditions should be established and the plant should have good effect and high degree of automation. This paper describes the automatic control system design suitable for small and medium sized sewage treatment plant. It takes a county sewage treatment plant as an example.This paper aims at the characteristics of the sewage treatment process which takes a three oxidation ditch as the core. It uses automatic control system composed of industrial computer(IPC)and programmable logic controller(PLC)to realize the automatic control to the whole process of sewage treatment.The control system uses the "centralized management, decentralized control" distributed architecture. The whole system is divided into three levels: management level, control level and the field level by the level involved in the production.Management level is the central control room. It uses KINGVIEW to develop monitoring system in industrial machines controlled by the central, in order to achieve centralized monitoring and remote control of the entire production process. Through the monitor image, users can real-time understand operating status of sewage treatment equipment, the required parameters, and whether there were alarm, fault and so on. Meanwhile, users can also search and print historical data and alarm information on IPC.Control level is on-site controllers. It has many functions, including automatic control of the whole production process equipments, on-site data collection, and communication with IPC. According to the sewage treatment process and field distribution, the system sets up three field control station. It uses Ethernet to achieve the connection between station and station, between station and IPC, to achieve the data exchange between IPC and PLC, between station and station. Site controllers use Siemens S7-300 series PLC as a field control and data acquisition unit. Through the site meter, level meter and other instruments, it can collect the data of the sewage flow, level, dissolved oxygen concentration etc. According to process requirements, it can control field devices, such as the grid machine, sewage lift pump, switch brushes, electric water weir gate, and sludge pumps. The sewage treatment process is a complex system. It is great impact by external disturbances, time-varying, nonlinear, so it can not establish a precise mathematical mode. Based on these characteristics, this paper proposed using a control method combined time sequence control with the expert control system on the oxidation ditch facilities and dissolved oxygen concentration control.This automatic control system has many advantages: simple hardware configuration, facilitate networking, easy to develop, high reliability and cost-effective. It ensures the water quality compliance, while also reduces power consumption, saves human resources, improves efficiency. So the system achieves the desired control objectives. |
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