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Application of Profibus Field Bus in Sewage Treatment Control Systems Author: Xie Kun, Shanghai Jiao Tong University. This article takes the software-redundant field bus control system of Haining Sewage Treatment Plant as an example to discuss the application of field buses and redundancy technologies in the field of sewage treatment. The hardware, software structure, and working principle of the system are explained. Tests have shown that this system can meet the requirements for automatic control in wastewater treatment, and it boasts advantages such as being advanced, reliable, and having good control performance. PROFIBUS is an **industrial fieldbus protocol standard developed in Germany in the early 1990s, designated as DIN19245. It is an international open fieldbus standard, namely the EN50170 European fieldbus standard. This standard provides the best protection for the investments of suppliers and users, while ensuring the independence of suppliers. PROFIBUS specifies the technical and functional characteristics of serial field buses, enabling distributed digital controllers to be connected from the field level up to the workshop and network infrastructure. PROFIBUS is divided into master devices (masters) and slave devices (slaves). The master station determines the data communication on the bus. When the master station gains control of the bus, it can send messages proactively without any external request. Slaves are peripheral devices; typical slaves include: input/output devices, valves, drivers, and measurement transmitters. They do not have bus control rights; they merely confirm the received information or send data to the master station when requested by it. 1 Process Introduction: The traditional activated sludge treatment process has issues such as high capital investment, high operating costs, and high electricity consumption. The SBR (Sequence Batch Reactor) sequential batch treatment process is a relatively advanced wastewater treatment method used internationally in recent years. Although this technology was developed abroad at the beginning of the last century, few wastewater treatment plants in China have actually adopted it to date. The main reason is that it has strict requirements regarding time and high demands on self-control. Wastewater treatment plants that use the SBR process are scattered in various locations and involve a large amount of equipment; strict coordination is required between different processing stages, as well as close cooperation among the various pieces of equipment. This precisely makes Profibus fieldbus an ideal solution. Figure 1 SBR wastewater treatment process. The SBR wastewater treatment control system mainly includes: the control system for the feed water pump room, the control system for the cyclone grit chamber, the control system for the main reaction tank, the control system for the effluent pump room, the control system for the blower room, and the control system for the dewatering room, among others. The wastewater collected by the urban sewage network is pressurized by pump stations and sent to the inlet pump house, where large solid impurities are filtered out by grates; thereafter, the relatively dilute wastewater is pumped to the cyclone grit chamber. Here, the wastewater has its solid particles removed thanks to the action of the mixer and the sand extractor. The next step is for the wastewater to first be sent to the anaerobic chamber of the SBR tank, where phosphorus and nitrogen are removed; thereafter, it flows into the aerobic chamber, where appropriate microorganisms are used to break down and purify the wastewater. The main conditions that sustain the sludge bacterial ecosystem in the reaction tank are organic impurities in the wastewater, and the supply of oxygen. Therefore, wastewater treatment plants are equipped with dedicated blower rooms, which use pipes at the bottom of the tanks to aerate the wastewater directly. The treated wastewater that meets the standards can be discharged directly, while the excess sludge is pumped to the dewatering room, where it is compressed into cakes through dewatering processes for use as fertilizer. 2 System Configuration and Instructions 2.1 Overall Control Requirements and Functions The requirements for the automatic control system in a wastewater treatment plant are to carry out automatic control and adjustment of the wastewater treatment process, so that the quality parameters of the treated water meet the specified standards. When an upload command is issued from the company’s control room, information such as the main operational parameters at that moment (water quality parameters, flow rate, liquid level, etc.), the operating status, and the key process curves over a certain time period are uploaded to the company’s control room. The functions are as follows: (1) Control operations: The controlled devices can be operated in real time from the central control room, such as starting or stopping a device, adjusting the values of certain analog outputs, and setting certain parameters of the PLC online. (2) Display function: Real-time graphical display of the operating conditions of the controlled devices at various sites, as well as the status parameters of those sites. (3) Data management: Based on the speed of change and importance of various operating parameters, a production history database is established to store raw production data for statistical analysis. By comparing and analyzing data from real-time databases and historical databases, some useful empirical parameters are obtained, which aid in optimizing the quasi-closed-loop control of SBR tanks; moreover, certain essential parameters and results are displayed on real-time screens and reports. (4) Alarm function: When the measurement value of a certain analog quantity (such as current, pressure, water level, etc.) exceeds a specified range, or when a certain digital switch (such as motor start/stop, valve opening/closing) changes state, alarms of different levels can be issued according to specific requirements. (5) Printing function: It enables the printing of reports and graphs, as well as the real-time printing of various events and alarms. Printing methods can be divided into: scheduled printing and event-triggered printing. 2.2 Control system network structure: If a conventional PLC centralized control approach is used, with field signals being connected to the PLCs in the centralized control room via cables, the long process pipelines and wide distribution of field control points require the installation of a large number of cables and cable trays. Moreover, due to the harsh field conditions, the construction process is extremely difficult. In light of this, the PROFIBUS-DP fieldbus technology was adopted; based on the process division, the system is equipped with three master stations, five slave stations, and two operator stations. The Siemens S7-300 series PLC is used, with the CPU315-2DP serving as the master station; it is equipped with one DP communication port and one MPI port. The substation uses the widely applicable ET200M field module for the acquisition and control of field data, and relies on Profibus (an industrial field bus) to facilitate the establishment of a control network system. The control system is divided into three levels: the field level, the control level, and the management level. 2.2.1 Management Level: The management level provides centralized monitoring of the operating status of devices at each branch station. For the management-level field bus, the Profibus-FMS bus is selected. Two redundant servers equipped with configuration software serve as the master stations for the PROFIBUS-FMS field bus; they are installed in the control room and can collect field data simultaneously. The server uses the WinCC configuration software, and server software package options are available. The SIMATIC S7-314 and MDS2701 radios are responsible for the company’s wireless data communication. 2.2.2 Control Level The main function of the control level is to receive parameters or commands set by the management layer, to control the wastewater treatment process, and to transmit the status of the system to the management layer. PLC1 and PLC2 are each composed of a power supply module PS 307 and a CPU315-2 DP module; they serve as hot backups for one another, handling all control algorithms and monitoring the automatic operation of the equipment. Centralized control enables complete coordination among all the components of the entire wastewater plant control system. 2.2.3 Field Level: Field-level control utilizes the PROFIBUS-DP field bus, making it very simple to achieve redundancy in the control system. The ET200M remote unit is connected to two PROFIBUS-DP buses via two IM153-2 bus interface modules respectively. The wastewater treatment plant is equipped with 6 ET200M remote units, two of which are located in the SBR tanks, while the remaining four are situated in the inlet pump room, outlet pump room, blower room, and sludge dewatering room respectively. Each ET200M unit consists of 2 IM153-2 bus interface modules along with several other digital and analog input/output modules. The IM153-2 bus interface module receives commands from the main control station via the bus to enable data acquisition and device control; only commands from the main PLC take effect, while commands from the standby PLC are ignored. The number and configuration of digital and analog input/output modules are determined by the number of points that need to be controlled and monitored locally. Figure 2: Control system structure diagram. Figure 3: System software. The control program for the main control unit S7-300 PLC is written using Step7 software. STEP7 is a programming software developed by SIEMENS for use with S7-300/S7400 PLCs; it runs on Windows NT or Windows 98 operating systems and offers user-friendly functions for various aspects of automation projects. The program adopts a modular programming structure, which facilitates system maintenance and expansion. Figure 3 Software flowchart 4 Conclusion This control system was completed and tested in September 2002, and it has been operating stably and reliably to this day. It has greatly improved labor productivity; this system effectively resolves many issues in production, such as reducing unexpected failures during the manufacturing process, shortening production preparation and repair times, and lowering the workload on workers. It brings significant economic and social benefits to sewage treatment plants, achieving the goals of energy savings and reduced consumption. Production management has reached a new level.