Integrated, fully distributed wastewater treatment control system based on industrial Ethernet

Abstract: This paper introduces the solution and implementation experience of the control system for the wastewater treatment project in Jiaxing City. The integrated fully distributed control system based on industrial Ethernet adopted features high decentralization, real-time performance, reliability, openness, and interoperability. It is also a system integrating data, voice, and video networks. Its successful application represents the latest development direction of fieldbus and control technology in the water treatment industry and has broad reference value for the implementation of similar systems.

Keywords: Fieldbus, Industrial Ethernet, Distributed Control, Water Treatment

1. Overview
As networking and information technology increasingly permeate the field of automation, the concepts and technologies for water treatment automation control are also constantly evolving. At the end of the last century, new fieldbus control systems overcame the shortcomings of closed systems where communication in DCS systems was achieved through dedicated networks, attempting to transform closed, proprietary solutions into open, standardized ones. Subsequently, distributed monitoring systems composed of host computers, PLCs, and fieldbuses gradually became the mainstream of water treatment automation. However, due to the different bus systems derived from different industries and applications, coupled with conflicting economic interests, the incompatibility between various fieldbus standards severely hampered the interconnection of equipment from different manufacturers, making fieldbuses a proprietary network restricted by manufacturers. With the continuous development of Ethernet technology, it has been widely used not only in office automation but also in industrial automation. Many controllers, PLCs, intelligent instruments and actuators, and even DCS systems, now have Ethernet interfaces. This signifies that industrial Ethernet has become the development direction of truly open and interconnected industrial networks, which will have a significant impact on water treatment automation technology.

The Jiaxing Municipal Wastewater Treatment Project is a wastewater treatment system for the centralized collection, transportation, treatment, and discharge of domestic sewage and industrial wastewater from Jiaxing City and some of its counties and cities into the sea (Hangzhou Bay). It includes the centralized sewage discharge network (implemented by each county and city) within the urban area of ​​Jiaxing City and the capital towns of Jiashan, Pinghu, and Haiyan, as well as Zhapu Town, along with transportation pipelines, sewage pumping stations, wastewater treatment plants, the discharge outlet, and a system monitoring and control center. The project commenced in November 1999 and was expected to be completed and operational by the end of 2002. Its primary service area is the urban area of ​​Jiaxing (including some suburbs), the capital towns of its counties and cities (Weitang Town in Jiashan County, Chengguan Town in Pinghu City, and Wuyuan Town in Haiyan County), and Zhapu Town, while also handling some scattered industrial point sources (industrial wastewater from industrial enterprises) and some rural domestic sewage. Based on the 2020 urban planning control land use, the area served, including the urban area and various towns, is 100.5 square kilometers, with a population of 1.03 million. Currently, there are 329 industrial enterprises involved.

In the wastewater treatment project in Jiaxing City, a distributed monitoring system consisting of a host computer, PLC, and industrial Ethernet was successfully applied. Practice has proven that such a control system is safe, reliable, and cost-effective for the water treatment industry, and it will become the mainstream of water treatment automation control technology.

2. System Network Structure

Due to its wide geographical distribution, the Jiaxing wastewater treatment system has two monitoring centers: a main monitoring center located in Jiaxing City and a central control room at the wastewater treatment plant in Haiyan County. The system passes through eleven pumping stations: Jia 1#, Jia 2#, Jia 3#, Jia 4#, Jia 5#, Jia 6#, Jiashan, Pinghu, Zhapu, Haiyan 3#, and Haiyan 4#, with the furthest distance between the two stations exceeding 60 kilometers. Considering the limitations of fieldbus communication, the development trend of industrial networks, and the system's scalability, the entire system adopts industrial Ethernet as the primary communication platform.

The main monitoring center, pumping stations along the route, and the wastewater treatment plant utilize 100Mbps Ethernet fiber optic rings (large rings) formed by DIN rail-mounted industrial Ethernet switches from Hirschmann, Germany. The central control room and the first and second sub-control stations within the wastewater treatment plant also use the same switches to form industrial Ethernet fiber optic rings (small rings). Redundant connections are established between the large and small rings using two shielded twisted-pair cables. Therefore, the entire network is essentially a mesh structure. Considering the long transmission distance, both the large and small ring lines use single-mode fiber optic cables. Each switch has two built-in 100Mbps single-mode fiber optic ports, and the wavelength of the transmitted light is 1300nm. At each network node, a star topology is used to connect local devices, such as computers, PLCs, VIP transmitters, and IP phones, to the local Ethernet switch.

The entire network solution has the following characteristics:

a. High bandwidth and fast transmission speed. The overall solution adopts a full-duplex 100Mbps fiber optic Ethernet ring network, with an actual network bandwidth of 200Mbps; it only takes 120μs to transmit a 1518-byte frame in one direction.

b. Switched Ethernet ensures system determinism. For real-time control, the network must be deterministic. Determinism requires that network transmission delay does not exceed the maximum allowable value of the system. Traditional Ethernet uses a carrier sense multiple access/collision detection (CSMA/CD) media access mechanism, and therefore does not inherently guarantee determinism in transmission. However, Ethernet can achieve determinism in three ways: limiting traffic, using master-slave communication, and employing Ethernet switches. This system uses switches to divide the entire network into multiple collision domains, thereby eliminating collisions and achieving determinism.

c. The ring redundancy scheme ensures system reliability. In the past, due to the lack of alternative solutions, redundant networks were mostly implemented using a dual-bus approach. The development of Ethernet and switching technologies has made it possible to establish redundant ring networks. With the same level of redundancy, the redundant ring reduces the concentration of risk and lowers implementation costs compared to the dual-bus approach. If the fiber optic cable in either the large or small ring fails, the ring structure will switch to a bus structure with full transmission capacity within less than 500ms. There are two twisted-pair channels between the large and small rings, one as the primary line and one as a backup line, achieving redundant connection. When the primary line fails, the backup line automatically activates within 500ms, thus ensuring reliable communication between rings.

d. Ethernet allows for continuous system expansion. Compared to fieldbus, Ethernet offers backward compatibility. For twisted-pair or fiber optic media, upgrading the transmission speed from 10Mbps to 100Mbps usually requires no changes to the existing cabling; only network equipment needs to be updated. Similarly, upgrading the system backbone from 100Mbps Ethernet to Gigabit Ethernet only requires upgrading the network transmission equipment, without the need to re-lay fiber optic cables.

e. Data, voice, and video "triple play". In this system, the network is used not only for control data transmission but also as a platform for voice and video transmission. Each pump station is equipped with a camera with a pan-tilt unit (PTZ). Its video output, audio signals, and PTZ control signals are compressed and converted into H.323 compliant digital multimedia signals by a VIP transmitter and connected to the Ethernet. At the monitoring end, dedicated decoding software enables remote video monitoring and recording, and allows remote control of the PTZ's movements.

f. Transparent Network System Based on MODBUS/TCP Application Layer Protocol. MODBUS/TCP, or MODBUS protocol based on TCP/IP, uses the standard Transmission Control Protocol (TCP) at the transport layer. It ensures reliable transmission through acknowledgment and retransmission mechanisms when erroneous data is received over the network, thus guaranteeing reliability during transmission. MODBUS itself is an open protocol supported by numerous vendors. Currently, MODBUS/TCP has become a de facto standard, providing a universal interface for other industrial control equipment. The network layer uses the IP protocol; users only need to know the IP address of the control device to achieve communication between devices, independent of lower-level network addresses and hardware. For the user, the existence of a specific network unit (hardware and software) is as if it does not exist; the entire network can be considered a transparent network.

3. Specific Implementation of the Control System

Traditional fieldbus control systems (FCS) are network-integrated, fully distributed control systems built on various fieldbuses. The Jiaxing wastewater control system, however, is a typical example of an integrated, fully distributed control system based on an industrial Ethernet platform. This system borrows features from fieldbus control systems, adhering to the principle of centralized management and decentralized control, employing two-level control for both the large and small loops. The first level is the management control station, located in the main monitoring center and the wastewater treatment plant's central control room. It consists of a microcomputer, monitor, printer, UPS power supply, PLCs, and an analog panel. Its purpose is to control and schedule the main process equipment, collect, monitor, optimize, and adjust process parameters during production, dynamically simulate and analyze the main process flow, and monitor the entire system's audio and video. The second level is the field control station. Each pump station, as well as the wastewater treatment plant's water zone (first sub-control station) and sludge zone (second sub-control station), is equipped with an independent PLC controller. These controllers collect process parameters such as flow rate and pressure in real time and control the operation of pumps and other equipment. The PLC controllers at each pump station communicate via Ethernet to achieve interlocking and linkage of actions.

Because the distance between the two furthest stations is relatively large, about 60 kilometers, in order to keep the distance between nodes balanced, it is not necessary to use high-power optical transceivers. During the optical cable laying process, a skip-style cabling method is adopted, so that the distance between adjacent nodes is no more than 20 kilometers.

Because the large and small loops are homogeneous networks, they are directly interconnected during implementation, instead of using the host computer in the wastewater treatment plant's central control room as a gateway. This allows the host computer in the main monitoring center to directly collect PLC data from the wastewater treatment plant, and also to view real-time and historical control data from the wastewater treatment plant through the host computer in the central control room. This enables the entire system to operate independently of the central control room. Simultaneously, it allows the central control room to serve as a backup monitoring center, controlling the entire system's operation in the event of a failure in the main monitoring center.

Compared to fieldbus communication, the industrial Ethernet platform supports the SNMP simple network management protocol and has real-time network status monitoring capabilities. In implementation, data exchange between the network management software and the host computer software integrates real-time network status data and alarms into the host computer monitoring system, thereby enabling the system to perform real-time alarm and diagnostic functions for network faults.
Communication between the host computer software and the PLC, as well as between the host computer software and the network management software, adopts the standard OPC (OLE for Process Control) communication method. OPC (OLE for Process Control) is an open interface standard and technical specification for information exchange between field devices and process management levels. Its purpose is to allow interoperability between automation and control applications, control devices, and business and office application software. Based on this standard, we transform different types of data from different hardware vendors into a unified OPC data format and provide it to customer applications via the OPC interface, thereby achieving system integration.

Considering that the selected PLC has its own web server, real-time data in the PLC can be viewed through a web browser during system implementation, giving the system a simple and convenient web monitoring function.

4. Conclusion

The integrated, fully distributed control system based on industrial Ethernet adopted in the Jiaxing City wastewater treatment project integrates the latest technologies in computers, communications, CRT displays, and industrial control (4C), representing the latest development in fieldbus control systems. Because it employs open and universal network technologies and standard communication interfaces, it simultaneously achieves the integration of data, voice, and video networks, providing a technologically advanced, fully functional, and reliable example for wastewater treatment. Furthermore, with the development of automation systems, more and more applications require the integration of automatic control systems, enterprise information systems, and video surveillance systems; therefore, this control scheme also has broad reference value for engineering applications in industries such as transportation, metallurgy, chemical engineering, and power.

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(3). Hu Jun, Industrial Ethernet and Internet-based Remote Monitoring Systems, World Instrumentation & Automation, pp. 43-45, Vol.6, No.2, 2002