Abstract: Circulating Activated Sludge System (CASS) is an advanced wastewater treatment process. A computer-automated control system for implementing the CASS process was designed. This system has good flexibility, high reliability, and maintainability. Keywords: Wastewater treatment, CASS process, distributed system, configuration, fieldbus. With the development of society and economy, the volume of wastewater and the types of pollutants in wastewater are constantly increasing. The purpose of wastewater treatment is to separate pollutants from wastewater or to transform and decompose them into non-toxic and harmless stable substances, thereby purifying the wastewater. Wastewater treatment technology has a history of over a hundred years, developing various wastewater treatment processes with different characteristics, including physical treatment, chemical treatment, and biological treatment. To make these methods work better, in today's era of rapid development in computer technology and automatic control technology, applying computer-automated control technology to wastewater treatment, and achieving semi-automatic or even fully automatic monitoring of wastewater treatment processes, is of great practical significance. 1. Introduction to the Cyclic Activated Sludge System (CASS) The Cyclic Activated Sludge System (CASS) is a pressurized-release intermittent activated sludge treatment process with nitrogen and phosphorus removal capabilities, operating in a sequential batch aeration-non-aeration mode. It completes the biodegradation of organic pollutants and sludge-water separation within a single reactor. The CASS process operates in a plug flow manner according to a specific time sequence, and its operation includes three stages: water filling-aeration, water filling-sludge-water separation, and supernatant decanting. These three stages constitute an operating cycle, and each cycle can be continuously cyclical. The overall system structure is shown in Figure 1. 2 System Analysis 2.1 System Logical Structure To achieve automatic control of the entire system, analysis reveals the following parameters require automatic monitoring: equalization tank high/low level; CASS tank high/low level; decanter position; sludge tank high/low level; group/fine screen start/stop; influent pump start/stop; sludge return pump start/stop; decanter operation/stop; filter press start/stop; aerator operation/stop. Besides fulfilling the measurement and control tasks, the system must also have a user-friendly interface. Therefore, the system's logical structure is shown in Figure 2. 2.2 System Technical Objectives Analysis As an industrial control system, in addition to fulfilling necessary monitoring functions and having a user-friendly interface, it should also consider aspects such as system safety, reliability, and maintainability, and take corresponding measures. 2.2.1 High reliability is essential to ensure long-term stable operation of the system, minimizing control errors and measurement/control failures. Even in the event of a failure in some components of the system, certain emergency measures must be provided to ensure continued production. 2.2.2 Good maintainability is essential. While we cannot expect system users to possess professional knowledge of industrial monitoring, they are still required to use and maintain the system. Therefore, system maintainability is crucial. The system should be able to quickly identify the cause and location of the fault and find solutions after a problem occurs. 2.2.3 A certain degree of flexibility is necessary. Since wastewater quality fluctuates, the system must provide certain functions, allowing users to adjust the system process appropriately based on wastewater quality and other external factors (such as temperature), and to allow for the use of multiple operating modes as needed. 3 System Structure and Implementation Strategy 3.1 System Structure Based on the above analysis, this design adopts a two-stage distributed control system. Each CASS pool is controlled by an interlocking machine, and all PLCs are managed and displayed by the same host computer (management machine). The measurement and control system structure is shown in Figure 3. 3.2 System Layer Description 3.2.1 Management Layer The management layer, as the highest level of the wastewater treatment system, mainly undertakes the following tasks: • Configuring the process control strategy of the interlocking layer PLC, enabling process personnel to easily change the control strategy and other process parameters online; • Displaying the process flow diagram, dynamically displaying the liquid level and the status of each equipment and the system status; • Providing a manual operation interface; • Implementing visual and voice alarms; • Implementing operation and system status recording and historical query; • Managing operators; • Providing a human-network interface for connection to the enterprise intranet. 3.2.2 Interlocking Layer The interlocking layer is the core of the system. The system's measurement and control interlocking functions are mainly completed here, including: • Detecting the liquid level and the status of each equipment; • Configuring control strategies based on process parameters and automatically controlling the entire wastewater treatment process accordingly; • Designing and developing dedicated control algorithms for field equipment, realizing the protection and control of field equipment. 3.2.3 Execution Layer This layer mainly consists of electrical execution equipment, achieving isolation between the PLC and control equipment, and executing the control commands issued by the PLC. 3.2.4 On-site mainly refers to the on-site execution and sensing equipment in the wastewater treatment system, including level controllers, pumps, fans, etc. 3.3 Equipment Selection Reliability is the primary consideration when selecting equipment for this system. An industrial control computer is selected as the upper-level management unit; a PLC is selected for the interlocking mechanism due to its high reliability, mean time between failures (MTBF) exceeding 50,000 hours, and strong anti-interference capabilities. 3.4 System Operation Modes The system provides three operation modes: • Automatic Operation Mode: The system automatically controls wastewater treatment according to the set process scheme; • Computer Manual Operation Mode: The upper-level computer displays the system status, and there is no interlocking relationship between the devices; • Mechanical Manual Operation Mode: This mode can be used when the system malfunctions or is under maintenance. In this mode, the operation of each device is not controlled by the management unit or PLC, but is achieved by physical switches on the frame. 4. Control Effect 4.1 System Features • This system is suitable for treating urban domestic sewage, winery wastewater, and food industry wastewater; • This system has certain configuration functions and can adapt to environments with significant changes; • It can provide protection and control for field equipment, extending the service life of field equipment and greatly reducing the occurrence of abnormal situations, thus improving reliability; • It has a high degree of automation, a user-friendly and intuitive interface, provides voice alarm functions, and is easy to use; it provides personnel management and historical record functions, facilitating accident analysis; • It has a two-level distributed structure, with each control unit in the system relatively isolated, making maintenance easy. 4.2 Actual Control Effect This system has been successfully applied to the Shengquan Beer Factory in Anhui Province, treating 7,000 tons of wastewater per day, effectively changing the factory's effluent quality and achieving compliant discharge. At the same time, it has reduced a large amount of pollutants, minimizing environmental harm. The annual reduction of its main pollutants is (based on GBS978-96B standard, annual operating time 300 days): CoDCr: 1942.5 tons/year BDD5: 987 tons/year SS: 630 tons/year Therefore, the actual operation effect is good and has been well received by users. It was successfully accepted in January 2000 and is ready to be promoted and used in large and medium-sized enterprises in China such as Xining Special Steel. With the increasing social requirements for sewage treatment, the development and application of economical, flexible and stable sewage treatment systems have been increasingly valued by researchers and enterprises. CASS process has good application prospects in both urban sewage and industrial wastewater treatment. In order to make this system more competitive, we propose to improve it in the following aspects: (1) The system adopts a bus-type distributed structure, sets up field controllers, and all detection equipment and motors are directly connected to their respective field controllers and connected by bus to form the system. This can reduce the system cost and make on-site construction extremely simple. At the same time, this structure is also more conducive to maintenance. (2) Install dissolved oxygen meter, pH meter, turbidity meter and other equipment in the system to monitor the water quality of CASS tank and sludge tank online. Based on the measured parameters, automatically adjust the process parameters in real time to achieve closed-loop control of the system, realize fully automatic control of sewage treatment, and ensure the optimal state of system control. (3) Connect the system management machine to the Internet through the enterprise intranet to realize remote diagnosis of the system and reduce maintenance costs.