Overview

　　1. Overview:

　　Relevant data shows that the United States has, on average, one wastewater treatment plant per 10,000 people, while the UK and Germany each have one plant for every 7,000 to 8,000 residents. In contrast, China only has one wastewater treatment plant for every 1.5 million people. This highlights the urgent need for wastewater treatment in China, while the market for wastewater treatment is also witnessing a vibrant and thriving scene.

　　2. Commonly Used Wastewater Treatment Processes:

　　There are several main methods for wastewater treatment, including physical, chemical, physicochemical, and biological approaches. Depending on the specific situation, these methods can be used individually or combined in various ways to treat different types of wastewater. Currently, biological treatment methods are generally the primary approach, often complemented by physical and chemical treatments. Commonly employed processes include the following:

　　(1) The conventional activated sludge process. This is the oldest wastewater treatment technology, with key components being the aeration tank and the sedimentation tank. During the retention period in the aeration tank, microorganisms adsorb most of the organic matter from the wastewater and oxidize it into inorganic substances. These microbial flocs then settle down in the sedimentation tank. After a certain period, clean water can be discharged, while to maintain the sludge concentration in the aeration tank, a portion of the activated sludge—after settling in the sedimentation tank—is returned back to the aeration tank.

 

 

　　The advantages of this process include: high organic matter removal rate; high sludge loading; small tank volume; low power consumption and reduced operational costs.

　　The drawbacks of this process include: the conventional aeration method requires a large land area, leading to high construction costs; it can only meet the relevant standards specified in wastewater treatment guidelines; it is prone to sludge bulking; and its removal rates for phosphorus and nitrogen are relatively low.

　　(2) The A/O Process. This wastewater treatment technology has evolved from the traditional activated sludge method. Here, "A" stands for Anoxic (anaerobic), and "O" represents Oxic (aerobic). The A/O process is an anaerobic-aerobic biological wastewater treatment system. By incorporating a nitrification-denitrification reaction system formed through alternating aerobic and anoxic tanks, this process effectively removes nitrogen from wastewater, delivering remarkable denitrification performance. However, the nitrification-denitrification reaction system requires precise control, which in turn places higher demands on operational management—this limitation has become one of the key drawbacks of the process.

　　(3) The oxidation ditch process is a variation of the conventional activated sludge method, classified as a low-load, extended-aeration system. In this process, the mixture of wastewater and activated sludge continuously circulates within a circular aeration channel, which is why it’s also referred to as a "circulating aeration tank." The oxidation ditch method offers several advantages, including a simple treatment process and infrastructure, no need for primary sedimentation tanks or nitrification tanks, high efficiency in organic matter removal, enhanced nitrogen and phosphorus removal (achieved by adding an anaerobic tank upstream of the ditch), minimal excess sludge production that’s easy to dewater, and stable treatment performance. However, it also has some drawbacks, such as low organic loading, large land footprint, high energy consumption, and relatively high operating costs. This method is well-suited for small- to medium-sized wastewater treatment plants with low organic loads. Depending on the specific design of the structure, the oxidation ditch process can be further categorized into: Carrousel oxidation ditch, Orbal oxidation process, and integrated oxidation ditch systems.

　　Additionally, there are the A2/O process, A/B process, SBR process, and UNITANK process. We won’t go into detailed explanations here; for more information, please refer to relevant technical resources.

　　3. Wastewater Treatment System Control Modes

　　(1) DCS System. Short for Distributed Control System, also known as a Distributed Computer Control System, it emerges from the mutual integration of computer technology, signal processing technology, measurement and control technology, communication network technology, and more. Composed of computers and field terminals, the system connects field control stations, monitoring stations, operator stations, and control centers via a network, enabling decentralized control along with centralized operation and management. Primarily used in various production processes, it helps elevate both production automation and operational management levels. Its key features are as follows:

　　① Adopting a hierarchical distributed control approach reduces the system's information transmission volume, making the system applications simpler.

　　② Achieved true distributed control, dispersing system risks and enhancing reliability.

　　③ Has strong scalability.

　　④ Abundant software and hardware resources, capable of meeting various requirements.

　　⑤ Excellent real-time performance with rapid response.

　　(2) Fieldbus Control System. Developed from DCS and PLC technologies, this system is an automation control system based on fieldbus technology. It uses open, standardized communication protocols to transmit and exchange data between intelligent devices and computers, enabling the integration of control and management into a unified automated system. Its advantages are outlined below:

　　① Can leverage the abundant software and hardware resources of computers.

　　② Fast response and excellent real-time performance.

　　③ The communication protocol is open, enabling different products to connect with each other.

　　(3) PLC System. The Programmable Logic Controller serves as the system's control unit, fulfilling the functional requirements of the control system. It can also utilize a computer as a host machine, connecting to the PLC via network for real-time monitoring of the production process. Its key features are as follows:

　　① Convenient programming, short development cycle, and easy maintenance.

　　② Highly versatile and easy to use.

　　③ Strong control ability.

　　④ Modular structure with strong scalability.

　　System Design

　　1. How a Wastewater Treatment System Works

　　(1) Overall control system framework diagram: As shown in Figure 2, the NA400 PLC serves as the core controller. It manages the operation, stopping, and speed control of related equipment by processing inputs from the operator panel buttons, various sensors, and relevant analog signals.

　　

 

　　Figure 2

　　(2) Working Process: In manual mode, the control of various equipment is managed directly through button inputs on the operation panel, with no logical restrictions—meaning control can occur regardless of the sensor status. In automatic mode, closed-loop control is implemented, where the system activates and deactivates the equipment based on the status detected by external sensors. The working process is illustrated in Figure 3.

　　① Connect the power supply, activate the automatic control mode, and start the submersible mixer and sludge scraper.

　　② Operate the coarse and fine screen machines intermittently—running them for a certain period, then stopping for a while, and repeating this cycle.

　　③ Control the operation and stoppage of the cleaning machine based on the feedback received regarding the liquid level difference.

　　④ The submersible pumps in the pumping station are controlled to start, stop, and adjust their operating quantity based on the liquid level height.

　　⑤ The rotating-disc aerator is controlled based on analog signals fed back from the dissolved oxygen meter, processed by the PLC, while simultaneously managing the operation and shutdown of the separator.

　　⑥ The operation and shutdown of the sludge return pump are controlled based on the liquid level height.

　　⑦ In the sludge dewatering system, the centrifugal dewatering machine is started using sequential control, activating its equipment one by one.

　　

 

　　Figure 3

　　 2. Hardware System Configuration

　　(1) PLC Selection. Based on the functional requirements and complexity of the electrical control system for the wastewater treatment process, as well as considerations such as cost-effectiveness and reliability, the NA-400 PLC has been chosen as the control unit for the system's electrical control framework. The system involves a large number of digital input/output points, and it also requires analog signal acquisition along with functionality to utilize analog signals for precise control. Therefore, additional analog input/output modules will be integrated into the setup. Specific configuration details are provided in Table 1.

　　

 

　　(2) PLC I/O Point Resource Allocation

　　① The digital input section—in this control system, the required inputs are essentially digital, primarily including various control buttons, rotary switches, and numeric inputs, totaling 31 digital inputs, as shown in Table 2.

　　

 

　　Table 2

　　② Digital output section. In this control system, the main devices controlled by the outputs include various contactors, valves, and more—totaling 19 output points, with the specific allocation as follows:

　　

 

　　Table 3

　　③ Analog Input Section. Since data from a dissolved oxygen meter needs to be collected, an analog input/output module has been added, as detailed in Table 4.

　　

 

　　Table 4

　　④ Analog Output Section. In this control system, the acquired analog signals need to be processed before being sent out through the analog output port to control the frequency inverter, thereby regulating the operation of other equipment, as shown in Table 5.

　　

 

　　Table 5

　　(3) Other Resource Allocation

　　① Contactor: This system requires a total of 17 contactors, including those for the screen machine, debris remover, submersible pump, separator, rotary disc aerator, submersible mixer, sludge scraper, and more.

　　② The inverter uses the SIEMENS MM430 inverter, which is specifically designed for fan and pump loads. In this control system, communication-based control of the inverter is required; therefore, it is necessary to first configure the inverter’s parameters, primarily adjusting the following parameters as shown in Table 6.

　　

 

　　For the sub-system's inverter, communication control is used during manual debugging, so it is necessary to adjust the inverter's parameters.

　　③ Various buttons, human-machine interfaces, level differential gauges, dissolved oxygen meters, and more.

　　3. Software System Design

　　During the program design process, many soft components such as intermediate relays, registers, and timers are used. To facilitate programming and modifications, it’s advisable to list the potentially required soft components beforehand, as shown in Table 7.

　　

 

　　(1) Manual Control Program: After the system is powered on, if the control mode is set to manual, each device can be operated individually via the buttons on the panel. Setting the system to manual mode is primarily convenient for debugging during initial production line installation, allowing you to verify whether each piece of equipment functions properly. The manual control program is outlined below.

　　

 

　　

 

　　(2) Automatic Control Program: In production, automatic processes are mostly used for control. The system regulates equipment start-up and shutdown, as well as speed adjustments, by relying on feedback signals from sensors. The automatic control program is outlined below.

　　

 

　　(3) Functional Programs: In this system, modular programming is employed, with functional blocks covering coarse and fine bar screens, submersible pumps, debris removal systems, sludge return systems, and VFD communication, among others. Here, we’ll highlight just a few of these blocks, with a particular focus on the functional block related to VFD communication.

　　① The coarse screen machine system program controls both the coarse screen machine and the debris removal machine. The system ladder diagram is shown below.

　　

 

　　② The aeration grit removal system program controls the operation, speed adjustment, and shutdown of the rotating-disc aerators. The ladder diagram program for the aeration grit removal system is as follows.

　　

 

　　

 

　　③ Inverter parameter reading program, used for the inverter's output frequency; the ladder diagram program for reading inverter parameters is shown below.

　　

 

　　④PID instructions, used for processing analog signals, output data AOUT to %QW0001, which is then used to send analog signals to control the operation of other devices.

　　

　　1. Hardware-related issues

　　In wastewater treatment systems, the main hardware issues include mechanical structures, as well as the peripheral circuit design and wiring connections for the PLC.

　　The primary issue with the mechanical structure lies in the installation position of the rotary disc aerator, which needs to be carefully placed according to the design of the oxidation tank. If positioned correctly, even a small number of aerators can effectively supply the necessary oxygen. In terms of the PLC's peripheral hardware wiring, the main focus is on adding protective devices. Since the outputs are connected to components like contactors, sudden switching—whether opening or closing—can generate transient waveforms that may damage the PLC's output terminals. To prevent this, it’s essential to install protective devices such as freewheeling diodes, which not only safeguard the system but also help extend the lifespan of the contacts.

　　2. Software-related issues

　　After the program is completed, it’s necessary to first perform software simulation on the computer to primarily check for any potential errors—this can be done using the simulation software included with NAPro. Next, simulate the hardware setup to identify any logic-related mistakes in the program. During debugging, you can debug each functional module separately according to its category, before finally conducting a comprehensive overall test.

　　Design Summary

　　This control system employs the NA400 series PLC from Nanjing Aoto Technology Co., Ltd. as its core control device. Within the system, long-term timing is achieved by combining timers and counters, while the inverter is controlled via analog signals, enabling closed-loop control through PID instructions—thus meeting the precise requirements of the wastewater treatment system. We would like to extend our heartfelt gratitude to Zhang Yunfeng, the electrical engineer from Shandong Feiyang Environmental Engineering Co., Ltd., whose invaluable assistance has significantly enriched the content of this article.