With the increasing environmental awareness of the public, wastewater discharged from power plant water treatment systems must meet discharge standards. Wastewater treatment is computer-controlled, with a master-slave architecture. Hardware uses an industrial computer and a Hongge I-8811 embedded controller, while software uses the general-purpose iFIX configuration software. The system structure is reasonable, simple, reliable, and easy to maintain; field equipment can be controlled from three locations. The alkali addition procedure uses a simulated manual alkali addition method, which is feasible.
Boilers, as the main equipment in thermal power plants, require safe and long-term operation. Scale buildup is a major factor affecting their lifespan, making them highly demanding in terms of water quality. Boiler water is heated at specific temperatures and pressures to produce steam, which serves as the medium for heat transfer and power generation. Low- and medium-pressure boilers have slightly lower water quality requirements, while high-pressure boilers have extremely high requirements. Impurities that can cause corrosion, scale buildup, and steam-water eutrophication in boilers, feedwater systems, and other thermal equipment, and can poison ion exchange resins, such as dissolved oxygen, soluble silica, iron, and residual chlorine, should be largely or completely removed. In boiler water treatment, ion exchange treatment of boiler makeup water is the most basic and important method, and our unit's three power plants all use this method. After ion exchange resins become ineffective, they must be reduced by adding alkali or acid, resulting in large amounts of acidic and alkaline wastewater. Direct discharge of this wastewater not only impacts the environment but also attracts monitoring from environmental protection departments, causing unnecessary losses and directly affecting economic benefits.
I. Acid and Alkali Wastewater Treatment Methods
The national standard for wastewater discharge is a pH value between 6.0 and 8.5. Therefore, the basic method for wastewater treatment is to neutralize acidic water with alkali and alkaline water with acid, achieving a pH value between 6.0 and 8.5 for safe discharge. Ideally, an online treatment method would be used, where acid or alkali is added simultaneously with discharge based on the detected pH value to achieve neutral water standards. This is the goal pursued by our technical staff. However, due to factors such as the quality of incoming water and the lag in pH value detection, there are currently no successful cases of its application in China. Before the renovation of our unit's No. 2 power station wastewater treatment system, the method was basically based on this principle and used a PLC programmable controller, but the results were unsatisfactory. Currently, static treatment methods are commonly used, where chemical wastewater is concentrated in a treatment tank, and neutralization is performed when the tank level reaches a certain height.
Based on the characteristics of reduced water in cation-cation beds, which are more prevalent than anion-cation beds, the wastewater in the treatment tank is generally acidic. Neutralizing it with an appropriate amount of alkali will bring it up to standard. How much alkali to add is the main issue we are researching.
The previous method relied on manual alkali addition. Without an online pH meter, the process was based entirely on intuition, resulting in errors of adding too much or too little. Therefore, our unit proposed upgrading the original system to a computer-controlled alkali addition process. The amount of alkali added would be automatically determined by the computer based on the online pH value. Due to the large volume of wastewater treated daily, the treatment time for each tank should be as short as possible; that is, the water in the tank should be quickly and evenly mixed after alkali addition. Mixing methods include:
① Add a mixer to the pool for mixing, such as the small neutralization pool scheme before the renovation of the No. 2 power station;
② Add an aeration head to the pool for stirring, such as the multi-point alkali addition and aeration head scheme adopted by a power plant;
③ Add a circulating pump to circulate the water, such as the alkali addition scheme in the circulating pipeline of the No. 2 power station. The alkali is not only mixed evenly in the pipeline, but also achieves uniformity through circulation in the pool.
II. Principles of Computer-Controlled Acid and Alkali Wastewater Treatment
The distance between the acid and alkali wastewater treatment site and the control room in the boiler water treatment system is relatively far. The high-voltage control cabinet is located in the field control room, and the computer is controlled through intermediate relays to prevent interference. The acid and alkali wastewater treatment site consists of a treatment tank, a storage tank, an alkali tank, an alkali addition pump, a circulating pump or blower for stirring the treatment tank, a water transfer pump between the treatment tank and the storage tank, electric valves, and various measuring instruments.
Data to be tested: liquid level in treatment tank and storage tank, liquid level in alkali tank, pH value in pipeline, pH value in tank, pH value of discharge and flow rate.
The equipment requiring control includes: circulating submersible pumps or fans, water pumps, dosing pumps, and electric valves. The principle is as follows: measurement data is directly input into the controller. The control of various motors is achieved by the controller's switching output controlling the activation of intermediate relays. The normally open contacts of these intermediate relays then control the contactors in each circuit of the field control cabinet, thereby starting or stopping the equipment. Simultaneously, a pair of normally open contacts of the contactors close, providing feedback to the input relays in the relay cabinet. The contacts of the intermediate relays serve as the controller's switching inputs, which are then fed to the computer to determine whether the equipment has started reliably and to assess its operating status.
III. Control System Design
1. System Hardware Configuration
The system employs a master-slave control method. The master computer is an industrial PC, communicating with the slave computer via RS-485. The master computer primarily displays the process flow diagram, collects real-time data at various points, dynamically displays the actual values on the screen, automatically saves the data, and performs automatic alarms, statistical printing, and report generation. The slave computer uses the I-8811 embedded controller from ICP DAX (Taiwan), composed of various AI, AO, DI, and DO cards. Its function is to transmit field signals via cables to a signal processing card, process them into 1-5V DC signals suitable for computer processing, perform A/D conversion, and then upload them to the master computer via RS-485. For each loop in the control circuit, the input signals are compared with the given signals transmitted from the master computer. If the deviation exceeds the set range, PID control is performed according to the PID coefficients transmitted from the master computer. The result is then processed by the output module to control the control valve.
2. System Software Design
The monitoring software for the host computer uses Interllution's iFIX version 2.6, an integrated software package based on multi-tasking, multi-platform compatibility, good real-time performance, and openness. It integrates state-of-the-art modern software technologies such as COM/DCOM, OPC, VBA, and ActiveX, enabling seamless integration of all application components into a single system and easy data sharing over networks. Systems built with it are stable and reliable and widely used in industrial control. ICP Faraday Future (Taiwan) not only provides the controller's testing software but also offers a free interface program with iFIX. The system uses the iFIX configuration software product, which we have further developed to accommodate the various functional modules required by the system, enabling it to complete our tasks. Configuration includes system configuration, real-time database configuration, historical database configuration, loop configuration, flowchart configuration, and alarm configuration.
Configuring the 8811 in the iFIX monitoring software is also quite convenient. The main contents include:
① Channel definition, which defines the communication path between iFIX's SCADA nodes and process hardware, including the baud rate of the network hardware and other communication settings.
② Device definition, which defines a single device, specifying its hardware type and configuration that differs from other devices. This includes the device name, device address, and device model.
③ Poll record definition: Within a device's data block, specified parameters are defined. These include the initial address and data type, as well as the polling time and wait time definitions.
3. Main functions of the control system
① Monitoring function: Detects and monitors parameters such as liquid level, acidity, and motor current in the treatment tank;
② Real-time dynamic process flow display function: This is the main screen for system operation and control. The opening status of each valve, whether the switching is successful, and whether the pump is turned on are displayed in real time on this screen. For control points, the set value and actual value of the point, liquid level, etc. are displayed.
③ Data processing function: The system can automatically process the collected signals and output corresponding control quantities;
④ Control function: Automatically controls the normal operation of the equipment according to the corresponding preset values before operation, and has manual/automatic switching function;
⑤ Trend module: You can see the pH value changing with dosage and time;
⑥ Alarm function: It can alarm for various parameters exceeding the limit or abnormal equipment status by means of visual and audible signals;
⑦ Reporting Function: Daily reports, monthly reports, and other tables can be automatically generated according to user requirements. Users can also select the print button on the screen to print reports. The report saving time needs to be determined, and the entire process parameters of automatic alkali addition can be queried and printed at any time.
4. Automated neutralization treatment of wastewater
Each treatment tank processes hundreds of tons of wastewater with significantly different initial pH values and varying sediment compositions, making it difficult to apply alkali using traditional PID control methods. We employ a simulated artificial alkali addition method. Before adding alkali, the wastewater is thoroughly mixed, the pH value is measured, and alkali is added based on the pH level. The mixture is then further analyzed after thorough mixing until the water quality meets the required standards.
The automated process for wastewater neutralization is as follows: When the treatment tank reaches the given water level, the compressor is started, and the aeration heads in the tank begin aeration and mixing. After thorough mixing (the time can be set), the pH value in the tank is measured. If the tank is acidic, the program automatically adds alkali to neutralize it, bringing the pH value to the discharge standard between 6 and 8.5. If the pH value is greater than 8.5, acidic water is pumped from the storage tank, or a portion of the alkaline water from the treatment tank is pumped to the storage tank, and then a portion of acidic water is introduced from the cation exchange bed. After thorough mixing to meet the discharge standard, the discharge valve is activated for discharge.
IV. Conclusion
This system has been applied in the wastewater neutralization treatment upgrades of our No. 1 and No. 2 power plants, achieving the expected results and improving the level of production automation. The method of simulating artificial alkali addition is feasible. The system is versatile and can be applied to industries such as power plants, pharmaceuticals, and textiles.
