Design Of Online Detection System For Total Phosphorus In Surface Water For Smart Cities

Smart water affairs are an important part of smart cities,and the intelligent detection of routine parameters of water quality is crucial to the realization of smart water affairs.The content of total phosphorus is one of the important indicators to evaluate the eutrophication degree of water body.At present,there are mainly offline methods and online methods for the detection of total phosphorus.The existing off-line detection instruments for total phosphorus are cumbersome and dangerous to operate.The existing commercial total phosphorus online detection instruments have problems such as long detection period,large volume,and high price.Accordingly,this study developed an online total phosphorus detection system with simple operation,small size,low cost,good accuracy and stability.Through the requirement analysis of the system,the overall hardware and software design scheme of the total phosphorus online detection system is proposed.Using ultraviolet light digestion method and phosphomolybdenum blue spectrophotometry combined with flow analysis technology,the system analysis flow path is designed.Especially for the problem that the Z-type flow cell is disturbed by air bubbles,theoretical and simulation studies have been done,and the angle between the solution outlet and the inlet of the Z-type flow cell has been optimally designed.The system adopts the concept of modular design,and designs peristaltic pump equipment modules,solenoid valve equipment modules,ultraviolet digestion equipment,syringe pump equipment modules and detection modules,etc.,to complete collection,digestion,mixing,detection and other functions.The system takes STM32F103RBT6 as the core,and designs the control thread of each device based on the RT-Thread embedded real-time operating system to realize the orderly management and control of each device.The TSL2581 optical digital sensor is selected to develop the detection module to realize the online detection of the total phosphorus concentration.Based on the Free Modbus protocol library,the Modbus protocol is used to complete the data transmission between the system master and slave devices.Use the Lab VIEW software platform to design the human-computer interaction program of the upper computer to realize the intelligent control of each device and real-time data collection and processing.Finally,the validity of the hardware circuit,software system and Modbus protocol are verified through software and hardware debugging.The results of the bubble experiment of the flow cell at different angles show that the angle has an influence on the discharge of the bubbles.Based on the experimental results,the optimal angle(75 °)was obtained and the structure design of the flow cell was optimized.System performance experiments show that the lower detection limit of the system is0.0004mg/L,which is far lower than the detection limit of the national standard method.The stability of the system was good,and the relative standard deviations of the determination results of phosphate samples with concentrations of 0.02mg/L,0.20mg/L,and 0.60 mg/L were 1.71%,0.87%,and 0.72%,respectively(n=13).The concentration of the water sample measured by the system is 0.046～0.825mg/L,and the concentration of the water sample measured based on the national standard method is0.048～0.792mg/L.The results of the two measurements are linearly correlated(y=0.9804x-0.0007,R2=0.9969,n=9),the slope of the fitting equation is close to 1,which shows that the results of the system measurement and the results of the national standard method have a high degree of approximation and small error.The practical application shows that the system meets the needs of online determination of total phosphorus,and can provide technical support for smart city construction and environmental protection.