Design of distributed water quality monitoring system under circulating water aquaculture mode of freshwater pearl mussels

Abstract: In China, the freshwater pearl mussel aquaculture plays an important role in agricultural production. The freshwater pearl mussel's growth process does not harm the environment, but the traditional aquaculture model has caused problems such as serious water pollution, eutrophication, and ecological environment destruction. Facing such a situation, it's necessary to develop a new model for freshwater pearl aquaculture. This study showed a new industrialized circulating water aquaculture model (using multi-layer dimensional cages and feed pearl mussel with high-quality microalgae). Due to the high density of aquaculture, this model had very high requirements for water quality, so it was particularly important to monitor the circulating water quality. This study developed a distributed monitoring system for the water quality monitoring based on ZigBee wireless sensor network. The system used the three-layer architecture of the field sensing layer, transmission layer and application layer, and mainly included the following 4 parts: water quality monitoring subsystem, meteorological monitoring subsystem, equipment control subsystem, and monitoring data center. The system used the ZigBee module to set up the wireless transmission network, the advantage of this method was flexible networking and easy operation, and it simplified the installation and maintenance work. In the field scene, we set up multiple monitoring points in different places, multi-parameter water quality sensors were used to monitor key circulating water quality parameters (including temperature, pH value, dissolved oxygen, ammonia nitrogen, calcium concentration, and algae concentration), and meteorological parameters (such as temperature, humidity, barometric pressure, illumination, wind direction, and wind speed) were collected at meteorological station. All the sensors sent real-time detection data to their related ZigBee routers by RS485 protocol, and through the ZigBee wireless communication network, the data (including water quality and meteorological parameters) would be gathered in the ZigBee coordinator. The ZigBee coordinator was the center of the wireless communication network, it was also connected to the PLC controller by RS485 protocol, so the PLC controller could collect real-time detection data from basic monitoring points. At the same time, the PLC controller and MCGS touch screen communicated through the serial port, so the water quality and meteorological parameters monitoring information could be displayed on the MCGS touch screen. According to the designed procedure, the MCGS touch screen displayed the water quality data change curve, and users could also issue equipment control commands at the screen, such as controlling the circulating water treatment equipment (including arc screen separator, microfiltration filter, protein separator, sand filter, and disinfection pool) and the algae feeding system device. By using the PLC controller, the study designed a program to control the start and stop status of aquaculture equipment and adopted a two-position control strategy for the algae feeding system. Besides, the PLC controller communicated with the host computer via the CP243-1 Ethernet communication module, so all the monitoring data was uploaded to the host computer in the monitoring center house. On the host computer, MCGS (network version) software and SQL Server database software were used to build a database to save the water quality data, and this database could provide a decision basis for scientific aquaculture. Through the practical test, the communication success rate of the whole system was more than 98%, and the average relative errors of dissolved oxygen, temperature, pH, and ammonia nitrogen were ±0.3 mg/L, ±0.4 ℃, ±0.3 and ±0.04 mg/L, respectively. It could be seen that this system met the monitoring needs of freshwater pearl mussel aquaculture. This system was conducive to solve the previous shortcomings and ensure the growth of production, it was very worthy of promotion and application, and it would open up a new situation for the establishment of a new environment-friendly and efficient aquaculture mode of freshwater pearl mussel.