## 2 Materials and Methods
### Variable-Flow Experiment Design  
Turbidity (NTU) is mainly influenced by water flow fluctuations and can only reflect the instantaneous transparency of the water body. This study proposes a technique for detecting turbidity in an RAS based on a micro-screen drum filter. The backwash frequency of the drum filter within a unit period (0.5 h) was used to represent overall RAS turbidity, and the variable-flow regulation model was constructed using the backwash frequency and various water quality data. The variable-flow regulation model can determine the operating frequency of the centrifugal pump for the next period using real-time data from the current period. The intelligent variable-flow RAS technology is implemented by controlling the RAS circulation rate by changing the circulating pump flow rate. The primary purpose of the variable-flow RAS is to implement a linkage control technology to  
Figure 1: The experimental recirculating aquaculture system: (**a**) the schematic of the experimental RAS control process; (**b**) the RAS in Dalian Huixin Titanium Equipment Development Co., Ltd.  
model the relationship between the micro-screen drum filter backwash frequency and the circulation flow rate. The total flow rate of the circulating pump was set to three levels: 55, 65, and 75 m\({}^{3}\)/h. The circulation rate was operated with a cycle of 24 h. A cycle started with a circulation rate of 55 m\({}^{3}\)/h and was adjusted to 65 m\({}^{3}\)/h after an interval of 24 h and then to 75 m\({}^{3}\)/h after the same interval (24 h). The drum filter controller collected backwash data every 0.5 h. Turbidity sensors were placed at the main return pipeline to monitor and record overall RAS water turbidity. Water quality indicators, including water temperature (T), dissolved oxygen (DO), pH, and salinity, were measured by sensors in real time using YSI ProPlus portable sensors. Total suspended solids (TSS), total ammonia nitrogen (TAN), and nitrite nitrogen (NO2-N) were measured daily with a Palintest 7500 water quality analyzer. The circulating pump was set to three circulating levels: slow (55 m\({}^{3}\)/h), medium (65 m\({}^{3}\)/h), and fast (75 m\({}^{3}\)/h). In the variable-flow RAS, the circulation rate was maintained at a medium level, and the control system read water quality indicators and backwash times from sensors at every unit period. The circulation rate for the next period could be adjusted to slow or fast levels. The circulation adjustment process could be operated in two ways: upshift and downshift. In the drum filter controller program, the backwash frequency was recorded for 48 periods in a day, using 0.5 h as a period. The circulating pump was utilized to determine the upshift/downshift for the next period by reading the current water quality sensors, current backwash frequency, and current circulating level. A water gauge controlled the drum filter backwash frequency; the backwash frequency reflects water turbidity in the RAS. Downshifts (\(-1\)) and upshifts (\(+1\)) of circulating pump frequency were used as indicators of circulation levels. The water quality indicators, current circulating pump frequency, and the drum filter backwash frequency were chosen as independent variables, and the downshifts (\(-1\))/upshifts (\(+1\)) data were considered as the dependent variable. As the whole culture process lasted for 90 days in the RAS, the total circulation rate was set to 55 m\({}^{3}\)/h for the first 30 days, 65 m\({}^{3}\)/h for the middle 30 days, and 75 m\({}^{3}\)/h for the last 30 days. Establishing a variable-flow circulation strategy was the core task of the experiment, and therefore the circulation rate regulation model was constructed using the optimal classification model based on machine learning to control the variable-flow circulation rate in the RAS. As shown in Figure 2, the drum filter controller was used to collect the backwash frequency, circulation flow rate, and water quality data that were then uploaded to the industrial PC through the RS485 protocol.