## 3 Results and Discussion
### pH, Temperature, and REDOX Monitoring  
One of the primary indicators of AD is the stability of the whole system. The pH value is considered to be one of the most important factors in assessing the stability of the entire process, as it is reported that the optimal pH values for methanogenesis are ranging between 6.5 and 8.2 [38; 39]. Since pH is relatively simple to measure and provides important information on system stability, it is often measured online in industrial biogas plants [40]. pH values, measured from the pH meter and the IoT pH sensor, are depicted in Figure 7. As can be easily observed, the pH value remained relatively stable throughout the experiment, with an average value of 7.68 \(\pm\) 0.01 and 7.55 \(\pm\) 0.04, as measured by the pH meter and the IoT sensor, respectively. Furthermore, from day 41 onward, the response of pH values was in accordance with the increase in OLR and VFA production. During this period, as the organic load led to increased accumulation of VFAs, the reactor's pH value dropped, as measured by both the instrument and the sensor. However, the pH fluctuation was smaller than the VFAs variation. Specifically, pH values drop by only 0.034 \(\pm\) 0.009 units as measured by the pH meter, and by 0.025 \(\pm\) 0.003 units as measured by the IoT pH sensor. This very slight change in pH with the increase in VFAs is attributed to the buffering capacity of the reactor [40; 41], due to the presence of carbonate salts in the BA medium. Overall, the proposed system appeared to perform well in monitoring the pH values of the reactor, as the average deviation between the pH meter and the IoT sensor readings was only 1.67% throughout the experiment. Thus, the presence of the IoT sensor provides a representative indication of the reactor's pH condition. However, as previously indi  
Figure 7: Bioreactor pH, temperature (\({}^{\circ}\)C), and REDOX measurements from the IoT device, along with pH and temperature readings from a bench pH meter and an analog thermometer over time (days). cated, monitoring the pH alone is not sufficient for a well-buffered system, given the low sensitivity in the pH response in these reactors [10]. Another AD process indicator that is often monitored in the liquid face is temperature. Temperature range has a significant impact on the biodegradability of different substances used in AD since it directly affects the metabolic activity of microorganisms [42]. The temperature of the digestion process influences the CH\({}_{4}\) content of the biogas and, thus, the overall CH\({}_{4}\) production [43]. It has been reported that a small change in the temperature can immediately alter the activity of the methanogens, which can lead to potential process imbalances [44; 45]. Therefore, temperature is one of the key indicators that are commonly monitored in anaerobic reactors. The temperature fluctuations measured from the analog thermometer and the IoT temperature sensor during the 48-day study period are presented in Figure 7. The average temperature recorded by the IoT temperature sensor was 37.19 \(\pm\) 0.31 \({}^{\circ}\)C, whereas the analog thermometer indicated an average temperature of 37.24 \(\pm\) 0.0 \({}^{\circ}\)C, resulting in a deviation of 0.15%. As can be easily observed from Figure 7, the IoT temperature sensor showed variations throughout the day since it recorded data four times every hour (every 15 min) all day long, making it more responsive to environmental changes. In contrast, readings from the analog thermometer were taken manually at roughly the same time each day. On the 43 rd day (Figure 7) of monitoring, the automated temperature notification system was evaluated.