## 3 Results and Discussion
### IoT bioreactor Performance
#### 3.1.1 Biogas Production Monitoring of the IoT Bioreactor  
The daily biogas production and total concentration of VFAs in the pilot IoT bioreactor are presented in Figure 4. Biogas production displayed an upward trend from the first days of bioreactor operation, stabilizing on day 16 as OLR reached the target level of 2 g VS LR\({}^{-1}\) d\({}^{-1}\). During the first five days, when the OLR was 1 g VS LR\({}^{-1}\) d\({}^{-1}\), the average biogas production was 19.74 \(\pm\) 1.04 L d\({}^{-1}\). In the following two periods, as the OLR gradually increased to 1.25 and 1.65 g VS LR\({}^{-1}\) d\({}^{-1}\), and biogas production was 25.61 \(\pm\) 1.50 and 37.21 \(\pm\) 1.59 L d\({}^{-1}\), respectively. When the OLR reached 2 g VS LR\({}^{-1}\) d\({}^{-1}\), biogas production stabilized at an average of 43.22 \(\pm\) 0.91 L d\({}^{-1}\). However, on day 41, the biogas production decreased due to the increased addition of oleic acid to the bioreactor, caused by the malfunction of the peristaltic pump mentioned earlier. Afterwards, it stabilized for a 7-day period at 38.02 \(\pm\) 1.76 L d\({}^{-1}\). Subsequently, a sudden reduction in biogas production was observed. This decrease may be attributed to several factors, including the introduction of oleic acid, which can inhibit microbial activity [32; 33]. Additionally, the absence of lignocellulosic materials in the substrate, which could have aided in microbial immobilization [34], combined with a low HRT for this type of substrate, probably contributed to the washout of microorganisms essential for biogas production. The concentration of total volatile fatty acids (VFAs) remained at relatively low levels until day 40, not exceeding 0.22 g L\({}^{-1}\). Following the malfunction of the oleic acid peristaltic pump (OLR 2.30 g VS L\({}^{-1}\) d\({}^{-1}\)), a noticeable increase in VFA concentration was observed, with levels nearly sextupling by day 48 (Figure 4). During the initial feeding period at 1.00 g VS L\({}^{-1}\) d\({}^{-1}\), acetic acid remained low and stable, persisting through day 40 (OLR 2.00 g VS L\({}^{-1}\) d\({}^{-1}\)) at 157.75 \(\pm\) 18.64 g L\({}^{-1}\). However, in the period with an OLR of 2.30 g VS L\({}^{-1}\) d\({}^{-1}\), acetic acid concentration nearly quadrupled. Propionic acid had an initial concentration of 4.95 \(\pm\) 2.88 g L\({}^{-1}\) and remained at low levels, not exceeding  
Figure 4: Daily biogas production and total Volatile Fatty Acids (VFAs) concentration (g VS L\({}^{-1}\) d\({}^{-1}\)) of the pilot IoT bioreactor for 48 days of continuous operation. 3.15 \(\pm\) 1.68 g L\({}^{-1}\). In the final period, propionic acid followed a similar trend to acetic acid, with its concentration reaching 62.24 \(\pm\) 11.17 g L\({}^{-1}\). VFA concentration is a key indicator of potential inhibition in AD processes. The observed VFA levels suggest that inhibition did not occur in the bioreactor during the anaerobic process. Typically, acetic and propionic acid concentrations rise significantly under inhibition; however, both remained within stable, low levels, indicating the absence of inhibitory effects. The observed VFA increase may be attributed to the wash-out of microorganisms from the bioreactor. The IoT bioreactor recorded each activation of the biogas meter with precise timestamps, generating a detailed database of biogas production events. Each event was logged with date and time down to the exact second. This setup allowed for the calculation of time intervals between biogas counter activations for each day. Figure 5 presents these intervals in minutes, four days before and four days after day 42, when a biogas production drop of approximately 11% was observed. Since the bioreactor received two substrate feedings per day via peristaltic pumps, the intervals displayed in Figure 5 focus on the first 40 biogas meter activations following each 12-h feeding cycle.