# Salinity Inhibition in Thermophilic Anaerobic Digestion of Organic Waste
## 3 Results  
### Continuous Exposure to Salinity  
The experiment lasted 238 days (Figure 1). The first 24 days were operated without salt added, and the average SBP (specific biogas production) was 0.562 m\({}^{3}\)/kg\({}_{\text{HCOD}}\) with 62.0% methane. The maximum tCOD removal efficiency was 79.9%. No significant amounts of SCFAs were detected, and the pH was well above 7.7. After the addition of salt at a concentration of 15 g/L chloride (21 g/L NaCl), the system showed a slight shock, but operation was not significantly hindered. SBP decreased to 0.490 m\({}^{3}\)/kg\({}_{\text{HCOD}}\), and tCOD degradation also decreased slightly. tSCFAs increased above 1 g/L, with propionic acid predominating. Concentrations peaked on day 43 with tSCFAs at 1.594 g/L (propionic acid 1.493 g/L and approximately equal traces of other SCFAs). After increasing salinity to chloride concentration of about 17.5 g/L (24.6 g/L NaCl) on day 61 and later to 20.0 g/L (28.2 g/L NaCl) on day 94, the operation did not change significantly. SBP was 0.512 m\({}^{3}\)/kg\({}_{\text{HCOD}}\) and 0.509 m\({}^{3}\)/kg\({}_{\text{HCOD}}\), respectively. The average methane concentration was 58.3 \(\pm\) 3.0% and did not vary significantly. Peak tSCFAs were 1.078 g/L (propionic acid 0.928 g/L) and 1.485 g/L (propionic acid 1.309 g/L), respectively. The pH ranged from 7.1 to 7.5. Looking at the SCFA data in Figure 1, it can be stated that before the NaCl concentration was increased to 24.6 g/L and 28.2 g/L, the SCFA values decreased significantly to values below 0.500 g/L, indicating that the process flow is stabilizing, and the microbial community is adapting to the conditions. Therefore, up to the NaCl concentration of 28.2 g/L, it can be concluded that no significant inhibition is observed, but the process is affected and requires about three HRTs to adapt to the salt conditions. The NaCl concentration of 28.2 g/L seems to be the boundary between stable operation and the occurrence of inhibition. Already at 28.2 mg/L NaCl (20.0 g/L chloride), the SCFA concentration no longer decreases. As far as the change in the concentration of organic acids, the following was observed. Acetic acid varied between 18.4 and 2023 mg/L, propionic acid between 2.4 and 1693 mg/L, and tSCFA between 28.8 and 3849 mg/L. It was also noted that there were periods when iso-butyric, butyric and valeric acids were not detected, while the highest concentrations observed were 117, 374 and 121 mg/L, respectively. After further increasing the chloride concentration, the process showed a partial inhibition, increasing tSCFA to more than 2.000 g/L and peaking at 2.986 g/L and 3.416 g/L at NaCl concentrations of 31.7 (22.5 g/L chloride) and 35.2 g/L (25.0 g/L chloride), re  
Figure 1: tCOD removal, pH, SBP, chloride concentration and SCFAs at continuous exposure to salinity. spectively. During most of this experimental period (from day 127 to day 190), propionic acid was still predominant (average at 1.462 \(\pm\) 0.126 g/L), but acetic acid began to increase to equal concentrations. Other SCFAs were less significant in the range between 0.060 and 0.150 g/L. Partial inhibition also decreased the SBP to 0.368 m\({}^{3}\)/kg\({}_{\text{ICOD}}\) and 0.296 m\({}^{3}\)/kg\({}_{\text{ICOD}}\) at NaCl concentrations 31.7 and 35.2 g/L, respectively. The tCOD removal also decreased below 60%. The pH was in the range between 6.9 and 7.3, and the methane concentration decreased to 51.1 \(\pm\) 2.3%. Further increase of salinity to NaCl concentration of 39.4 g/L (28.0 g/L chloride) initially increased the symptoms of partial inhibition. SBP decreased to 0.256 m\({}^{3}\)/kg\({}_{\text{ICOD}}\), tCOD removal continued to decrease (minimal peak at day 199, where removal was 43.6%), and tSCFAs continued to increase. Propionic acid was in the same range as before, and acetic acid increased massively, reaching a peak concentration of 2.023 g/L on day 210. In addition, significant amounts of butyric acid occurred, reaching a peak of 0.374 g/L, confirming the strong inhibition of AD.