# Salinity Inhibition in Thermophilic Anaerobic Digestion of Organic Waste
## 3 Results
### Shock Exposure to Salinity  
The experiment lasted 303 days (Figure 3). The first 33 days were operated without the addition of salt, and the average SBP was 0.595 m\({}^{3}\)/kg\({}_{\text{ICOD}}\) with 63.2% methane. The maximum tCOD removal efficiency was 83.1%. No significant amounts of SCFAs were detected, and the pH was well above 7.6.  
Figure 2: UPGMA dendrogram of T-RFLP profiles of bacterial (**left**) and archaeal (**right**) communities at continuous exposure to salinity. The numbers above the dendrogram represent the percent of similarity.  
In terms of the concentration of organic acid, the following was observed. Acetic acid varied between 17.2 and 680 mg/L and tSCFA between 20.1 and 2101 mg/L. It was also noted that there were periods when propionic, iso-butyric, butyric and valeric acids were not detected, while the highest concentrations were 1938, 119, 126, and 112 mg/L, respectively.  
The microbial community handled shocks of salinity much better than continuous load. For each shock, an increase in tSCFAs up to 1248 mg/L was observed, with propionic acid (1153 mg/L) being the most dominant. Of the other SCFAs, acetic was the most prevalent, with the others present at less significant concentrations. Each spike showed a decrease in biogas production with a brief inhibition followed by a rapid recovery. The trend in SBP was stable, with an average value of 0.590 \(\pm\) 0.159 m\({}^{3}\)/kg\({}_{\text{tCOD}}\). The average methane concentration was 63.2 \(\pm\) 2.7%. The degradation of tCOD was stable at 75.2%. The  
Figure 3: tCOD removal, pH, SBP, chloride concentration and SCFAs at Shock exposure to salinity.  
pH varied from 7.1 when the salt shock occurred to 7.8, indicating normal operation. Only after four consecutive shocks did prolonged inhibition occur, which was a consequence of the build-up of salt concentration. The minimum SBP was 0.250 m\({}^{3}\)/kg\({}_{\text{GCCD}}\) and the pH decreased to 7.0, and the corresponding methane concentration was 50.8%. After the salinity was reduced, the process recovered by itself and reached its initial parameters. This clearly shows that only prolonged and continuous exposure to salinity strongly affects the anaerobic process. It can be concluded that concentrations up to 25.4 g/L NaCl (18 g/L chloride) allow safe operation in the thermophilic range. Thermophilic AD can tolerate shock loading at higher levels of up to 39.4 g/L NaCl (28 g/L chloride) if the salt concentration drops to baseline between shocks. It takes 1 to 1.5 HRT to completely normalize after a shock.  
In this experiment, shifts in bacterial communities were clearly influenced by salt addition (Figure 4, left). Obvious shifts were detected one to three days after the salt shock, as indicated by the clustering of samples taken on the first and third days after the salt shock. During the first 260 days, the bacterial community recovered and reached up to 80% similarity to the community prior to the salt shock. The response of the bacterial community to the salt shock was correlated with a rapid partial inhibition of SBP and increased propionic acid concentrations, followed by rapid recovery after approximately 3 days. A similar trend was evident during the shock period from day 260 until the end of the experiment. Although the final bacterial community differed from the initial one by about 34%, it did not lose functionality and provided continuous methane production throughout the experiment (303 days). The diversity of the bacterial community was not highly reduced during the experiment. TRFLP electropherograms showed that two dominant peaks persisted throughout the experiment, indicating the existence of halotolerant species that may have originated from the initial inoculum.