# Salinity Inhibition in Thermophilic Anaerobic Digestion of Organic Waste
## 5 Conclusions and Future Perspectives  
The results of the experiment, in which salt addition was gradual, show that bacteria and archaea responded rapidly to salt addition. Although the structure of bacterial and archaeal microbiomes differed and the diversity decreased at the end of the experiment, the resumption of methane production was still possible after the decrease of salt concentration at the end of the experiment. The results of the experiment in which there was rapid salt addition show that the shift of the bacterial community was significantly affected by the salt addition. However, it was also observed that the bacterial community recovered and reached a great similarity with the original community before the salt shocks. As for the archaeal community, the results show non-uniformity and lower diversity compared to the bacterial community. The archaeal microbiome responded rapidly to the salt shocks by reducing the salt-sensitive species. The maximum COD removal achieved in the gradual salt addition and rapid salt addition experiments was 79.8% and 83.1%, respectively. In the first case, the maximum NaCl concentration was 28.2 g/L, whereas in the second case, higher concentrations of up to 39.4 g/L were tolerated. SPB production with gradual salt addition varied between 0.490 and 0.562 m\({}^{3}\)/kg\({}_{\text{ICCOD}}\) depending on the salt concentration, while SPB production with rapid salt addition was relatively stable at 0.590 m\({}^{3}\)/kg\({}_{\text{ICCOD}}\). This was also underlined by the fact that short-term rapid salt shocks were not lethal to the microbial community, which quickly recovered and adapted to the above conditions. This suggests that both the bacterial and archaeal communities adapted well to the increased salinity and recovered quite well between salt shocks. These observations suggest that the selected bacterial and archaeal communities are robust and well adapted to high salt concentrations under the conditions tested, which in turn implies that these microbial communities have the potential for anaerobic digestion in hypersaline environments. We can argue that our thermophilic methanogenic microbiota, resistant to high salinity, developed during lab-scale experiments one and two have great potential. Following the pilot experiments with the developed salt-resistant biomass, we plan to use it as a source of microorganisms for bioaugmentation of anaerobic digestion processes on an industrial scale where high salinity substrates are degraded under thermophilic conditions like waste from the fish industry and food waste. Successful bioaugmentation experiments under mesophilic conditions were conducted by [PERSON] et al. [28], using microbial consortia derived from marine sediments to reduce inhibition of methane production under NaCl stress. There is almost no way to find natural microbial consortia for bioaugmentation of thermophilic digestion processes, but there is a way to develop them, as shown in the present work and proposed in one of the recent articles on high salinity wastewater treatment technology [48]. When the results of future pilot scale bioaugmentation experiments are collected, the sustainability assessment of the developed process for anaerobic degradation of high saline organic substrates needs to be performed using tools suggested by [PERSON] et al. [49]. In future experiments, the metagenomic sequencing of the developed microbial consortium is planned, as performed by [PERSON] et al. [26], to identify the predominant thermophilic salt-resistant bacterial and methanogenic species in order to explain and understand the phenomenon of microbial acclimation to high salinity stress. The results suggest that anaerobic digestion has great potential for high-salinity wastes. Given the relatively sparse literature on this topic, further investigation of the effect of salinity on anaerobic digestion will provide insights that will optimize waste management for these specific properties.