# Salinity Inhibition in Thermophilic Anaerobic Digestion of Organic Waste
## 4 Discussion
The obtained TRFLP results showed that on the first day after salt shocks, the same peaks in the TRFLP electrophergrams disappeared, decreased (two peaks), or increased noticeably (one peak). Salinity shocks likely reduced the salt-sensitive species to levels not detectable by the TRFLP method, resulting in lower archaeal diversity after the shock. The dominant microbial groups are present throughout the shock period (three peaks) and may be the key groups for the resilience of AD. It seems that both bacterial and archaeal microbiomes acclimated well to increased salinity and recovered quite well between the salt shocks. These TRFLP results are in good agreement with [PERSON] et al. [41] and can be compared to the results of [PERSON] et al. [26], although these experiments with increased salinity were performed under mesophilic conditions and the microbial communities were studied in more detail by metagenomic sequencing. [PERSON] et al. [12] reported that microbial community redundancy, rather than resilience or resistance, was the main strategy behind anaerobic digestion process recovery. This idea is also supported by [PERSON] et al. [42]. However, some methanogens, such as _Methanosarcina_ spp., form irregular cell clusters that could increase their tolerance to high salt concentrations, and some of them (such as _Methanosaeta_) can adapt to higher salt concentrations [15]. The salt-adapted archaeal community showed up to 83% similarity with the community before each salt shock. The archaeal community at the end of the experiment with salt shocks differed by 25% from the initial community and, together with the bacterial community, allowed relatively effective methane production until the end of the experiment, except for short periods immediately after the salt shocks. SPB levels partially decreased immediately after salt addition but recovered quickly and remained at virtually the same level until the end of the experiment (Figure 3). The results of the presented experiments on thermophilic anaerobic digestion under high salinity conditions are difficult to compare with the results of other authors for several reasons. There are only a few studies dealing with the influence of high salinity conditions on thermophilic anaerobic digestion. Most of the experiments were performed under mesophilic conditions. The salinity is represented in different ways in the different experiments: as NaCl concentration, chloride concentration, Na+ concentration or conductivity. The majority of the experiments are short-term (approximately 30 days), while the experiments presented in this study are long-term experiments (238 and 303 days). The volumes of experimental bioreactors range from a few milliliters to several liters, and the experiments are performed at different ORLs, etc. For anaerobic degradation under high salinity at mesophilic conditions, it was observed that a NaCl concentration of 5 g/L inhibits the CH\({}_{4}\) formation rate [9], although the studies on anaerobic treatment of seafood processing wastewater indicate the possibility of anaerobic digestion even at salt concentrations close to the seawater salinity of 30 g/L NaCl [10]. [PERSON] et al. [9] observed that during the mesophilic process, an increase in NaCl concentration from 0 to 20 g/L inhibited methanogenesis but did not affect acidogenesis when the NaCl concentration was below 20 g/L. [PERSON] and [PERSON] [43] reported that methanogens are affected at NaCl concentrations as low as 6 g/L, whereas acidogenic bacteria are affected only at NaCl concentrations higher than 20 g/L. NaCl concentrations greater than 28.2 g/L were found to inhibit methane production under thermophilic conditions and continuous salt loading in the experiment. The results of the experiment with salt shocks showed that thermophilic AD could even tolerate shock loadings with high levels up to 39.4 g/L NaCl when the salt concentration drops to baseline between shocks and that concentrations up to 25.4 g/L NaCl do not cause inhibition of methane production.