# Salinity Inhibition in Thermophilic Anaerobic Digestion of Organic Waste
## 4 Discussion
These high inhibitory concentrations (compared with mesophilic concentrations) were not expected for the thermophilic process, which is normally considered more susceptible to inhibition than the mesophilic process. The low inhibition could be due to the very robust and flexible microbiome with high redundancy, which originated from a long-standing anaerobic reactor where the microbial biomass could be exposed to several salt shocks before being used for the experiments. Therefore, the microbiome could already be partially acclimated to the high salinity. The papers of [PERSON] et al. [22; 44] are some of the few scientific reports on thermophilic AD under saline conditions. They tested the anaerobic degradation of halophyte biomass in a mixture with organic waste for biogas production comparatively at mesophilic and thermophilic temperatures. They obtained biogas yields (equal to SBP) of up to 0.366 m\({}^{3}\)/kg dry matter at mesophilic conditions and up to 0.480 m\({}^{3}\)/kg dry matter at thermophilic conditions with organic matter degradation of 60-70% at 55 \({}^{\circ}\)C during the 30 days of the incubation period, although thermophilic AD is generally considered to be more sensitive to different inhibition factors [45]. Their results show slightly lower values than in the present work (SBP was 0.562 m\({}^{3}\)/kg\({}_{\text{COD}}\), and the tCOD removal efficiency was 79.9%), which is probably due to the fact that more degradable organic waste is mixed with less degradable halophyte biomass. According to [PERSON] et al. [46], a comparison of thermophilic and mesophilic anaerobic treatment of waste from a mussel cooking factory exhibited better performance of the mesophilic reactor due to the faster adaptation of the mesophilic sludge to the high salinity. In addition, sodium is more toxic to propionic acid-utilizing microorganisms than to acetic-acid-utilizing ones. According to [PERSON] and [PERSON] [47], sodium proved to be the most potent inhibitor on a molar basis among the metal cations. Sodium showed moderate inhibition at 3.5-5.5 g/L and strong inhibition at 8 g/L. The authors reviewed by [PERSON] [48] showed different limits of salt inhibition and acclimation, with IC50s in the range of 5.6-53 g/L depending on the adaptation period, antagonistic/synergistic effects, substrate, and reactor configuration. Nevertheless, it can be concluded, from this research, that the process adapts very well up to 20 g/L chloride. Even at higher concentrations, only partial inhibition occurs, while immediate recovery occurs when the salinity is reduced. [PERSON] and [PERSON] [29] conducted a review study in which they concluded that the right amount of salt could boost microbial activity, which in turn can increase biogas production, and that sodium salt concentration should be below 8 g/L to prevent inhibition of methane production. Furthermore, [PERSON] et al. [30] studied the effects of a complex protein substrate and individual amino acids on anaerobic digestion in a saline environment and found that the addition of individual amino acids in the saline environment disrupted the granule form. The methane content produced during the anaerobic digestion of acidic wastewater can be optimized by dilution with municipal wastewater [31]. Finally, [PERSON] et al. [32] studied the anaerobic digestion of brackish water fish sludge (salinity 12 g/L) and observed the highest biomethane potential of 564.2 NmL CH\({}_{4}\)/g VS. In summary, based on the conducted studies and in comparison, with the results found in the relevant literature, it can be concluded that a saline environment imposes some limitations on anaerobic digestion. However, the presence of salt is not lethal to the microorganisms, suggesting that careful optimization and future research are needed for better understanding.