# Salinity Inhibition in Thermophilic Anaerobic Digestion of Organic Waste
## 1 Introduction
To overcome the negative effect of salinity on anaerobic digestion, [PERSON] et al. [7] proposed co-digestion with other biomass feedstocks to dilute the salt and/or acclimate the microbial biomass to operate at a higher salinity in the AD process by gradually optimizing the microbial community composition and metabolism. During this optimization process, salt-tolerant microorganisms are preferentially selected in the biomass. While short-term exposure to NaCl concentrations below 4 g/L improved the anaerobic digestion of kitchen waste, higher salt concentrations inhibited biogas production, although some microbial communities had the potential to adapt to a hypersaline environment [15]. Among the first studies on anaerobic treatment of highly saline wastewater were mainly experiments with wastewater from seafood processing [16]. [PERSON] et al. [17] studied the effects of high NaCl concentrations on methanogenic activity in semi-continuous flow-through reactor systems at mesophilic temperatures (30-35 \({}^{\circ}\)C). At 95 g/L of NaCl, 50% inhibition was observed after the adjustment period. [PERSON] et al. [18] studied the inhibitory effect of Cl ions on anaerobic up-flow digestion for acidified brine from reverse osmosis treatment of sewage at 20 \({}^{\circ}\)C. 2.1 g/L chloride ions in the brine did not negatively affect the treatment. [PERSON] et al. [19] reported various inhibitory effects of sodium at neutral pH on the methanogenic activity of the granular sludge in the UASB reactor. Sodium concentrations of 5, 10, and 14 g/L of Na\({}^{+}\) caused 10, 50 and 100% inhibition, respectively, relative to the maximum specific acetoclastic methanogenic activity of the granulated sludge containing _Methanothrix_ sp. as predominant acetoclastic methanogen. In addition, no adaptation of _Methanothrix_ sp. to high sodium concentrations was detected over a 12-week period. [PERSON] et al. [10] studied the effect of sodium on the methanation of short-chain (volatile) fatty acid (SCFA) mixtures for three different sludges. Sodium concentrations that caused 50% inhibition ranged from 3 to 16 g/L in the absence of nutrients or other salts. Higher sodium tolerance was evident for sludges originating from the digesters that treated highly saline effluents after adaptation. In contrast, [PERSON] [20] was unable to achieve sludge stabilization in the mesophilic anaerobic treatment of saline fish farm effluents when using 10-10.5 g/L sodium in the continuously stirred tank reactors (CSTR) at 35 \({}^{\circ}\)C. [PERSON] et al. [21] investigated the anaerobic digestion of desugared molasses for biogas production in batch mode and in a CSTR under mesophilic (37 \({}^{\circ}\)C) and thermophilic (55 \({}^{\circ}\)C) conditions. The latter contains two to three times higher ion concentration than normal molasses, which could inhibit the biogas process. Furthermore, a 50% inhibition of manure digestion occurred at sodium and potassium concentrations of 11 and 28 g/L, respectively. Several papers in the older and recent literature deal with the effect of salinity on mesophilic anaerobic digestion, but very rarely with anaerobic digestion under thermophilic conditions [22; 23]. A list of recent work in this area is given in Table 1. The thermophilic temperature range in anaerobic digestion has many advantages compared to the mesophilic range, such as a high level of pathogen elimination, improved hydrolysis of complex organic materials, better destruction of organic solids, and higher potential for biogas production [24]. However, a disadvantage is also that the process is more sensitive to inhibition than anaerobic digestion in the mesophilic range. Therefore, this study investigates the limits of successful thermophilic anaerobic digestion of organic wastes at high salt concentrations to determine the concentration limits for successful operation.