# Salinity Inhibition in Thermophilic Anaerobic Digestion of Organic Waste
## 1 Introduction  
Anaerobic digestion (AD) is a preferred treatment for waste streams heavily loaded with organic matter, allowing the waste volume to be reduced and energy to be recovered at the same time. According to current biogas trends in the literature reviewed by [PERSON] et al. [1], the main parameters that influence biogas production include process, pretreatment, and feedstock, which is why the proper balance of these parameters is needed to achieve the highest biogas yield. In addition, current trends in anaerobic digestion contribute to the circular economy, which can also, in turn, benefit wastewater treatment [2]. Much of the research on biogas production accentuates lab-scale production [3], currently used in biogas production, which is why the assessment of its applicability needs to be considered [4]. Advantages of anaerobic digestion over other treatment processes include low operating and maintenance costs, low excess biological sludge accumulation, low nutrient requirements, and high methane production efficiency. The disadvantage of anaerobic digestion also includes sensitivity to inhibitors. Many nutrients necessary for operation can have an inhibitory effect in excess amounts [5]. Regular table salt (NaCl) is one ofthe common compounds found in nature, especially when the substrates for anaerobic digestion come from the sea. When seaweed was used as a substrate for AD, the inhibitory effect of NaCl was found to be slightly below a salt concentration of 3% w/w (concentration of salt in seawater), as no biogas was produced under these conditions [6]. Salt-tolerant plants (halophytes) used as energy crops on saline soils also accumulate salt in their vacuoles, increasing the salt concentration in the biomass [7]. During the anaerobic digestion of food waste, table salt used as a food additive may interfere with anaerobic digestion, and therefore, the optimal NaCl concentration in the anaerobic digestion of food waste has been reported to be 2-4 g/L [8]. [PERSON] et al. [9] found that a NaCl concentration of 20 g/L during anaerobic digestion of food waste did not affect acidogenesis, but it did affect methanogenesis. [PERSON] et al. [6] found that increasing salinity up to 20 g/L NaCl had no significant effect on biogas production from oily halophilic algal residues. Anaerobic treatment of saline waste streams is hindered by ions present in the substrates, especially monovalent cations such as Na\({}^{+}\), which cause inhibition of methanogenic archaea [10]. All other influencing factors that inhibit the anaerobic digestion process in general and for different types of waste streams are detailed in the review article by [PERSON] et al. [5]. Indeed, high salt concentrations lead to the plasmolysis of microbial cells due to strong osmotic stress on the cell wall, resulting in the desiccation of microbial cells or, to a greater extent, cell death [7]. Anaerobic bacteria have varying degrees of saline tolerance. _Staphylococcus aureus_ can grow in solutions containing up to 65 g/L NaCl, while _E. coli_ is inhibited at much lower levels [11]. The dominance of the order _Clostridiales_ was observed at higher salinity in biogas reactors, and under the same conditions, a shift in the archaeal community from acetoclastic _Methanosaeta_ to hydrogenotrophic _Methanocorpusculum_ was detected [12]. According to [PERSON] and [PERSON] [13], NaCl toxicity increased in the following order: lignocellulosicoses-degrading bacteria > acetate-utilizing bacteria > propionate-utilizing bacteria > H\({}_{2}\)/CO\({}_{2}\) utilizing organisms. In studying waste degradation in landfills with leachate recirculation, [PERSON] et al. [14] found that a salt concentration of 35 mS/cm of electrical conductivity inhibited methane generation under mesophilic conditions, while salt concentrations below 21 mS/cm electrical conductivity did not affect methane production. It has been postulated previously that the total conductivity in AD should be kept below 30 mS/cm to avoid salt stress [5].