## 2 General Aspects of Dry Anaerobic Digestion
### Influence of Water Activity on Dry AD  
There may not be enough free moisture to conduct dry AD with a high TS concentration in a substrate [29]. The production of biogas is reduced in proportion to a TS concentration increase from 10 to 25%. Anaerobic digestion is unstable at a TS concentration equal to 30%, and dry AD with a TS concentration of 35% is completely suppressed [52]. Even under thermophilic conditions (where moisture is extra bioavailable), the amount of biologically available water is insufficient [53]. The same effect on CH\({}_{4}\) production of water availability is observing for anaerobic processes in soils [54].  
Water activity is the ratio of the vapor pressure of the water in the material to that of pure water at a given temperature [55]. According to [PERSON], this is the availability of free water in the system [56]. It means that lower water activity of the substrate leads to decreasing available moisture for microorganisms [55]. Water distribution, which depends on both the water content and the water interactions with the medium compounds, determines water bioavailability and kinetic rates [57]. Biological activity can be used to estimate the possibility of using anaerobic digestion for treating waste [57]. The presence of biologically active water can affect population growth by increasing the entropy of the cell systems [58].  
It has been proven that anaerobic digestion of substrates with a TS concentration of 20% lacked biologically active moisture due to low water activity for the formation of biogas by microorganisms at 20, 37, and 55 \({}^{\circ}\)C [29].  
The activity of most microorganisms is inhibited when the water activity value is less than 0.6. Bacteria, unlike yeast and micromyectes, are less osmotolerant and their optimum growth is reached with a value of water activity in the range of 0.99-0.995 [55]. Bacteria growth is possible in the range of water activity of 0.85-1 [59]. According to [PERSON] [60], the value of water activity for high-speed hydrolysis should be more than 0.91.  
An increase of TS concentration from 5 to 19% leads to a sharp decline in water activity from 0.956 to 0.93, and below the last value, water activity decreases more smoothly. Thus, the value of water activity at a TS concentration of 25% is 0.928, and at a TS concentration of 35%, it is 0.925 [61].  
Water activity is affected by temperature. An increase in temperature leads to increased water activity. Thus, growth of microorganisms under thermophilic conditions is up to 60% more intense as compared with the mesophilic ones, in particular, due to higher water activity. There is more biologically active moisture at 55 \({}^{\circ}\)C than at 35 \({}^{\circ}\)C [29]. However, it is often impossible to use high temperatures, due to inhibition by ammonium, as described in Figure 3.  
Figure 2: The proportion of free ammonia depending on pH and ambient temperature.  
An increase in water activity under dry AD is possible by spraying water into the upper part of the reactor [29]. Thus, [PERSON] et al. [62] studied top-down and down-top modes of fluid motion under dry AD and found that the top-down mode improved biogas yield.