## 2 General Aspects of Dry Anaerobic Digestion
### Inhibition of AD by Ammonia  
The main problem of providing dry AD of chicken manure is inhibition due to the high content of nitrogen- [30] and sulfur-containing [31] compounds formatted in the process of hydrolysis. However, it has been determined that chicken manure digestion is primarily limited by the content of nitrogen-containing compounds [32; 33; 34], which can be present in a substrate in the forms of ammonium and non-dissociated ammonia [35]. Under dry AD, inhibition of anaerobic digestion by ammonia is much more intensive, which limits its use. However, the advantages of dry AD generate interest in its use with high-nitrogen substrates. Ammonia also causes engine corrosion and environmental pollution [36; 37]. To decrease the effect of ammonia inhibition, it is necessary to provide an anaerobic digestion process with high hydraulic retention time (HRT) [38] and, consequently, low organic load rate (OLR) [39].  
Ammonia has a strong inhibitory effect on methanogens, especially on acetate-utilizing methanogens [40]. The most sensitive to increasing ammonia concentration are representatives of Methanomicrobiales, particularly the Methanoculleus sp. [41]. Methanosaetaceae are also more sensitive to inhibition by ammonia than representatives of Methasorsarcinaceae [42].  
According to [PERSON] et al., chicken manure consists of 35.16% C, 4.83% H, 30.12% O, 5.44% N, and 0.84% S and its gross formula is CH\({}_{1.648}\)O\({}_{0.642}\)N\({}_{0.133}\)S\({}_{0.009}\). Therefore, decomposition of 1 g of volatile solids (VS) of chicken manure delivers 0.087 g of NH\({}_{3}\) (0.072 N-NH\({}_{4}\)+) [43]. Therefore, the production of NH\({}_{3}\) is estimated to be from 39 to 46 g/kg (32-38 N-NH\({}_{4}\)+ g/kg) in the case of 25% TS and 60-70% VS of chicken manure. According to [PERSON] [44], the anaerobic digestion is inhibited at any pH when ammonium content is more than 3 g/L. However, even a concentration of ammonium from 1.5 to 2.5 g/L causes inhibition [45]. It has been proven that the process is unstable when NH\({}_{3}\)-N content is more than 1 g/L [41]. Figure 1 shows potential ammonia content in case of full decomposition at different TS contents.  
As shown in Figure 2, potential ammonium content in the case of full decomposition at conditions of dry anaerobic digestion (TS content more than 16 %) is much higher than the safe level.  
Mechanism of Inhibition by Ammonia  
The most toxic substance for anaerobic bacteria is non-dissociated ammonia. It is diffused inside a cell and ionized to NH\({}_{4}\)+. This process leads to pH imbalance inside and outside a bacteria cell, which provides incensing of cell energy usage, decreasing of fermentative activities, violation of the transport of substances, proton imbalance, or potassium deficiency [43; 46; 47]. Inhibition of anaerobic metabolism can also be due to the accumulation of volatile fatty acids [48]. [PERSON] et al. found that an increase in ammonium concentration and increased temperature led to a decrease in the proteolytic activity of the anaerobic consortium [49].  
It has been proven that increasing both temperature [50; 51] and pH [30] has led to an increased inhibition effect by ammonia. Figure 2 shows the proportion of free ammonia depending on pH and ambient temperature.  
Figure 1: Potential ammonia concentration in case of full decomposition at different total solids (TS) contents.