## 2 General Aspects of Dry Anaerobic Digestion
### Technologies of Dry AD  
Dry AD can be carried out at an industrial scale when the TS content is 20% or higher, due to the accumulation of solid particles in reactors [22]. More than 90% of the reactors used in Europe are single-stage reactors. Dry and wet AD are distributed equally [70]. Dry AD can be carried out in continuous or batch mode. The companies that offer dry AD technologies that are available in the market, include Dranco, Kompogas, Valorga, and Biocel (see Figure 4). Additionally, installations are divided into stirred and unstirred. These technologies are intended for the digestion of organic components of municipal solid waste, agricultural waste, and food waste. Dranco reactors are vertical containers that move and mix a substrate from the upper part to the bottom of the reactor [13; 22]. They work at a TS concentration equal to 20 to 50% with a hydraulic retention time (HRT) of the reactor in the range of 15 to 30 days [1]. The inoculate is preparing by mixing six parts of substrate with one part of effluent [15; 22; 74]. The substrate is loaded to the top of the reactor and moves to the conical bottom. After that, the effluent is withdrawn from the reactor, and then solid matter is dehydrated and pressed [13]. The technology works under thermophilic or mesophilic conditions [74; 75]. Optimum thermal conditions for fermentation are from 55 to 60 \({}^{\circ}\)C. However, in practice, this temperature is not always provided due to inhibition by ammonium [13]. [PERSON] et al. [13] pointed out that the Dranco technology could provide a biogas yield that equalled 103 to 147 mL/g of wet mass, and [PERSON] et al. [1] noted that biogas yield ranged from 100 to 200 mL/g. The reactor that provides the Dranco process is shown in Figure 5.  
Kompogas reactors are horizontal digesters, which operate continuously with a HRT between 15 and 20 days at a TS substrate content equal from 23 to 28% under thermophilic conditions [13; 15]. The Kompogas reactors operate similarly to the Dranco reactors, with the difference that the substrate is mixed by a vortex flow in a horizontal position [13; 76]. The horizontal position contributes to the homogenization, degassing, and mixing of particles in the reactor by using impellers [13; 22; 77]. Such systems require that the TS concentration in the substrate is controlled close to 23%. Particles such as sand and glass accumulate in the reactor when the TS concentration decreases, and it can lead to disruption of mixing when the TS content increases [76]. The biogas output ranges from 110 to 130 mL/g of wet mass [13]. Inhibition by ammonium is characteristic of fermentation of nitrogen-containing substrates. For example, inhibition is typical of systems for protein waste fermentation by Kompogas systems [78]. The reactor that provides the Kompogas process is shown in Figure 6.  
Figure 4: Dry AD reactors available in the market. The Strabag technology is very similar to the Kompogas technology but differs by the method of mixing. Mixing is providing by several transversal paddle agitators. This technology is devoted to processing the substrate with a TS content of 15-45% [79]. The reactor that provides the Strabag process is shown Figure 7[80]. According to [PERSON] et al., the Valorga technology provides fermentation of the substrate with a TS concentration between 25 and 30% of TS with a HRT up to 3 weeks under thermophilic and mesophilic conditions [13]. [PERSON] noted that the technology operated at 25 to 32% with HRT between 18 and 25 days, and removed particles with a size of more than 80 mm [15]. The reactor consists of a vertical cylinder with an inner wall extending to about two-thirds of the tank diameter [13,15]. In this case, the substrate moves from one side of the wall to the other side through the partition [15]. Mixing is achieved by injecting the biogas under high pressure into the bottom of the reactor every 15 min through a network of biogas injectors [13,15,68]. Such mixing works satisfactorily, and consequently, there is no need for effluent recycling.