# A Review on Anaerobic Digestion of Lignocellulosic Wastes: Pretreatments and Operational Conditions
## 2 Overview of AD Process
### Solid-State (ss) and Wet-State Anaerobic Digestion
#### 2.4.2 Solid-State AD (ss-AD)  
Solid-state anaerobic digestion (ss-AD) provides the ability to process high-solid-content substrates; typically, with a solid content above 15%. The process is recognized as robust and advantageous compared to wet AD because it requires less energy for heating, less water, smaller digester volume, minimal material handling, and because of its low moisture content, the digestate of ss-AD is more coherent and can be used as fertilizer or peltetized fuel, which is much easier to handle than the effluent produced by wet AD [14; 34; 35]. These advantages contributed to the large industrial development of ss-AD, with the rapid emergence of full-scale plants [31]. Nonetheless, it should be noted that regardless of the various advantages of ss-AD technology and advancements in system design, there are still a number of aspects that need to be deeply investigated and improved for further commercialization of the technology. Water content is noted as a key factor in deriving microbial activity, and low content of this parameter in ss-AD would lead to accumulation of the microbial end products, and thus inhibition of methanogenesis [38]. In this regard, and because of slow mass transportation, longer retention time (up to three times) has been documented for ss-AD compared to wet AD [38; 39; 40]. According to [41], water availability presents two distinct forms: (i) \"free water\", which can act as a solvent for soluble compounds and salts; and (ii) \"bound water\", which is unable to act as a solvent, as it has more structural bonding (chemical and physical interactions) than liquid or free water. The variation in the composition and structure of the organic matter has a major role in determining the dominant form of water and consequently the rate of degradation. In this context, it should be considered that when the TS content increases, the quantity of free water decreases; thus, rheological behavior of the substrate is modified and the diffusion of metabolic compounds within the substrate at a macroscopic level is lowered [42; 43], meaning the transport of soluble content within the substrate can become a limiting factor [43]. As illustrated in the literature, substrate degradation and consequently biogas production are reduced by increasing the TS content [38; 39; 42]. One study [44] demonstrated that an increase from 20% to 30% in TS content was associated with a decrease in organic waste degradation by 17%. Also, another study [45] reported reduced methane yields and cellulose hydrolysis rates of lignocellulosic biomass in ss-AD compared to those of wet AD. The limitation of ss-AD might be explained by the slow diffusion of dissolved inhibitory products inside the organic matrix causing local accumulation at the microbial scale [46; 47; 48]. In a recent study [38], methanogenesis inhibition was observed at high solid content levels, which was attributed to gas transfer limitation, and more particularly by a local accumulation of gases (H\({}_{2}\) and CO\({}_{2}\)) leading to VFA accumulation. In this context, and because of thermodynamic reasons, anaerobic oxidation (acetate formation) can only take place at low partial hydrogen pressure, meaning the balance between the hydrogen-forming and hydrogen-consuming microorganisms is very important. In order to avoid stress in the acidogenic bacteria, the partial pressure of the hydrogen gas should always be less than 1-4 atm [49]. Solid-state AD under mesophilic and thermophilic conditions has been investigated, and mesophilic AD is considered more stable and better established than thermophilic AD [14]. However, for lignocellulosic wastes, thermophilic conditions usually show better initial performance than mesophilic AD, since high temperatures improve the hydrolysis of substrates, being \"the major rate-limiting step\" [14].