# A Review on Anaerobic Digestion of Lignocellulosic Wastes: Pretreatments and Operational Conditions
## 1 Introduction  
Organic wastes are produced in large amounts worldwide. Mismanagement of such wastes will lead to various problems for both human beings and the environment. Among the different technologies used to treat this type of waste is the anaerobic digestion process (AD) [1]. This process is a biological treatment that allows the decomposition and stabilization of a wide spectrum of organic wastes, from complex lignocellulosic materials to easily degradable food waste, while simultaneously producing renewable energy, recovering fibers and nutrients for soil amendments, and offsetting greenhouse gas emissions [2; 3].  
The AD process is a well-established technology that transforms the organic fraction into renewable fuels, such as methane. However, the process is recognized as a complex one, and many research studies have indicated that hydrolysis and methanogenesis can be the rate-limiting steps in AD because of the accumulation of undesirable volatile fatty acids (VFA) [4; 5; 6]. Nevertheless, controlling and optimizing the major operational parameters can improve the process efficiency and obtain stable methane generation. Recently, numerous studies have been performed to enhance the biogas production and to mitigate the inhibition effect caused by various compounds and situations. In this regard, pretreatment methods of feedstock, including physical, chemical, biological, thermal, and combined approaches, were investigated to overcome kinetic disadvantages and increase processpotential, mainly for lignocellulosic biomass, as this kind of waste represents a promising source of alternative energy since it has high potential for biogas production [7; 8].  
Monodigestion of substrates has been applied in AD, and this approach proves to work effectively for a narrow range of generated wastes, especially for easily degradable fraction waste. However, introducing a mixture of different substrates together, which is known as codigestion, is able to improve the process performance and make it the more attractive alternative in waste management hierarchy. In this context, codigestion provides a great chance for recycling a wide spectrum of wastes, supporting the synergic effects of microbial activities through improving nutrient balance (mainly carbon/nitrogen (C/N) ratio), and consequently, the enhancing the process stability in general [9]. Various mixtures of agricultural wastes, municipal wastes, industrial wastes, sewage sludge, etc., have been investigated by researchers using the codigestion approach and satisfactory results have been obtained [10]. Importantly, selecting an adequate co-substrate and mixing ratio, organic loading rate, co-substrate characteristics, and co-substrate-prompted inhibitions are essential factors to be considered, as they may greatly influence the process performance [11].  
This current review provides a comprehensive overview of the process of AD, which is considered to be one of the most viable options for recycling the organic fraction of solid waste. Feedstock and degradation pathways, AD systems, pretreatment technologies, process stability, and codigestion for lignocellulosic materials are introduced and discussed thoroughly in this review.