# A Review on Anaerobic Digestion of Lignocellulosic Wastes: Pretreatments and Operational Conditions
## 5 Codigestion of AD  
As discussed in the aforementioned sections, providing optimal operating conditions is a crucial point for an effective AD. It is well-known that the deficiency of some nutrients could lead to low performance or even the inhibition of the process [53]. In this regard, codigestion arises as an efficient and cost-effective way to overcome some of the difficulties during this process [14; 134; 135]. The codigestion process simply involves mixing two or more substrates together, which would improve the utilization of nutrients through balancing them (mainly the C/N ratio), enhancing the microbial diversity, improve buffer capacity, and dilute inhibitors; consequently, codigestion may achieve positive synergistic effects and lead to higher methane yield than monodigestion of substrates [53; 159; 160]. In this context, lignocellulosic biomass is categorized as a great prospect for anaerobic digestion due to its high content of cellulose and hemicellulose. However, its elevated C/N ratio (Table 1) and the complexity of its biomass structure (contains 10-25% lignin) inhibits the anaerobic digestion process when it is used individually [13; 161; 162]. On the other hand, animal waste, such as manure, contains high amounts of organic nitrogen [163; 164], which would lead to accumulation of ammonia, and potentially inhibition of the microbial activities because of a low C/N ratio [163; 165; 134; 148]. Therefore, monodigestion of lignocellulosic biomass or animal wastes would result in less stable performance and low methane production as a result of the inappropriate C/N ratio. Creating an optimum C/N ratio through codigestion of lignocellulosic biomass with appropriate animal waste can maximize methane production and make the AD process more stable [162; 164].  
Cellulose, hemicellulose, and C/N ratio potential values for rice straw, wheat straw, corn stover, and switch grass are 32%, 24%, and 47; 38%, 21%, and 60; 37%, 22%, and 63; and 38%, 26%, and 93, respectively. These values indicate the high potential of these substrates to produce biogas, however, they are still considered difficult to digest, as they contain a significant fraction of lignin, whereas the codigestion of these substrates with a highly nitrogen-rich substrate, such as manure, would result in a successful way to utilize these materials in an anaerobic digestion process, since the materials complement each other when used together and the potential for inhibition is reduced [166]. Additionally, for the codigestion of hay with soybean processing waste at 25:75 ratio under solid-stateAD process, the methane yield was 258 L/kg VS, which is 50% and 148% higher than the amount obtained by each substrate individually [117]. Co-digestion of barley straw with pig manure produced 233.4 L/kgVS methane, but it produced only 192 L/kg VS methane when it was codigested with cow manure [167]. Corn stover and dog food at an equal ratio (1:1) produced a methane yield of 304.4 L/kg VS, which led to an increase of 129% compared to corn stover alone and 9% compared to dog food digestion alone [116]. Rice straw with swine manure, wheat straw with chicken manure, corn stover with chicken manure, and crop silage with cow manure produced 350, 235, 223-298, and 249 L/kg VS methane, respectively, in an AD codigestion process with optimum C/N ratio. In this context, codigestion resulted in a 31% increase of the expected yield, which was calculated from the methane potential of the individual fractions [168]. Food waste codigestion with sewage sludge can produce a high yield of biogas of 708 L/kg VS [169]. In Table 3, other examples of codigestion experiments are resumed.