# A Review on Anaerobic Digestion of Lignocellulosic Wastes: Pretreatments and Operational Conditions
## 4 AD Stability or Inhibition
### Effect of Ammonia
#### 4.1.2 Mitigating Ammonia Inhibitory Effects  
Even though the ammonia inhibition mechanism has been determined by different researchers, unfortunately until now there has been no direct approach to avoid ammonia toxicity, especially when it exceeds the threshold inhibition level [6]. However, some measures could be employed to mitigate ammonia inhibition, which are described below. (a) Inoculum Selection and Adaptation of Microorganisms  
The inoculum source and adaptation of methanogenic consortia to high ammonia levels have been recognized as effective and feasible tactics for enhancing the anaerobic digestion process. In this context, acclimation of acetoclastic methanogens to high concentrations of TAN reduce their susceptibility to any increase in TAN and improve their tolerance to a wider range of pH levels [96]. Methane was successfully produced in dry fermentation of chicken manure at about 8000 to 14,000 mg TAN/kg after adaptation time of 254 days at 37 \({}^{\circ}\)C and pH of 7.3-8.8. It was concluded that spontaneous acclimation of the methanogenic biomass to high concentrations of ammonia could occur and result in production of methane, even under a high TS content (25%) and high concentration of ammonia [6; 35; 96; 97; 110]. The research conducted in [111] investigated the effect of six different inoculum sources on the digestion of rice straw using BMP tests under the same conditions, and significant differences in the biogas yields were observed, caused by the different inocula sources. Methane yields were 180 mL/g VS, 160 mL/g VS, 75 mL/g VS, 125 mL/g VS, 15 mL/g VS, and 5 mL/g VS for dairy manure, swine manure, chicken manure, granular sludge, municipal sludge, and paper mill sludge inocula, respectively. Reactors inoculated with digested manure had higher methane yields compared to others, and this was attributed to the availability of macro- and micronutrients in manure, which enhanced the digestion of rice straw. One study [112] demonstrated that using diary manure digester effluent as the inoculum improved methane yields from corn stover up to 30% and 100% compared to inoculum obtained from food waste and sewage sludge digesters, respectively. This variation was attributed to the higher populations of hydrolytic microbes in dairy manure effluent. Also, best inoculum selection was able to reduce the lag phase from 20-30 days to 2-5 days [113]. (b) Substrate/Inoculum Ratio (S/I)  
Providing an optimal substrate to inoculum (S/I) ratio is considered to be important in AD process. However, this ratio may vary with types of substrates under consideration, because while the digestion of food waste may be susceptible to VFA accumulation because of its composition and high biodegradability, other substrates may have buffering capacities that reduce the potential VFA accumulation [114; 115]. In ss-AD of lignocellulosic biomass, a wide range of S/I ratios have been investigated under both mesophilic and thermophilic conditions. Ratios between 2 to 3 (VS basis) were found to be relatively robust for ss-AD mesophilic conditions, whereas a relatively high S/I ratio was preferred under thermophilic conditions. For instance, the highest biogas yield for corn stover in mesophilic ss-AD was obtained with an S/I ratio of 2.43 (VS basis), while the highest yield under thermophilic conditions was achieved at an S/I ratio of 4.58 [7; 116; 117; 118]. According to [115], and in accordance with the proposed guidelines by [PERSON] et al. [119], a minimum S/I of 0.5 is recommended for highly degradable substrates, such as food waste, while an S/I of 1 may be used for less degradable substrates, such as lignocellulosic waste. As will be illustrated in the following sections, other parameters, such as pH and C/N ratio, also directly affect ammonia toxicity [6; 120; 121].