## 4 Enhancement of Energy Recovery (Biogas/Methane Yield) from the Anaerobic Digestion of Rice Straw
### Organic Loading Rate (OLR)  
Process stability and efficiency are proved to be the basic criteria to evaluate the performance of AD [78]. OLR is an important operation parameter that greatly influences the performance of the AD, and it can be described as the amount of substrate added to the digester per day per unit volume. OLR is expressed as Equation (1):  
\[OLR=\frac{C}{t_{HR}} \tag{1}\]  
where C is the feeding concentration of substrate in g.VS/L, and t\({}_{\rm HR}\) is the hydraulic retention time in days (d).  
Higher OLR can produce higher biogas because of the higher quantity of substrate for digestion [79]. With a higher OLR, the rate of hydrolysis/acidogenesis could be higher than methanogenesis, and the accumulation of VFAs can eventually cause process failure [80]; therefore, a suitable OLR is always necessary for stable operation.  
Thermophilic co-digestion of RS with animal manure ( PM, CoM, and CM) was carried out at different OLRs of 3.0, 3.6, 4.2, 4.8, 6.0, 8.0, and 12.0 g VSL\({}^{-1}\) d\({}^{-1}\) by [PERSON] et al. [40]. Results from co-digestion of RS and PM illustrated that a stable biogas yield, on average, 434 L/kg VS was achieved without inhibition by ammonia or VFAs while, for the co-digestion of RS with CoM, the average yield was 455 L/kg VS at all OLRs. In contrast, the AD process was inhibited by ammonia when the OLR was \(\geq\) 6 g V S L\({}^{-1}\) d\({}^{-1}\) during the co-digestion of RS with CM and the specific biogas yield was 422 L/kg VS at 4.8 g VSL\({}^{-1}\) d\({}^{-1}\).  
Similarly, the effect of various OLRs on the anaerobic mesophilic co-digestion of RS with the PM [41], CoM [28], and CM [67] have been investigated, and at 6-8, 6, and 4.8 g VS L\({}^{-1}\) d\({}^{-1}\), there was no inhibition in the digesters due to the accumulation of VFAs or ammonia, respectively. However, co-digestion of RS with PM and CoM was severely inhibited by VFAs at OLR of 12 g VS L\({}^{-1}\) d\({}^{-1}\). The co-digestion of RS with CM was inhibited at OLR \(\geq\) 6 g VS L\({}^{-1}\) d\({}^{-1}\) due to the accumulation of ammonia.  
The effect of various OLRs on the yield of biogas from the AD of RS, and their recommended values from the literature are summarized in Table 6.  
The recommended OLRs for commercial co-digestion of RS with PM, CoM, and CM at mesophilic conditions are 6.0-8.0, 6.0, and 4.8 g VS L\({}^{-1}\) d\({}^{-1}\), while recommended values for thermophilic conditions are 12.0, 12.0, and 4.8 g VS L\({}^{-1}\) d\({}^{-1}\). From the results of these studies, it can be concluded that a choice of a suitable OLR is necessary for attaining a good biogas yield from the AD of the RS. The optimal OLR is influenced by the type of substrate (e.g., manure) and the operating temperature. Higher OLRs with stable biogas yield can be attained at thermophilic conditions compared to mesophilic ones. Moreover, the cause of inhibition is also different in different co-digestion systems. For CM, the process inhibits by free ammonia; VFAs inhibit co-digestion of PM and CoM at higher OLR. The co-digestion system of RS assisted with in situ removal of ammonia can be considered to avoid inhibition by ammonia at higher OLRs [40]. The methods considered for the removal of ammonia are immobilization and adaptation of microorganisms, ultrasonication, microwave, hollow fiber membranes, and microbial fuel cell applications [82].