## 4 Enhancement of Energy Recovery (Biogas/Methane Yield) from the Anaerobic Digestion of Rice Straw
### Bioavailability of Trace Elements (TEs)  
The presence of a particular TE in the AD does not necessarily imply that it can be taken up by microorganisms and incorporated into the catalytic center of the enzymes [118]. Bioavailability is defined as the degree to which a substance is available for metabolic activities [119]. The bioavailability of TEs can be grouped and approximated by sequential extraction techniques [120]. Water-soluble and exchangeable fractions of TEs are commonly considered as highly bioavailable. TEs bound in carbonates, sulfides, or organic complexes are less bioavailable as their mobility depends on the aqueous solubility of the compounds. The residual fraction is not available to microorganisms because it is non-extractable and non-dissolvable [120]. [PERSON] et al. [120] examined the applicability of the sequential extraction technique on slurries of AD plants. These slurries were obtained from three biogas plants: an agricultural biogas plant utilizing PM and MS\({}^{\text{a}}\), a plant using grass silage (GS) and MS\({}^{\text{a}}\), and a plant processing the slaughterhouse waste (SHW). Their results are presented in Table 10. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  
**TEs**, Trace elements; **AD**, Anaerobic digestion; **RS**, Rice straw; **MS\({}^{\text{a}}\)**, Maize silage; **CS**, Corn stover; **MS**, Maize straw; **PS\({}^{\text{a}}\)**, Phragmites straw, \({}^{\text{b}}\)**, Biogas enhancement.  
\end{table} 
Table 9: Effect of addition of TEs on the methane yield in the AD of RS and other straws. Fe has been added alone and incombination with Co to determine whether their bioavailability could be a limiting factor for CH\({}_{4}\) yield using RS as a substrate in the presence of agro-zootechnical digestate as an inoculum in a study carried out by [PERSON] et al. [110]. The results pointed out that the amount of highly bioavailable Fe was increased from 11% to 23% when it was added alone and from 11% to 20% when it was dosed with Co. Similarly, Co addition resulted in an increase of the highly bioavailable fraction from 8% to 48%. The CH\({}_{4}\) yield was the same with and without the addition of TEs. Therefore, the direct relationship between the increased highly bioavailable fractions of TEs and enhanced CH\({}_{4}\) yield in the AD of RS was not observed. [PERSON] et al. [109] conducted a similar study to investigate the changes in five fractions of Fe to assess its bioavailability and whether the system was deficient in Fe during the AD of RS. Their results indicated an overall very low (1.7-3.1%) bioavailability of Fe, and most of the Fe was not readily available to microorganisms in the digestion system. Thus, there was a deficiency of bioavailable Fe which may have reduced CH\({}_{4}\) yield. The effect of the bioavailability of Fe, Ni, Co, and Mn on the CH\({}_{4}\) yield from the AD of MS in the semi-continuous CSTR was studied by [PERSON] [121]. The results indicated a decreasing trend of highly bioavailable fractions of TEs over time. At the end of the experiment (after 96 days), the highly bioavailable fractions of Fe, Ni, Co, and Mn decreased by 23%, 32%, 33%, and 20%, respectively, compared to day 1. The results confirm that fewer TEs were directly available for microbial uptake at the end of the process leading to an 11% decrease in CH\({}_{4}\) yield. The results of the studies summarized in Table 10 show that the sequential extraction is a useful method to assess the bioavailability of TEs available in various AD systems. The determination of the bioavailability of TEs is a more appropriate approach compared to total available amounts concerning their requirement to monitor the process to achieve a stable and efficient process. The results of one study show the direct relationship between the reduced amount of highly bioavailable fractions of TEs and reduction in CH\({}_{4}\) yield in the AD of MS.