## 4 Biomethane Production via Anaerobic Digestion (AD)
### Biochemistry of Anaerobic Digestion
This will lead to the formation of biohydrogen and biomethane from POME, which helps in stabilizing the system through sludge diminishing. Currently, anaerobic digestion systems are springing up like a mushroom. For POME, the most recommended digestion process includes UASB, UASFF, an anaerobic sequencing batch reactor (ASBR) and a continuous stirred tank reactor (CSTR) [87]. Tables 4 and 5 show some comparison studies using a single-stage bioreactor and integrated bioreactor for POME treatment, respectively, while Table 6 summarized the preferences and drawbacks of each bioreactor. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Anaerobic Treatment** & **Advantages** & **Disadvantages** & **References** \\ \hline UASB & \begin{tabular}{c} High COD removal efficiency \\ and CH\({}_{4}\) production rate \\ \end{tabular} & \begin{tabular}{c} High dependable on \\ sludge settling property \\ \end{tabular} & [41] \\ \hline UASFF & \begin{tabular}{c} Higher biomass retention, a \\ shorter start-up for \\ sludge granulation \\ \end{tabular} & \begin{tabular}{c} Reactor stability and \\ efficiency depend on the \\ feed flow rate, internal \\ packing, up-flow velocity \\ and effluent recycle ratio \\ \end{tabular} & [94] \\ \hline ASBR & \begin{tabular}{c} Simple operation, flexible and \\ no separate clarifiers needed. \\ \end{tabular} & \begin{tabular}{c} Low treatment capability \\ under higher OLR \\ \end{tabular} & [73] \\ \hline CSTR & \
\begin{tabular}{c} Inexpensive and easy \\ to handle \\ \end{tabular} &
\begin{tabular}{c} Poor gas production \\ under high OLR and \\ short HRT \\ \end{tabular} & [95] \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Advantages and disadvantages of the anaerobic treatment system commonly used for POME treatment using different bioreactor configurations. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Inoculum** & \ \begin{tabular}{c} **Ingetated** \\ **System Used** \\ \end{tabular}  &
\begin{tabular}{c} **Organic Loading** \\ **Rate  
Theoretically, the same mechanism of fermentation and anaerobic digestion applied in treating different organic wastes, regardless of the types of bioreactor used. However, the characteristics of the microbes, bioreactor configurations, conditions used and process parameters might affect the growth of hydrogen-producing bacteria (HPB), reactor stability, pH and temperature in the bioreactor. Extensive research has been done using an integrated system for biohydrogen and/or biomethane production using various lignocellulosic wastes. The studies conclude that in comparison to a single-stage, the two-stage system could increase energy yield [92], stabilize hydrolysate and improve energy recovery [96; 97], stabilize using a high organic loading rate [41] and achieve operational stability [98]. The economic benefits of the waste treatment also could be improved by having a phase separation for H\({}_{2}\) and CH\({}_{4}\) in the respective systems [99]. Studies on biohydrogen/biomethane using an integrated system in Malaysia are also increasing every year [100; 101]. This review is important for researchers and industries despite studies treating POME using the integrated system that is new in Malaysia. This is because anaerobic digestion is the cheapest technology for biohydrogen/biomethane production compared to photo-fermentation or electro-hydrogenases. The integrated process of DF-AD can also be applied for different organic wastes such as food wastes for hydrogen/methane production in Malaysia. Meanwhile, Table 7 shows studies done in different countries treating organic wastes for biohydrogen and biomethane production implementing a two-stage system. The big finding of using the integrated system is that either effluent concentration could be reduced (high COD removal efficiency), or two valuable gases could be produced (H\({}_{2}\) and CH\({}_{4}\)) or both.