## 1 Introduction
### POME Treatment Systems in Malaysia
The implementation of a closed anaerobic system has drawn many changes towards the regulatory standard. It was reported that covered lagoon or closed-tank anaerobic digester has been widely used to treat POME [36]. Table 3 shows different methods studied to treat POME, especially in Malaysia, in order to get the highest removal of BOD, COD and suspended solids (SS) and/or total suspended solids (TSS) as possible. On the other hand, a hybrid system that combines the conventional and alternative methods such as the anaerobic filter [37], up-flow anaerobic sludge blanket (UASB) [38], sequencing batch reactor (SBR) [39], up-flow anaerobic sludge fixed-film reactor (UASFF) [40] and anaerobic fluidized bed reactor (AFBR) [41] were studied and used to obtain higher efficiency and ensure lesser treatment time. These hybrid reactors were proven to reduce hydraulic retention time (HRT) when studied on a laboratory scale. Above all, all palm oil millers must meet the standard requirement provided by the Malaysian DoE. The transition of the treatment method makes conventional POME treatment system becomes outdated and the new requirement for BOD discharge limit of 20 mg L\({}^{-1}\) seems hard to be fulfilled by the respective mills. However, a lot of POME treatment technologies have been studied as an alternative to the above-mentioned problem. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Treatment Method Used** & \multicolumn{2}{c}{**Parameters**} & \multicolumn{2}{c}{**References**} \\ \hline \hline  & BOD removal (\%) & COD removal (\%) & Total suspended solids/suspended solids removal (\%) & \\ \hline \multicolumn{5}{c}{Lab scale} \\ \hline Using biosorbent & 97.41 & 100 & 100 & [42] \\ \hline Ultrasonic-assisted membrane anaerobic system & 74 & 95 & 91-99.5 & [43] \\ \hline Attached growth on rotating biological contactor & 91 & 88 & 89 & [44] \\ \hline \multicolumn{5}{c}{Large scale} \\ \hline Anaerobic expanded granular sludge bed (EGSB) & 88.24 & 94.89 & 64.65 & [45] \\ \hline Combined high-rate anaerobic reactors & - & 93.50 & \textgreater{}90 & [46] \\ \hline Ultrafiltration membrane & 86.33 & 85 & 99.86 & [47] \\ \hline Activated carbon as bioadsorbent & 83 & 68 & 90 & [48] \\ \hline Green synthesis & - & 94.70 & 51.50 & [49] \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Different methods studied for POME treatment in Malaysia. Despite the importance of DF and AD process for biohydrogen and biomethane production, there has been extensive research on two-stage DF or AD for biohydrogen and biomethane production using different bioreactor configurations. However, combining the DF and AD process for biohydrogen and biomethane production utilizing POME is uncommon. Therefore, the major aim of this paper is to highlight current integrated systems treating POME. The sole purpose of this review is to highlight the importance of POME and how an integrated system could turn POME into valuable end products. This review will not focus on various aspects of POME polishing, processing and purification and its treatment methods using different bioreactor configurations as they have been reviewed previously [50; 51].