## 1 Introduction
### POME Treatment Systems in Malaysia  
The anaerobic process has become the most suitable method in treating POME due to its high organic properties. The high concentration of lipid, nitrogenous compounds, carbohydrates, protein and minerals in POME can be converted to valuable products by using microbial process [30]. Due to that, treating POME using the ponding system has been used in an earlier stage for the palm oil industry. Despite the fact that POME is non-lethal, there is a concern that economic expansion, environmental protection and sustainable development need to be balanced due to the fact that POME is a potential cause of pollution [31]. To ensure that this industry remains sustainable and environmentally friendly, POME needs to be managed properly and cannot directly be discharged into a water body as it can contaminate the water and endanger the aquatic ecosystem [32]. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Parameter** & **Unit** & **Raw POME** & **Digested POME** & **References** \\ \hline Oil and grease & mg L\({}^{-1}\) & 7000 \(\pm\) 550 & 183 \(\pm\) 10.1 & [21, 22, 26] \\ \hline Ammonium nitrogen & & & & & \\ (NH\({}_{3}\)-N) & mg L\({}^{-1}\) & 63 \(\pm\) 24 & 25 \(\pm\) 5 & [22, 23, 25] \\ \hline Biochemical oxygen & & & & & \\ demand (BOD) & mg L\({}^{-1}\) & 28,000 \(\pm\) 6750 & 19,000 \(\pm\) 5500 & [21, 22, 26, 27] \\ \hline \end{tabular} \({}^{\star}\) S.D. = standard deviation  
\end{table} 
Table 1: _Cont._ \begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Parameter** & \multicolumn{6}{c}{**Limits Required Based on the Period of Discharge**} \\ \cline{2-7}  & **1 July** & **1 July** & **1 July** & **1 July** & **1 July** & **1 July 1982–31** & **Future Standard** \\  & **1978–30** & **1979–30** & **1980–30** & **1981–30** & **December** & **1984–2015** & **Discharge Limit** \\  & **June 1979** & **June 1980** & **June 1981** & **June 1982** & **1983** & **1983** & **(2015 onwards)** \\ \hline pH & 5–9 & 5–9 & 5–9 & 5–9 & 5–9 & 5–9 & 5–9 \\ \hline Temperature (\({}^{\circ}\)C) & 45 & 45 & 45 & 45 & 45 & 45 & 45 \\ \hline Oil and Grease & & & & & & & \\ (mg L\({}^{-1}\)) & 150 & 100 & 75 & 50 & 50 & 50 & 5 \\ \hline Total Solids (mg L\({}^{-1}\)) & 4000 & 2500 & 2000 & 1500 & - & - & - \\ \hline Suspended Solids (mg L\({}^{-1}\)) & 1200 & 800 & 600 & 400 & 400 & 400 & 200 \\ \hline Total Nitrogen (mg L\({}^{-1}\)) & 200 & 100 & 75 & 50 & - & - & 150 \\ \hline Ammonium Nitrogen (mg L\({}^{-1}\)) & 25 & 15 & 15 & 10 & 150 & 100 & - \\ \hline COD (mg L\({}^{-1}\)) & 10,000 & 4000 & 2000 & 1000 & 100 & 100 & - \\ \hline BOD (mg L\({}^{-1}\)) & 5000 & 2000 & 1000 & 500 & 250 & 100 & 20 \\ \hline \end{tabular}
\end{table} 
Table 2: POME discharge limit from 1978 to 2015 and thereafter [29]. Therefore, a lot of studies have been done by researchers to treat POME using alternative methods. This is because conventional methods such as the aerobic/anaerobic system, open decomposing tank, anaerobic system, closed anaerobic decomposition tank and advanced ventilation system requires extensive land area and producing a foul odor, which results in environmental pollution [33; 34]. Due to the presence of untreated palm oil residue, raw POME consists of a high value of degradable organic matter [35]. Biological treatment with the aerobic, anaerobic or facultative process is the most suitable method to degrade/treat POME. This is because biological treatment requires less energy demand, does not liberate foul odor, can minimize sludge accumulation and can produce hydrogen and methane gas by anaerbes under fermentation and digestion processes. Moreover, methane gas produced can further be used for electricity generation. However, the open ponding system could cause methane gas being released into the atmosphere. This contributes to the thinning of the ozone layer that resulted in the greenhouse gas (GHG) effect. Even though less operational energy and small capital are required, an open ponding system involves a longer retention time (20-60 days) and large area [13].