## 2 Biogas Production from POME
### Challenges Using POME Wastewater  
Raw POME is composed of lignocellulosic material types that make it hard to degrade. A biological pre-treatment, either using specific bacteria or mixed, will take a longer time compared to using the chemical pre-treatment. A study on the pre-treatment of brewery seed sludge for biohydrogen production using raw POME as a substrate was done with the end goal to determine the best pre-treatment strategy for biohydrogen efficiency [60]. Among all the studied strategies, heat-shock pre-treatment was found to produce the highest cumulative hydrogen with highest COD removal efficiency. This is because homoacetylens in the seed sludge (inoculum) had been suppressed during the heat-shock, thus enabling hydrogen-producing bacteria (HPB) to grow.  
[PERSON] et al. uncovered that the obtained results were higher than the study done by [PERSON] et al. using dairy wastewater as a substrate, regardless of the pre-treatment method used [61]. It shows that even though the hydrogen production using raw POME is not as high as another study [62], but this carbohydrate-rich material contains a large amount of starch, simple sugars and cellulose, therefore makes it a suitable substrate for biogas production, especially in Malaysia. Considering the above matter, dark fermentation is clearly the key innovation for producing hydrogen from agricultural wastes. Such wastes, which are complex substrates, can be biologically degraded by complex microbial ecosystems. Furthermore, biological pre-treatment is preferable as it is much cheaper compared to chemical pre-treatments.  
Meanwhile, [PERSON] et al. were reported on having a long start-up period using POME [63]. They needed 123 operating days for microbial adaptations and to evaluate the performance of a single-stage up-flow anaerobic sludge batch (UASB) reactor during a temperature shift. This is on account that the UASB reactor can treat high-strength wastewater that contains high levels of suspended solids and deliver a high measure of biomethane. However, a study done by [PERSON] et al. demonstrated that the two-stage anaerobic high-rate bioreactor could abbreviate the start-up period to just about two months for biohydrogen and biomethane generation [18]. Using a two-stage up-flow anaerobic sludge fixed-film (UASFF) bioreactor, they found out that the start-up period could be shortened by initially acclimatizing the digested POME and using fresh raw POME as a substrate. However, the up-flow velocity in the bioreactor, influent and effluent flow rate, as well as the internal packing material play important roles for the reactor stability and efficiency.  
In Malaysia, the current situation does not prepare for the implementation of biohydrogen production technology from POME. The main problems lead to the constraints of up-scale biohydrogen production, which are the HRT, storage and safety problems, and the reactor engineering [64]. However, the conventional POME treatment does require wide land area, longer HRT, mass sludge production and low treatment effectiveness. Therefore, the inexpensive high-rate anaerobic treatment, together with the steady and well-organized bioreactor (in terms of biogas capture), raises an important consideration for oil palm industries.