## 4 Biomethane Production via Anaerobic Digestion (AD)  
### Biochemistry of Anaerobic Digestion  
In the absence of oxygen, a process by which microorganisms will breakdown biodegradable material is called anaerobic digestion. Since it can provide a significant reduction in the mass of the input material (substrate), therefore, anaerobic digestion is mostly used for wastewater treatment or any organic wastes. In the anaerobic digestion process of organic polymeric materials, there are seven sub-processes involved [83]. Complex organic materials will be hydrolyzed at first, followed by fermentation of amino acids and sugars in the second phase. The oxidation process will occur next in long-chain fatty acids and alcohols. In the fourth phase, short-chain fatty acids take place in anaerobic oxidation (except acetate), followed by the production of acetate from carbon dioxide and hydrogen in the fifth phase. Then, acetate will be converted to methane. Finally, methane will be produced by carbon dioxide and the hydrogen reduction process [83]. However, even though there are seven sub-processes involved, the principle of bacteria classes is only divided into three categories [83];  
I--Bacteria that is responsible for hydrolysis. These bacteria hydrolyzed the substrate and breakdown the insoluble organic polymers (e.g., carbohydrates) and make them accessible for other bacteria. II--Acid-producing bacteria. There are two acid-producing bacteria involved in this pathway. The first one is acidogenic bacteria while the other is acetogenic bacteria. The former will convert sugars and amino acids into CO\({}_{2}\), H\({}_{2}\), ammonia and organic acids while the latter will then convert the produced organic acid into acetic acid (along with ammonia, H\({}_{2}\) and CO\({}_{2}\)). III--Methane-producing bacteria. In the end, these bacteria convert the products into CH\({}_{4}\) and CO\({}_{2}\). Methane formation is strictly under anaerobic condition in this phase and the reaction is exergonic. It is also reported that not all methanogens will degrade the substrates [84]. Meanwhile, substrates that are acceptable for the methanogenesis process are divided into three groups as mentioned below:  
(i) Acetoclastic methanogenesis will convert acetate to CH\({}_{4}\) + CO\({}_{2}\) (5) (ii) Hydrogenotrophic methanogenesis will convert H\({}_{2}\) + CO\({}_{2}\) to CH\({}_{4}\) (6) (iii) Methylotrophic methanogenesis will convert methanol to CH\({}_{4}\) + H\({}_{2}\)O (7)  
There are two biochemical components for methanogens that makes them unique; the mechanism of H\({}_{2}\) oxidation and CO\({}_{2}\) reduction. Methanogens will utilize H\({}_{2}\) with acetate, formate, CO\({}_{2}\) and methanol as substrates under the methanogenesis process [79]. Next, they will use CO\({}_{2}\) as the thermal electron acceptor before producing CH\({}_{4}\)[84]. An Integrated System as an Innovative Approach for Biohydrogen, Biomethane Production and Wastewater Treatment  
A large amount of water was consumed during palm oil mill processing. This contributed to the mass production of POME wastewater that leads to water contamination because of its high BOD and COD content. However, through anaerobic digestion, POME has become one of the potential and valuable sources of bioenergy, viz. biohydrogen and biomethane. Every oil palm industry in Malaysia should consider having a renewable and sustainable bioenergy strategy, as well as the in-house wastewater treatment system [85]. The production of methane and CO\({}_{2}\) by the action of active microorganism requires multi-stage processes for organic matter degradation, i.e., hydrolysis, acidogenesis, acetogenesis and methanogenesis [41]. During the early stage of hydrolysis and acidogenesis, acid-forming bacteria will convert fresh raw POME to volatile fatty acids (VFAs), before being converted to CH\({}_{4}\) and CO\({}_{2}\) in methanogenesis under the anaerobic digestion process [86].