## 3 Anaerobic Digestion of Food Waste
### AD Process  
During the anaerobic degradation process, the OM content of FW (or any substrate) is transformed into biogas in a chain process that includes mainly four consecutive steps [85]. These four reactions are (a) hydrolysis, (b) acidogenesis, (c) acetogenesis, and (d) methanogenesis [86]. Figure 2 shows two different and complete schemes of AD reactions and development.  
The process is carried out by numerous microbiological species and different bacteria working through syntropy (the growth of one partner is improved, or depends on the nutrients, growth factors, or substrate provided by the other partner). This means the positive development of the process is related to a correct balance of bacterial populations. Some bacteria that are associated to hydrolysis are capable of degrading the OM either in aerobic or anaerobic conditions. That is why they are named facultative bacteria. Otherwise, bacteria that are responsible for the other reactions are strictly anaerobic [87].  
To completely degrade OM from substrates, present as lipids, carbohydrates, and proteins (organic polymers), it has to firstly be hydrolysed in order to reduce those macromolecules and transform them into other soluble compounds such as aminoacids, sugars and long chain fatty acids. This process is known as hydrolysis, an extracellular reaction carried out by enzymes that should be primarily adsorbed on the surface of solid substrates [88; 89]. This is the reason why hydrolysis is considered the stage-limiting step for AD development.  
Secondly, the small molecular materials created during hydrolysis are converted during the acidogenesis phase into volatile fatty acids (acetic, propionic, and butyric acids), and other by-products, like NH\({}_{3}\), CO\({}_{2}\), and H\({}_{2}\) gases, and small parts of alcohols, aldehydes, and ketones. Acetogenesis is an intracellular reaction (as all the following reactions are) that is carried out by fermenting bacteria.  
Thirdly, products obtained in the previous reactions are transformed into acetate, H\({}_{2}\), and CO\({}_{2}\) during the acetogenesis phase through acetogenic bacteria. These ones are suitable applicants to be transformed into methane and carbon dioxide, among other gases that constitute biogas.  
Finally, in the methanogenesis phase, methanogens transform short-chain fatty acids, alcohols, and gases (CO, CO\({}_{2}\), and H\({}_{2}\)) into methane (CH\({}_{4}\)). This can occur via acetoclastic methanogenesis or hydrogenotrophic methanogenesis. In the first case, acetoclastic methanogens mainly use acetate to form methane, whilst hydrogenotrophic methanogens use H\({}_{2}\) and CO\({}_{2}\). In addition, carbinol and CO can be also transformed into methane [34]. All of these reactions are shown in Figure 2.  
During methanogenesis, a large number of intermediate products are formed by a unique bacteria population. It is an extremely complex process: on one hand, it uses a reduced substrate group, and, on the other hand, methanogens are the most strictly anaerobic microorganisms (methanogens growth only occurs under a red-ox potential below \(-330\) mV).  
In general terms, reference [90] states that carbohydrates are degraded into saccharides and sugars to produce VFA, alcohols, and organic acids. Lipids are degraded into glycerol, glycerine, and long-chain fatty acids, to further be transformed into VFA. Proteins, which are mainly considered as the encapsulation of nitrogen, are transformed from peptides to amino acids, and finally into VFA.  
AD is an extremely complex process that should simultaneously digest all substrates in order to produce the substrate for the next phase reaction. This requires a stable environment for the different  
Figure 2: Anaerobic digestion process and reactions. (**a**) Simplified way and (**b**) adapted from [75] for ADM1 Simulation.  
microbial growth. It is therefore important to maintain the key parameters of AD process within the correct range, to ensure the appropriate operation of AD.