## 3 Anaerobic Digestion of Food Waste
### Anaerobic Mono-Digestion of FW  
AD is a synergistic process between microbes' population. AD mono-digestion means that the substrate, FW in this case, is treated solely with the consortium of anaerobic microbes, under anaerobic (non-air and non-oxygen) conditions. AD is more suitable for food waste treatment than other conventional treatments, because of its composition, as it recovers energy, and, in addition, digestate can be applied for agricultural uses. Biogas production and methane production from FW AD can be estimated through its composition, previously reviewed. Biogas yield is affected mainly by LPCH content and its balance. Some theoretical results in terms of biogas yield and biogas composition from degradation pf proteins, lipids, and carbohydrates, can be determined through methane production equations, especially the Buswell-Muller formula [165] and the Boyle formula [166] when elemental and chemical composition of compounds are known:  
\[\mathrm{C_{a}H_{b}O_{c}+\left(a-\frac{b}{4}-\frac{c}{2}\right)H_{2}O\ \rightarrow\ \left(\frac{a}{2}+\frac{b}{8}-\frac{c}{4}\right)CH_{4}+\left(\frac{a}{2}- \frac{b}{8}+\frac{c}{4}\right)CO_{2} \tag{2}\]  
\[\mathrm{C_{a}H_{b}O_{c}N_{d}S_{e}+\left(a-\frac{b}{4}-\frac{c}{2}+\frac{3d}{4} +\frac{c}{2}\right)H_{2}O}\]  
\[\rightarrow\ \left(\frac{a}{2}+\frac{b}{8}-\frac{c}{4}-\frac{3d}{8}-\frac{c}{4} \right)CH_{4}+\left(\frac{a}{2}-\frac{b}{8}+\frac{c}{4}+\frac{3d}{8}+\frac{c} {4}\right)CO_{2}+dNH_{3}+eH_{2}S \tag{3}\]  
According to this procedure, to obtain the theoretical biogas yield and composition, it is interesting to know the chemical composition of LPCH, as shown in Table 2. This provides the maximum biogas yield that can be obtained, assuming that the entire substrate is transformed into biogas, although part of the substrate is converted into biomass (3-10%) [167] and it is thus not available for biogas formation. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline
**C** & **H** & **N** & **O** & **S** & **Ref. ** &Carbohydrates ((CH\({}_{2}\)O)\({}_{n}\)) exhibit the fastest conversion rate but the lowest biogas yield [168; 169; 13], around 750 NI per kilogram of total solids, with a biogas composition of 50% CH\({}_{4}\) and 50% CO\({}_{2}\). Carbohydrate-rich substrates, in FW terms, such as fruit and vegetable waste or sugar industry wastes, are enriched with simple sugars and disaccharides [75]. Anaerobic degradation of simple sugar may result in VFA formation and accumulation, leading to a pH decrease and methanogenesis inhibition [75]. Mixing them with low organic content substrates [124; 170] might balance operational conditions and biogas production [171]. On their behalf, proteins (C\({}_{a}\)H\({}_{b}\)O\({}_{c}\)N\({}_{d}\)S\({}_{e}\)) represent a fast conversion to biogas and a biogas yield [169; 172] slightly above carbohydrates (800 NI per kilogram of total solids) with a higher methane composition, 60% CH\({}_{4}\) against 40% CO\({}_{2}\). As protein structures are different from each other, biogas content may vary significantly. CH\({}_{4}\) content that is given in Table 2 is typical for amino acids. Since many proteins can be present in a less oxidised form, methane content can increase up to 70%. Not only is methane released, but also ammonia and hydrogen sulphide can be obtained, as they are the only compounds that present N and S in their composition (a common point between amino acids is that those have an amine group (-NH\({}_{2}\)) [75]). If an excess of ammonia is released it can deliver in a process inhibition [173]. FW protein rich materials, such as meat residue, slaughterhouse waste, or manure require a suitable adjustment of C/N ratio and an optimum key parameter control to avoid process inhibition and system failure due to problems described. Finally, lipids (CH\({}_{3}\) (CH\({}_{2}\))\({}_{n}\)COOH) show the highest biogas yield, as they have a higher carbon composition (C:16-C18 typically), providing 1390 NI of biogas per kilogram of total solids of which around 72% is CH\({}_{4}\).