# Food Waste as Feedstock for Anaerobic Mono-Digestion Process
## 2 Physicochemical Properties of Food Wastes
Figure 3: Physicochemical properties of FW reported in the literature: (**a**) pH (N = 71); (**b**) TS (N = 69), vs. (N = 82) and VS/TS (N = 41); (**c**) sCOD (N = 28), (**d**) NH4\({}^{+}\)-N (N = 22), VFA (N = 19); (**e**) C/N (N = 56). N—number of data. As can be clearly seen in Table 1, pH is the most frequently analyzed parameter of FW. Roughly speaking, it can be attributed to the fact that pH is linked with concentrations of VFAs; hence, it plays a key role in the pH balance management during the AD process [134] (Section 3.1). According to the results of the present study, it was found that the pH values of the FW reported in the literature were in a wide range. Indeed, it was from 3.65 \(\pm\) 0.06 [110] to 8.51 \(\pm\) 0.2 [11] (Figure 3a). The lowest value indicated above was noted for FW collected from a dining hall, while the highest value was obtained for food waste products from the distribution points of big retail chains. It is worth noting that the average value of reported pH was 4.88, while the median was equal to 4.71. TS is defined as the mass sum of dissolved and suspended solids [28]. In general, the AD process is classified based on the TS content in the substrate. Indeed, wet digestion is characterized by TS \(<\) 15%, while for dry digestion, TS \(\geq\) 15% [120; 135]. Hence, it must be stressed that in the case of a wet process performed with the use of FW characterized by high TS content, the addition of a large amount of water is required [91]. [PERSON] et al. [75] pointed out that conventional anaerobic digesters require feed material with the TS concentration below 10%, while modern systems can be operated with the feed characterized by TS content higher than 20%. A similar indication was presented by [PERSON] et al. [91] who suggested that the AD process is usually conducted with the application of substrates with the TS content below 10%. [PERSON] et al. [45] investigated the impact of TS on the biogas production via the single-stage anaerobic mono-digestion process. For this purpose, the FW collected from the canteen was used (Table 1). The process was performed at a temperature of 30 \({}^{\circ}\)C. The obtained biogas production rate was equal to 0.150, 0.162, 0.143, 0.129 and 0.109 L/day for the solid concentrations of 5%, 10%, 15%, 20% and 25%, respectively. Hence, it can be concluded that the above-mentioned authors have demonstrated that the use of FW with 10% ensures the highest biogas yield. Finally, it should be pointed out that analysis presented in the current study has shown that the values of TS reported in the literature were from 4.24% [112] to 42% [68] (Figure 3b). In addition, it has been determined that the average value and median of the TS were equal to 22.49% and 23.45%, respectively. It is interesting to note that different values were indicated in a study [13] wherein it has been pointed out that the typical FW contains 7-31% of total solids. Hence, it should be pointed out that the TS content should be controlled since it may strongly affect the biogas performance. In addition, technically, a too high TS concentration may result in insufficient mixing during the AD process and finally, an expensive mixing procedure may be a challenge [25]. VSs are known as a part of the TSs present in the substrate [136]. As recognized in the literature, the VS content is another parameter determining the biogas yield during the AD of food waste. Indeed, its high content may lead to the rapid hydrolysis process and, finally, a severe acidification and inhibition of methanogenesis may occur [64]. The dataset created in the present study indicated that the VS values documented in the literature were in the wide range. Indeed, the noted vales of VS were from 4.1% [112; 114] to 99.26% [55] (Figure 3b). The average VS value was equal to 33.95%, while the median was 22.50%. The analysis of the VS/TS clearly showed that it was in the range from 78.89% [81] to 97% [112] (Figure 3b).