# Food Waste as Feedstock for Anaerobic Mono-Digestion Process
## 2 Physicochemical Properties of Food Wastes
Moreover, it has been found that the average value and median of the VS/TS were equal to 91.19% and 92.90, respectively. It is worthy of note that this finding is in agreement with the indication presented by [PERSON] et al. [56] who demonstrated that typically, the VS/TS ratio ranges from 80 to 97%. On the other hand, the range reported in the present study was slightly wider than that shown by [PERSON] et al. [45]. Indeed, the above-mentioned authors pointed out that usually, the VS/TS is from 85% to 96%. COD is a very useful and important parameter. Indeed, it allows to determine the amount of available organic matter as well as calculate the digestion efficiency [137]. In the present study, it was recognized that the values of sCOD in the FW collected from various sources was in the range from 5.84 \(\pm\) 0.05 [105] to 152 g/L [45], while the average and median were equal to 65.92 and 77.85 g/L, respectively (Figure 3c). Volatile fatty acids are intermediate products of the AD process containing mainly short-chain fatty acids, such as the fatty acids acetate, propionate and iso-butyrate. As it has been indicated in [134], VFAs are produced from monomers in the acidogenesis step. VFAs can be used as a source in the biological nutrient removal (BNR) process [126]. It is well known that FW is a highly biodegradable feedstock that tends to accumulate large amounts of VFAs. Consequently, it leads to a decrease in pH and inhibition of the AD process [138; 37; 120]. As a result, it may buffer the AD system capacity and lead to its failure [72]. [PERSON] et al. [139] studied the impact of VFAs on the biogas production via the AD process of kitchen wastes. The above-mentioned authors have found that CH\({}_{4}\) generation has been completely limited when the VFAs concentration was from 5.8 to 6.9 mg/L. In turn, the results obtained in the present study have demonstrated that the VFAs concentration in the FW collected from various sources around the world was up to 8.79 g/L [91] (Figure 3d). The average value and median of this parameter were equal to 2.76 and 2.5 g/L, respectively. Roughly speaking, during the AD process, ammonia is generated via the degradation of the nitrogenous matter present in the feedstock [140]. It is generally acknowledged that ammonia plays a key role during the AD process. Indeed, it is a nutrient required for the growth of bacteria involved in the AD process. In addition, NH\({}_{4}\)+N allows to maintain the required alkalinity and consequently ensures sufficient buffering capacity for the system [59; 132; 140]. Nevertheless, according to [94; 141; 140; 132], a higher concentration of NH\({}_{4}\)+N inhibits the enzymatic activity of the methanogens, leading to a decrease in the CH\({}_{4}\) production. In [59; 94], it has been highlighted that the inhibition effect of ammonium would occur under its concentration in a reactor higher than 2 g/L. Therefore, in the present study, the analysis of NH\({}_{4}\)+N concentration in the FW collected from various sources has been performed. It has been found that it was in the range from 14 [105] to 1143-1343 mg/L [70] with the average value and median equal to 411 and 166 mg/L, respectively (Figure 3d). The C/N ratio is an indicator of the availability of nutrients present in a substrate [96]. According to the literature, it is a key parameter which affects the microorganisms' activity. Generally, low values of this parameter are not suitable for the AD process mainly due to the inhibition from total ammonia nitrogen present in FW [48]. In a paper by [PERSON] et al. [14], it has been pointed out that in general, FW is characterized by a C/N equal to 9-21. On the other hand, [PERSON] et al. [137] showed that for FW, this parameter is in the range of 9.3-24.5. In a study by [PERSON] [133], it has been indicated that the C/N of food waste is in the range between 2 and 30. However, the above-presented indications are not in agreement with the finding presented in the present study.