# Food Waste as Feedstock for Anaerobic Mono-Digestion Process
## 3 Performance Profile of Single-Stage Anaerobic Mono-Digestion Process
### Temperature and pH  
Invariably, the AD process can be conducted under different temperature conditions, such as (i) psychrophilic (12-16 \({}^{\circ}\)C), (ii) mesophilic (35-37 \({}^{\circ}\)C) and (iii) thermophilic (50-60 \({}^{\circ}\)C). Among them, mesophilic and thermophilic conditions are more favorable since a higher temperature may result in a higher hydrolysis rate [28]. Moreover, it has been widely acknowledged that the operating temperature has a significant impact on the biogas stability and the bacterial and methanogenic communities [151; 152]. More precisely, it affects the activity of both enzymes and coenzymes as well as hydrolysis development [22]. It is important to note that although the thermophilic AD process is known to be more efficient than mesophilic one [25; 66; 75], the anaerobic mono-digestion of FW was operated mainly under mesophilic conditions (about 35 \({}^{\circ}\)C) (Table 2). On the other hand, thermophilic conditions were applied in the studies [39; 54; 81; 89; 101; 102; 119; 147; 148; 149; 150]. The performance of the process under mesophilic conditions is generally more stable than that obtained for a thermophilic AD process [86]. The advantages and disadvantages of the above-mentioned conditions have been thoroughly discussed in the study [27]. It is important to note that performing the literature review allowed to find that investigations on the biogas production via an anaerobic mono-digestion process under psychrophilic conditions (below 20 \({}^{\circ}\)C) are unavailable. Hence, a review of the relevant literature found that the existing knowledge in this area is incomplete.  
As it has been indicated earlier, pH is the important parameter governing the AD process performance. Indeed, this finding has been clearly indicated in a number of publications, e.g., [69; 90; 126; 134]. Roughly speaking, pH has a significant impact on the microorganisms' growth and the hydrolysis of particulate organic matter [126]. For the degradation of carbohydrate and proteins, the most suitable pH is in the range between 6-9 and 7-8, respectively [134]. As already stated in the literature, in order to ensure the stability of the AD process, the adjustment of initial pH and its continuous control are required [105]. Based on the data presented in Table 2, it can be clearly seen that the AD experiments presented in the literature were performed under a range of pH values between 4 [127] and 8.3 \(\pm\) 0.1 [150]. According to [28; 142], the optimum values of biogas production via the AD process are around 7. As demonstrated in Section 2, most of the FW generated throughout the world and analyzed in the present study was characterized by pH values lower than 7. Therefore, it is important to point out that according to previous research, if in the continuous reactor, the controlling of pH is required, sodium bicarbonate (NaHCO\({}_{3}\)) and sodium hydroxide (NaOH) can be added to the system [142]. For instance, investigations performed by [PERSON] et al. [78] clearly highlighted that NaHCO\({}_{3}\) is an effective alkalizing and buffering agent for the AD of food waste. Indeed, it interacts with large amounts of acids produced during the process, relieving the inhibition effects.