# A Review of the Role of Critical Parameters in the Design and Operation of Biogas Production Plants
## 3 Process Parameters Involved in a Biogas Production Plant
### Temperature  
The AD temperature depends on the tolerance of the microorganisms and is categorized as psychrophilic, mesophilic, thermophilic, or extremophilic [177] established at temperature ranges of 4-25 \({}^{\circ}\)C, 30-40 \({}^{\circ}\)C, 50-60 \({}^{\circ}\)C, and \(>\)65 \({}^{\circ}\)C, respectively [177; 178]. Although anaerobic digestion can be achieved with temperatures lower than 20 \({}^{\circ}\)C [179], below 10 \({}^{\circ}\)C, the degradation is three times slower than the normal mesophilic process with methanogenesis becoming the rate-limiting step [180]. With higher temperatures, such as those between mesophilic and thermophilic regimes, anaerobic digestion tends to have a perfect environment for biological degradation resulting in high rates of hydrolysis and consequently high biogas yields. The correlation between the reaction rate and the biological process with temperature is typically represented by Equation (8) below [181]:  
\[\text{k}_{\text{T}}\text{=k}_{\text{20}}\theta^{\text{(T-20)}} \tag{8}\]  
where \(\text{k}_{\text{T}}\) denotes the reaction rate constant at temperature T, \(\text{k}_{\text{20}}\) represents the reaction rate constant at 20 \({}^{\circ}\)C, \(\theta\) is the temperature activity constant, and T denotes the temperature. The methane production amount in thermophilic AD is almost identical to mesophilic AD, but higher temperatures improve the production rate [182], and reduces the requirement of high operational  
Figure 5: Two novel configurations (**a**,**b**) of airlifting bioreactor agitating sludge without having an installed mixer—reproduced with permission from the original authors [175; 176]. (**a**) biogas-lift reactor (BLR): right upward arrow- first biogas lift included with sludge and water; middle downward arrow—flow of sludge and water; left upward arrow—second biogas lift with separation of sludge and water. (**b**) gas-lift membrane bioreactor: the flow directions of materials are indicated with the arrows and texts in the body of the figure. HRT and consequently reducing the reactor size [183]. However, thermophilic digestion system can be energy intensive, unstable, and sensitive to inhibition [184] (see Section 3.5), which, in addition to the robustness of the operation, is why mesophilic processes are presently the most preferred techniques implemented at industrial scale AD [185]. For a given operational temperature type, the fluctuation of a few degrees of temperature can have a severe impact on methane yield, as microorganisms adjust to one certain temperature and re-adaptation corresponding to a different temperature requires an alternated microbial structure. Noticeably, a variation in mesophilic temperature of \(\pm\)4 \({}^{\circ}\)C and thermophilic temperature of \(\pm\)1 \({}^{\circ}\)C was found to result in a sharp decrease in biogas yield [19; 186]. Within physio-microbial activities, some AD processing phases are more influenced by the temperature than others. For example, the temperature requirements for optimizing the growth of methanogens bacteria, specifically the mesophilic methanogens species, may differ from the temperature requirements for optimizing hydrolysis or acidification [19]. Investigation of phase-specific temperature effects has been conducted and it was found that the temperature-phased (70 and 55 \({}^{\circ}\)C in the two successive stages) digesters, separating hydrolysis and methanogenesis, produced better results in terms of biogas yield and organic matter removal than the thermophilic or mesophilic digestion alone [187]. Similar results were demonstrated in previous research works by the authors [PERSON] et al. [188; 189]. Moreover, a staged digester combining thermophilic (55 \({}^{\circ}\)C) and extremophilic (68 \({}^{\circ}\)C) temperatures was reported to accelerate hydrolysis of recalcitrant organics, resulting in higher biogas yields compared to a thermophilic or extremophilic reactor alone [190].