# 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
### Feedstock
#### 3.1.3 Pretreatment  
Pretreatment increases feedstock utilization towards AD. Substrates composed of high recalcitrant matter, or not readily biodegradable matter, are considered for pretreatment. The technique used for pretreatment depends on the type of substrate and varies to a wide degree of methods, but, generally, thermal [59], chemical [60], physical/mechanical [61], ultrasound [62], microwave [63], biological [64], and metal addition [11] methods are used to perform pretreatment. The primary feedstock pretreatment categories are shown in Figure 3.  
Thermal pretreatment occurs at a vast range of temperature, which across various studies spans between 70 \({}^{\circ}\)C and 275 \({}^{\circ}\)C [59; 66; 67; 68]. Before anaerobic digestion, the thermal treatment can enhance feedstock hydrolysis resulting in increased anaerobic digestion rate and extent. Substrates like waste activated sludge [66], food waste [69], municipal solid waste [70], agricultural byproducts [71], grass [72], and algae [73] showed a positive effect on methane yield as a result of thermal pretreatment. However, the methane yield from thermally pretreated substrates is not always higher than those that are untreated. [PERSON] et al. [74] showed no improvement in methane yield from the solid fraction of pig manure at a pretreatment temperature of over 100 \({}^{\circ}\)C, while [PERSON] et al. [75] observed that a pretreatment temperature of 190 \({}^{\circ}\)C was found favorable for improved biogas yield from the total liquid and solid fractions of a similar substrate. Steam explosion is another thermal pretreatment technique mostly applied for treating lignocellulosic biomass. Through steam explosion, the fibers in lignocellulosic biomass open-up and become more accessible for biological degradation. According to the literature [76], biogas yield from lignocellulosic biomass such as Salix woodchips can be maximized with the steam explosion method at 210 \({}^{\circ}\)C for about 10 min. Steam explosion pretreatment is usually defined by a severity factor that is calculated from the temperature and duration of the process. The relation between the steam explosion severity, duration, and temperature are expressed by Equation (7) below [77]. A similar trend to the Salix woodchips was observed for birch wood chips [78], where an approximately two-fold increase in methane yield was achieved compared to untreated woodchips due to the steam explosion with the severity of 4.5 at temperature 220 \({}^{\circ}\)C. For agricultural biomass such as wheat straw, the different severity of steam explosion had shown no positive impact on methane yield, but the degradation rate was found to be increased [79]. The severity factor of steam explosion for thet majority of feedstocks usually lies within the range of 3.14-3.56 [77]. \[\text{logR}_{\text{o}}=\text{log}\Big{(}\text{t-e}^{(\frac{\text{(T-100)}}{ \text{LL}\text{V}^{2}})}\Big{)} \tag{7}\]  
where,  
logR\({}_{\text{o}}\): the severity factor as a function of treatment time;  
T: the temperature in \({}^{\circ}\)C;  
t: is the residence time in (min); and  
14.75: the activation energy where the process obeys first-order kinetics and the Arrhenius temperature dependence  
Apart from thermal pretreatment, thermochemical pretreatment combining heat and chemicals/additives can contribute to enhancing AD productivity by increasing the COD (chemical oxygen demand) solubility [80], reducing feedstock particle size [81], and increasing volatile solid reduction [82]. The chemicals predominantly used for thermochemical pretreatment include alkali [83], acids [84], biological additives [85], and ozone [86]. Acid pretreatment, performed with acids such as HCl, HNO\({}_{3}\), H\({}_{2}\)SO\({}_{4}\), and H\({}_{3}\)PO\({}_{4}\), accelerates hemicellulose solubilization into oligomers (i.e., molecules  
Figure 3: Primary feedstock pretreatment categories—modified from Ref. [65].