# 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
### Pressure  
The effect of pressure on anaerobic digestion has not been studied extensively. Although anaerobic digestion typically occurs at atmospheric pressure, due to the accumulation and exchange of different  
\begin{table}
\begin{tabular}{l l l l} \hline \hline
**OLR** & **CH\({}_{4}\) Yield** & **Aim** & **Feedstock** & **Reference** \\ \hline
0.4 to 3.1 kg & Maximum (0.46 LCH\({}_{4}\)/gCOD\({}_{\text{removed}}\)) at & & Rice straw and pig & [257] \\ COD/m\({}^{3}\) & OLR of 2.5 kg COD/m\({}^{3}\) & & manure & [258] \\
1 to 4 gVSI/d for & Methane production maximized at OLR & & Macro-algae & \\ methane reactor; 3 & 2 gVSI/d and thereafter decreased. H\({}_{2}\) & & _Laminitaria digitata_ & \\ to 12 gVS/L/d for & production maximized at OLR of & and CH\({}_{4}\) & _Arthurgena_ & [258] \\ H\({}_{2}\) reactor & 6 gVS/L & & _plantensis_ & \\ \hline
30, 60 and 90 gVS/L & & & \\  & & & \\  & & & \\  & & & \\  & & & \\
2.5 to 27.7 gVS/L & Ammonia inhibition at OLR \textgreater{} 20 gVS/L & Ammonia inhibition & municipal solid & \\  & & & waste, chrome & \\  & & & \\
0.4 to 0.7 gCOD/L/d & Methane yield decreased with & Pilot scale two stage & Slaughter house & \\  & increased OLR & AD & waste & \\  & & & \\
1.5 to 4.3 g/L/d & Maximum methane yield at OLR of & Methane production & Protein rich fish & \\  & & & \\  & & & \\  & & & \\  & & & \\  & & \\  & & & \\  & & & \\  & & & \\  & & & \\  & & & \\  & & \\  &  
gases into the reactor headspace, over or under pressure on a liquid surface can be developed. Based on the previous findings, it was reported that lower pressures on liquid surfaces resulted in higher biogas yields, as CH4 solubility increases with pressure [272]. In an earlier study [273], the pressure change effect of various gas species was investigated using reaction models. The results showed that the partial pressure increase of CO2 led to increasing the digester pH level, which consequently lowered the nonionized hydrogen sulfide concentration, reducing gas toxicity. Additionally, the hydrostatic pressure level within the digester can also affect the production of methane. Past experiments [274] observed that the methanogenesis activities maximized at digester depth of 4-5 m (400-500 mm H2O) for a digester of over 10 m height. By reducing the height of this digester (i.e., making it horizontally oriented), a reduced hydrostatic pressure can be established, resulting in improved methanogenesis.  
Anaerobic reactors working with high pressures (i.e., pressurized reactors), have been developed and investigated in lab-scale studies. As high pressure increases CO2 solubility, the biogas released from the anaerobic digester partially strips of CO2, giving net rise in CH4 concentration. By using a high pressure within an anaerobic digester, a methane level of above 95% was achieved [275]. However, in some cases, elevated pressures in an anaerobic reactor did not produce a satisfactory improvement in biogas production. Technical challenges associated with leakages in the reactor system, pH decrement, and investment costs are identified as a few of the drawbacks of the pressurized reactor concept.