# 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
### pH  
The pH is measured to indicate the health of anaerobic microorganisms and the performance of AD system [17]. A variation of pH in the magnitude of 0.5 can result in a substantial change in microbial metabolism influencing reaction kinetics and produced gas yield. Although feasible anaerobic digestion can be achieved within the pH of 5.5 and 8.5 [194], methanogens are highly sensitive to the pH change and generally optimize at a pH of close to 7 [177]. A pH under 6.3 or over 7.8 can adversely affect methanogenesis with a tendency of process failure to occur [195]. However, unlike methanogenesis, the other processing steps (e.g., hydrolysis and acidogenesis), can potentially optimize between pH of 5.5 and 6.5 [196]. With regard to the intermediates, specifically the VFAs produced during the acidogenesis if not metabolized into products, the pH can be significantly lowered (\(<\)3), eventually leading to process collapse. Process disturbances can also result due to high alkalinity caused by the formation of total ammonium nitrogen (TAN) during hydrolysis. Generally, the pH range of extreme acidic (\(\sim\)3 or less) or extreme alkaline (\(\sim\)12 or higher) can be inhibitory to the acidogenesis [197] and limit the rate of hydrolysis.  
The development of various chemical species during biochemical interaction (e.g., NH\({}_{3}\), CO\({}_{3}\)[2]-, and CH\({}_{3}\)COO\(\bar{\phantom{\rule{0.0 pt}{0.0 pt}}}\)) can cause a considerable impact in digestate pH variation. For example, the formation of ammonium carbonate ((NH\({}_{4}\))\({}_{2}\)CO\({}_{3}\)) or reduction of CO\({}_{2}\) from the digestate liquid can directly lead to a pH increase [198; 199]. Also, the production of basic cations (e.g., Mg\({}^{2+}\), Ca\({}^{2+}\), K\({}^{+}\)) or the reduction of multivalent anions (e.g., SO\({}_{4}\)[2]-, Fe(OH)\({}_{3}\)) can cause electric charge imbalances in the digestate liquid resulting in a pH rise. Similar to observations in pH increases, the precipitation of carbonates (e.g., calcite CaCO\({}_{3}\)) or the fatty acids as a consequence of high organic loading could be the critical factors influencing pH reduction. [PERSON] et al. [200] showed that the protein-rich substrates (i.e., meat extracts), tend to wane buffering capacity at an elevated organic loading input resulting in pH drop. The tendency of the various ions originating from various degradation pathways to influence pH is shown by Figure 7.  
To control pH fluctuation and to operate AD process at the desired pH range, modern biogas plants are often installed with an automatic pH controller. The primary objective of the controller is to help balancing the system buffer (alkaninity) by adding an appropriate neutralizing agent at an appropriate concentration when necessary. Strong bases (e.g., NaOH) or carbonate salts (e.g., Na\({}_{2}\)CO\({}_{3}\) and NaHCO\({}_{3}\)) and acids (e.g., HCl) are primarily the components used as chemicals to achieve a pH increase or decrease. Moreover, indirect pH control can be applied by manipulating operating parameters including a reduction in organic loading rate [201], digestate recirculation [202], HRT variation [203] and codigestion [204].  
Figure 7: Interaction of degradation pathways and the formation of products influencing pH.  
The controlling of pH, whether achieved by the direct addition of a buffer or by the manipulation of the operating conditions, requires a careful and sophisticated application to avoid disturbances to methanogenesis and further to biogas yield. Robust controlling of pH by incorporating multiple parameters towards an optimum methane yield using sensor technologies is a growing field of research in AD.