According to San Thy et al., (2003) biogas fermentation requires an environment with neutral pH and when the value is below 6 or above 8 the process will be inhibited or even cease to produce gas because of toxic effect on the methanogen population. The optimum for biogas production is when the pH value of the input in the digester is between 6 and 7. Increasing the amount of feedstock or a change in the fermentation material is likely to acidify the fermentation system because of the accumulation of volatile fatty acids (VFA). In this way pH can be used to indicate if the system is being overloaded. In the initial period of fermentation, as large amounts of organic acids are produced by the acid-forming bacteria, the pH in the digester may fall below 5 causing inhibition of the growth of the methanogenic bacteria and hence reduced gas generation (Da Silva, 1979). Acetate and fatty acids produced during digestion tend to lower the pH of the digester liquid (Marchaim, 1991). Hansen et al., (1998) stated that acetate-utilizing methanogens are responsible for 70% of the methane produced in biogas reactors.

Buren (1983) pointed out that the micro-organisms involved in anaerobic biodigestion require a neutral or mildly alkaline environment, as a too acidic or too alkaline environment will be detrimental. The work stated that a pH between 7 and 8.5 is best for biodigestion and normal gas production. The pH value for a digester depends on the ratio of acidity and alkalinity and the carbon dioxide content in the digester, the determining factor being the density of the acids. Buren (1983) noted further that for the normal process of digestion, the concentration of volatile acid measured by acetic acid should be below 2000 ppm, as too high a concentration will greatly inhibit the action of the methanogenic micro-organisms. Results of a study by Jantrania and White (1985) further confirm the foregoing. The study compared the performance of a number of digesters processing poultry wastes and found that the pH of the residue from digesters that failed were between 6.1 and 6.7, while the pH from the successful digester was 7.5. The digesters which stopped producing any appreciable amount of gas after 54 days had higher hydrogen sulfide content (over 200 ppm) than the successful digester.