pH-stability in anaerobic bioreactors treating methanolic wastewaters

Methanol is the main pollutant in evaporator condensate wastewaters from the kraft pulping industry. These wastewaters have no alkalinity. As methanogenesis is known to work best at neutral pH, additional alkalinity is needed to prevent the pH from dropping, and consequently, cause reactor instability. In anaerobic environments, methanol can either be directly converted to methane by methylotrophic methanogens or be converted to acetate by acetogens. The COD removal efficiency and stability of anaerobic reactor treating methanolic wastewaters are dependent on which route methanol is degraded. In this study, a mathematical model was made and tested. The model estimates pCO₂ and pH expected from a certain stoichiometry yield of acetic acid and methane from methanol and the amount of alkalinity added. Continuous experiments were performed in five UASB (Upflow Anaerobic Sludge Blanket) reactors to check if the theoretical and experimental data matched. Methanol (5 g COD/l) was the only substrate used and NaHCO₃ and K₂HPO₄ were the sources of added alkalinity. The amount of added alkalinity varied from 0 to 50 meq/l. The experimental data fitted very well with the model. With this model it is possible to predict the pH in the reactor and the CO₂ composition of the biogas.