Substrate competition between methanogens and acetogens during the degradation of methanol in UASB reactors

To evaluate the effect of methanol concentration and inorganic carbon on the competition between acetogens and methanogens for methanol, eight upflow anaerobic sludge blanket reactors were operated continuously with different levels of sodium bicarbonate at variable methanol loadings. The results indicated that methylotrophic methanogens will predominate in a broad pH range (4.2-7.2) if either the methanol concentration in the effluent or inorganic carbon is low. Continuous steady state methane production resulted in chemical oxygen demand (COD) removals of 99.8-54% at organic loadings ranging from 7.6 to 22 g COD·l^-1·d^-1. On the other hand, significant acetogenesis occurred if exogenous inorganic carbon was supplied and if the methanol concentration in the effluent was greater than 1000 mg COD·l^-1. The observation that acetogenesis only occurred in the presence of high methanol concentrations in the effluent confirms the previously determined 60-times-higher substrate affinity of methanogens. Steady state acetogenesis was only observed when unionized VFA, which inhibited the methanogens, were present at high levels for prolonged periods of time. Addition of moderate levels of NaHCO₃ (15 meq·l^-1) were found to create such conditions since VFA accumulation exceeded the buffering capacity. Continuous acetogenesis from methanol at organic loading rates of 21 g COD·l^-1·d^-1 resulted in poor COD removal (16.3%). In contrast, high levels of NaHCO₃ supplementation (≥25 meq·l^-1) favured methane production since the alkalinity was strong enough to prevent toxic levels of undissociated acids even in periods when VFA accumulated as a result of organic overloading.