The fate of methanol in anaerobic bioreactors

Methanol is an important component of certain industrial wastewaters. In anaerobic environments, methanol can be utilized by methanogens and acetogens. In wastewater treatment plants, the conversion of methanol into methane is preferred because this conversion is responsible for chemical oxygen demand (COD) removal, whereas with the formation of volatile fatty acids (VFA) little COD removal is achieved. Moreover, the accumulation of VFA can lead to reactor instability due to pH drops, in weakly buffered systems. The undesirable formation of VFA has previously been associated with the presence of trace elements and bicarbonate in the medium.This thesis investigates the environmental factors that lead to the predominance of acetogens over methanogens during anaerobic wastetewater treatment of methanol. For this purpose, batch and continuous experiment were carried out using a model medium composed of methanol and defined mineral nutrients. The main factors studied were: the effect of trace elements, the reactor pH, the bicarbonate level, and the methanol concentration in the reactor.In Chapter 1 an introductory review is given about anaerobic methylotrophic metabolism in environmental biotechnology. The natural and anthropogenic sources of methanol and other methylated compounds are outlined. In addition, the anaerobic methylotophic microorganisms and the biochemistry of methanol metabolism in methanogens and acetogens are briefly described. Finally, the previous experience in environmental biotechnology with the anaerobic biodegradation of methylotrophic substrates is reviewed. Included is a summary of the reactor types, efficiencies achieved and organic loadings applied for the treatment of wastewaters containing methanol.In Chapter 2, the effect of trace elements on the anaerobic conversion of methanol was studied. Cobalt was the only trace element tested which greatly enhanced methanogenesis from methanol. In continuous experiments, less acetate was formed in a cobalt-deprived reactor than in a cobalt supplemented reactor. These results suggested that cobalt levels could be used to prevent acetate formation from methanol. Therefore, in Chapter 3 the effect of cobalt addition for each individual trophic group was evaluated. Using specific inhibitors, specific activity and the kinetic parameters, μmax and Ks, were determined. Methylotrophic methanogens and acetogens were the only trophic group stimulated by cobalt addition, while the other trophic groups utilizing downstream intermediates, H 2 /CO 2 or acetate, were largely unaffected. At a low cobalt concentration, both methylotrophs had similar growth rates, whereas at high cobalt levels, acetogens grew slightly faster. The optimal cobalt concentration for both methylotrophic populations was around 0.05 mg · l -1.In Chapter 4, the possibility of the anaerobic treatment of methanol without addition of any alkalinity was investigated. Methanol can successfully be converted into methane, regardless of the low pH value of alkalinity-free medium. An astonishing high specific methanogenic activity of 3.57 g COD · g -1VSS · d -1was evident, which enable the conversion of 13 g COD · l -1· d -1to methane during continuous operation at a reactor pH 4.2. A Methanosarcina- like organism was the main methanogen responsible for the conversion of methanol to methane.Chapter 5 presents the effect of inorganic carbon, alkalinity addition and reactor methanol concentration in anaerobic reactors. The results indicated that acetogenesis occurred only when exogenous bicarbonate was added, when unionized VFA accumulated and in the presence of high methanol concentrations, confirming the previously determined 60 times higher substrate affinity of methanogens.The results of this dissertation are summarized in Chapter 6. Methylotrophic methanogens will predominate over a broad pH range (4.2 to 7.2) if either the reactor methanol concentration, inorganic carbon content, or the cobalt concentration is low. Significant acetogenesis can only be expected to predominate if the reactor methanol concentration is high (>1000 mg COD · l -1), exogenous inorganic carbon is supplied, cobalt is available and methanogens are inhibited, e.g. by undissociated VFA. All these four conditions have to be met. Addition of moderate levels of NaHCO 3 (approximately 10-20 meq · l -1) were found to create such conditions if the reactor was overloaded.