Anaerobic treatment of methanolic wastes

Abstract

Although it is well known that methanol can be fermented directly by a specific species of methane bacteria, viz. Methanosarcina barkeri, until now little information was available about the effect of important environmental factors on the anaerobic fermentation of methanol. As methanol can be the main pollutant in some specific chemical wastes and may be formed under natural conditions (e.g. in marshes) as intermediate in the decomposition of organic matter, more information about the effect of environmental factors is required. In this study the effect of pH, bicarbonate alkalinity and trace elements on the methane fermentation of pure aqueous solutions of methanol has been investigated. Methanol was found to be still fermented at pH values as low as 3.5. The optimal pH range is presumed to lie between pH 5.5 and 6.0. No exact figures can be given yet about the optimal pH range, because pH values higher than 6.0 could not be maintained in a well digesting system (due to the formation of volatile fatty acids (VFA) from methanol), except if no trace element solution was supplied to the influent. In the absence of trace elements the formation of VFA from methanol proceeds fairly slowly, but the conversion of methanol to methane is also slower in that case. In the presence of trace elements a significant fraction, directly depending on the HCO 3 − concentration, is converted to VFA, mainly consisting of acetic acid and butyric acid. The VFA formation from methanol was found to be more sensitive to a low pH shock (e.g. from pH 3.5 to 4.0) and slows down more rapidly with decreasing pH than the formation of methane. Concerning the anaerobic treatment of wastes containing methanol as the main pollutant, it is not yet possible to decide upon a one-step or a two-step process, e.g. with a separate acid formation and methane fermentation step.