Effects of Acetic, proponic and Butyric Acid concentrations on Methanogenesis in Mesophillic Anaerobic Reactors

Melchora Andrade-Bautista

Thesis (MS Environmental Engineering) - University of the Philippines Diliman-2007

Abstract

In view of developing waste treatment systems involving energy recovery from waste. this study investigated the effects of acetic, propionic and butyric acid concentrations on methanogenesis (biogas production). These volatile fatty acids (VFAs) are intermediate products of anaerobic digestion of high-COD wastewater (such as bioethanol production and food processing), landfill leachate and gigesters treating organic fraction of municipal solid waste. Understanding their effects on methane formation is critical in the design and operation of waste treatment systems especially when energy recovery is to be optimized. Three sets of duplicated batch reactors were set up at varying acetic, propionic and butyric acid levels under mesophilic conditions. The specific methanogenic activity (SMA) was determined for each volatile fatty acid (VFA) level and the Monod equation kinetic parameters such as the half-saturation constant (Ks) and maximum specific growth (Umax) were determined from acetic acid. Results showed that methanogenesis was not inhibited within the range of concentrations (50 mg/L-2500 mg/L) used in the experiment and the rate of methagenesis increases with increasing aceti and butyric acid levels. However, prioponic acid was found to inhibit methagenesis at prioponic acid levels greater than 1000 mg/L. The SMA of sludge was found lowest with prioponic acid as the substrate for methane production and highest with butyric acid. This study showed that the conversion of prioponic acid to acetic acid is rate-limiting in the overall process of methane production starting from priopolic acid as shown from the results of gas chromatography where prioponic acid was found to be present in the solution until after 12 days of digestion.