Organic acid degradation inhibition by high concentrations of hydrogen in an in-situ anaerobic methanation reactor

Abstract

Biological in-situ methanation is a promising method to produce highly pure methane out of organic residuals and hydrogen from surplus energy. Two lab scale digesters were operated continuously to demonstrate biological in-situ methanation (BM) using raw sludge from a municipal wastewater treatment plant as substrate. The degradation of organic acids and organic macro molecules during the conventional anaerobic digestion process, provides the necessary CO2 for the methanation reaction. Additional hydrogen for the methanation reaction is provided by water electrolysis and is introduced into the reactor via a fine porous silicon diffuser. However, it was proven that degradation of propionic acid needs very low concentrations of hydrogen to be performed (<10-4 atm). Otherwise the reaction is inhibited through thermodynamic reasons. (Fukuzaki et al., 1990; Siriwongrungson et al., 2007). Fang and Xiao-Shan (1998) showed that the presence of H2 (between 10-5 and 10-4 atm) does not affect the butyrate degradation, but enhances the formation of propionate (regardless of the initial butyrate’s concentration). But it was also demonstrated that the addition of high quantity of H2 (1 atm) in a batch fed with manure inhibits the propionic acid and butyric acid degradation only under high mixing intensity while more than 90% of H2 is converted to methane. The aim of this study is to examine the kinetic behavior of propionic acid, acetic acid and butyric acid degradation during continues operated in-situ methanation under high concentrations of hydrogen. Degradation rates and gas production rates are presented for each acid. Inhibition factors for the degradation of acetic acid, propionic acid, and butyric acid are presented. Keywords: biological methanation, degradation inhibition, hydrogen, kinetic, organic acids