Biogas Production from Waste Glycerol with Inhibition from Carboxylic Acids

Abstract

One of the ways for waste glycerol utilization is biogas production. This biogas could be used for partial energy supply of the main biodiesel plant. An important hinder for this application is the accumulation of carboxylic acids leading to strong inhibition of the methanogenesis and shift to production of gas with very low methane content. The present paper studies the possibility of biogas production from waste glycerol in a multistage anaerobic digester composed by consecutively situated cells separated by baffles. This cascade-type bioreactor enables the separation of zones with different pH and organic acid content and therefore to achieve flexible operation. The experiments show stable pH-profile along the bioreactor stages with minimum pH-values in the middle cells. Identification of the intermediate products resulting of the digestion was carried out. A strong correlation between the acid content and the pH-value in each reactor stage was observed.

Microbiological study shows that the microbial profile along the reactor varies with the progress of fermentation. The initial glycerol biodegradation is accomplished by aerobes and facultative anaerobes, whereas the methanogenesis starts after the first stage and it is represented consecutively along the stages by the genera Methanosarcina, Methanobacterium and Methanobrevibacter. Their concentrations are strongly sependent on the pH values. It was shown that overloading with glycerol leads to gradual decrease of methane in the produced biogas with an un-desired long period of further restoration of the normal reactor functioning.