Anaerobic biodegradation of long chain fatty acids: a review

Abstract

If an effluent with high lipid content is fed to a continuous high rate anaerobic digester, a significant accumulation of Long Chain Fatty Acids (LCFA) onto the sludge aggregates (flocs or granules) is observed, without further mineralization to methane. That accumulation is progressive and, after some time, the sludge becomes surrounded by whitish foam and tends to float, leading, most of times, to a severe washout. This happens not only with granular sludge reactors, but also with fixed bed reactors. This fact, associated to the accepted idea that LCFA were highly toxic to the anaerobic consortia, imposed that lipids had to be removed from any wastewater before the anaerobic treatment step. Therefore, the high methane potential of these compounds was lost. However, it was observed that anaerobic sludge had the capacity to mineralize the LCFA associated to the cells by mechanisms of adsorption, precipitation or entrapment. The mineralization of these biomass-associated LCFA was inhibited by oleic acid present in the feed. Furthermore, after the mineralization of these biomass-associated LCFA, the specific methanogenic activity of the sludge was enhanced, being able to accumulate and mineralize a new LCFA load. These facts contradict the accepted theories about the severe and irreversible toxicity of LCFA and clearly suggest that only a reversible inhibition occurs, probably a simple physical transport limitation effect. The main practical result of this research is that a sequencing batch reactor (SBR) based process will be the most suitable way to efficiently mineralize effluents with high lipids content. The new SBR concept is under development and was recently awarded with the Lettinga Award 2004. A review of the most important results on anaerobic LCFA biodegradation is presented in this paper.