Conversion of C_n-Unsaturated into C_n-2-Saturated LCFA Can Occur Uncoupled from Methanogenesis in Anaerobic Bioreactors

Abstract

Fat, oils, and grease present in complex wastewater can be readily converted to methane, but the energy potential of these compounds is not always recyclable, due to incomplete degradation of long chain fatty acids (LCFA) released during lipids hydrolysis. Oleate (C18:1) is generally the dominant LCFA in lipid-containing wastewater, and its conversion in anaerobic bioreactors results in palmitate (C16:0) accumulation. The reason why oleate is continuously converted to palmitate without further degradation via β-oxidation is still unknown. In this work, the influence of methanogenic activity in the initial conversion steps of unsaturated LCFA was studied in 10 bioreactors continuously operated with saturated or unsaturated C16- and C18-LCFA, in the presence or absence of the methanogenic inhibitor bromoethanesulfonate (BrES). Saturated Cn-2-LCFA accumulated both in the presence and absence of BrES during the degradation of unsaturated Cn-LCFA, and represented more than 50% of total LCFA. In the presence of BrES further conversion of saturated intermediates did not proceed, not even when prolonged batch incubation was applied. As the initial steps of unsaturated LCFA degradation proceed uncoupled from methanogenesis, accumulation of saturated LCFA can be expected. Analysis of the active microbial communities suggests a role for facultative anaerobic bacteria in the initial steps of unsaturated LCFA biodegradation. Understanding this role is now imperative to optimize methane production from LCFA.

Insight into the Role of Facultative Bacteria Stimulated by Microaeration in Continuous Bioreactors Converting LCFA to Methane

“…This is in accordance with the predominance of these bacteria in bioreactors where oleate to palmitate conversion occurred. 12,13…”

“…In AnR Syntrophomonas and Syntrophobacter became less abundant as well, and the relative abundance of facultative anaerobic bacteria (e.g., Pseudomonas, Stenotrophomonas, and Aeromonas) increased to 21%. 12 showed that oleate conversion to palmitate can occur independently of methanogenesis and that facultative anaerobic bacteria are enriched under such conditions. In this work, we applied high organic loading rates and low hydraulic retention times (Table 3) to selectively washout the microorganisms with slower growth rates (including methanogens) and simultaneously to enrich for microorganisms capable of performing oleate to palmitate conversion.…”

Enhanced Methanization of Long-Chain Fatty Acid Wastewater at 20°C in the Novel Dynamic Sludge Chamber–Fixed Film Bioreactor

“…The DSC-FF reactors consistently removed the saturated and unsaturated LCFAs in SDW to concentrations below 50 mg/L in the effluent, at HRTs as short as 18 h. The saturated LCFAs, palmitate and stearate, were partially removed at 12 h. During treatment of wastewaters with high lipid loads, LCFA accumulation constituting of palmitate or stearate has often been encountered in various reactor types (Pereira et al, 2005;Cavaleiro et al, 2009;Dereli et al, 2015;Ziels et al, 2015Ziels et al, , 2017Duarte et al, 2018) due to the fast conversion of unsaturated LCFAs (linoleate and oleate) to palmitate (Cavaleiro et al, 2016).…”

“…The standard Gibbs free energy changes of reaction ( G • ') for the degradation of LCFAs (linoleate, oleate, stearate, and palmitate) present in the feed (SDW) were calculated at 20 (Table 3), as numerous studies evaluating lipid or LCFA degradation at mesophilic conditions have used 37 • C as the operational temperature (Ramos et al, 2014;Dereli et al, 2015;Jensen et al, 2015;Cavaleiro et al, 2016).…”

“…Up until recently, the high-rate anaerobic treatment of LCFAs has been studied extensively at mesophilic or thermophilic conditions (e.g., Hwu et al, 1998a;Kim et al, 2004;Jeganathan et al, 2006;Ramos et al, 2014;Dereli et al, 2015;Cavaleiro et al, 2016;Duarte et al, 2018;Szabo-Corbacho et al, 2019) but not at low ambient temperatures. An evaluation of the anaerobic LCFA treatment at low temperatures will bring new insights into the treatment of lipid-rich wastewaters.…”