Principles, Advances, and Perspectives of Anaerobic Digestion of Lipids

Abstract

Several problems associated with the presence of lipids in wastewater treatment plants are usually overcome by removing them ahead of the biological treatment. However, because of their high energy content, waste lipids are interesting yet challenging pollutants in anaerobic wastewater treatment and codigestion processes. The maximal amount of waste lipids that can be sustainably accommodated, and effectively converted to methane in anaerobic reactors, is limited by several problems including adsorption, sludge flotation, washout, and inhibition. These difficulties can be circumvented by appropriate feeding, mixing, and solids separation strategies, provided by suitable reactor technology and operation. In recent years, membrane bioreactors and flotation-based bioreactors have been developed to treat lipid-rich wastewater. In parallel, the increasing knowledge on the diversity of complex microbial communities in anaerobic sludge, and on interspecies microbial interactions, contributed to extend the knowledge and to understand more precisely the limits and constraints influencing the anaerobic biodegradation of lipids in anaerobic reactors. This critical review discusses the most important principles underpinning the degradation process and recent key discoveries and outlines the current knowledge coupling fundamental and applied aspects. A critical assessment of knowledge gaps in the field is also presented by integrating sectorial perspectives of academic researchers and of prominent developers of anaerobic technology.

Underestimation of Extracellular Lipid Contribution to Fouling in Anaerobic Membrane Bioreactors

“…However, QQ could negatively affect the lipid degradation microorganisms. Specifically, the abundance of LCFA-degrading bacteria Syntrophus was retarded by 54.17% in the presence of QQ , (Figure S1), and it might be attributed to the disruption of QS of Syntrophus, as AHLs were implicated to change the metabolic processes of Syntrophus . Thus, these collectively led to the unaffected balance between lipid accumulation/degradation (i.e., lipid content) in QQAnMBR, as compared to the Control AnMBR.…”

“…However, QQ had no impact on lipid profiles and the lipid content of EPS (Figure a). The probable reasons were that the effects of QQ on lipid contents in AnMBRs were complex, as multiple specific functional microorganisms could be involved in lipid production and lipid degradation . For example, QQ could disrupt the QS systems of hydrolytic–fermentative bacteria and thereby retard the metabolic activities such as hydrolysis of cells and lipid accumulation.…”

“…Specifically, the proportion of lipids was only around 1% in EPS and 3% in SMP of activated sludge in a full-scale AeMBR . The lipid fraction of the cake layer on membrane surfaces in AeMBRs was suggested to be insignificant as compared to other fouling constituents (e.g., proteins and carbohydrates). , In anaerobic bioreactors, extracellular lipids were mainly investigated as microbial metabolites, precursors of methane production, or inhibitors of methanogens. − However, the significance of lipid-related membrane fouling has rarely been studied and compared to that of typically recognized foulants such as proteins and carbohydrates in AnMBRs. Different from aerobic bioreactors, extracellular lipids and long-chain fatty acids (LCFAs) are produced by both prokaryotes and eukaryotes during the digestion of microbial cells and cell lysis in anaerobic bioreactors (e.g., hydrolysis of cells), ,− which could inevitably lead to the accumulation of lipids in AnMBRs. ,, For example, lipids were found to be the dominant organics of anaerobic sludge, which accounted for 21–44% in the mixed liquor .…”

“…However, the effect of QQ has previously been mainly focused on the retardation of proteins and carbohydrates in AnMBRs, , which was attributed to the disrupted QS activity of these QS microorganisms. The effects of QQ on the accumulation and degradation of lipids/LCFAs in AnMBRs, which are highly dependent on the specific and complex interactions of functional microorganisms, have yet to be elucidated.…”

Co‐substrate composition is critical for enrichment of functional key species and for process efficiency during biogas production from cattle manure

“…In this study, the added fat was well degraded (~85%, Figures S4 and S5 ) and the reactors showed no sign of VFA accumulation. However, co‐digestion with fat‐rich material can be challenging and, if fat is included at high rates, it can result in accumulation of fatty acids and inhibition of methanogenesis (Holohan et al, 2022 ). This was clearly illustrated in a study by Wang et al ( 2021 ) where CM was co‐digested with glycerol trioleate (fat, oil, grease (FOG)) or glucose, applied in an increasing load from 3.2 to 5 g VS L −1 day −1 .…”