Perspectives in anaerobic digestion of lipid-rich wastewater. B. C. Holohan 1,2* , M. S. Duarte 3* , M. A. Szabo Corbacho 4* , A. J. Cavaleiro 3 , A. F. Salvador 3 , C. Frijters 5 , S. Pacheco-Ruiz 6 , M. Carballa 7 , M. A. Pereira 3 , D. Z. Sousa 8 , A. J. M. Stams 3,8 , V. O’Flaherty 1 , J. B. van Lier 4,9 and M. M. Alves 3 . 1 Microbiology, School of Natural Sciences and Ryan Institute, National University of Ireland, Galway. (b.holohan1@nuigalway.ie; vincent.oflaherty@nuigalway.ie) 2 NVP Energy Ltd, Mervue Business Park, Galway, Ireland. 3 Centre of Biological Engineering, University of Minho, Campus de Gualtar, 4710057, Braga, Portugal. (salomeduarte@ceb.uminho.pt; acavaleiro@deb.uminho.pt; asalvador@deb.uminho.pt; alcina@deb.uminho.pt; madalena.alves@deb.uminho.pt) 4 Department of Environmental Engineering and Water Technology, UNESCO-IHE Institute for Water Education, Delft, The Netherlands; (a.szabocorbacho@unesco-ihe.com) 5 Paques B.V., T de Boorstadt 24, 8561 EL, Balk, The Netherlands; (c.frijters@paques.nl) 6 Biothane, Veolia Water Technologies, Tanthofdreef 21, 2623 EW, Delft, The Netherlands. (santiago.pacheco-ruiz@veolia.com) 7 Department of Chemical Engineering, Institute of Technology, Universidad de Santiago de Compostella, Spain; (marta.carballa@usc.es) 8 Laboratory of Microbiology, Wageningen University and Research, The Netherlands. (diana.sousa@wur.nl; fons.stams@wur.nl) 9 Delft University of Technology, Sanitary Engineering Section, The Netherlands. (j.b.vanlier@tudelft.nl) *Contributed equally to this paper. Corresponding author: b.holohan1@nuigalway.ie Abstract Lipid-rich wastewaters are ideal sources for methane production, but lipids are generally separated and removed prior to anaerobic treatment to avoid sludge flotation and microbial inhibition. In this work, we review the major technological and microbiological advances in the anaerobic digestion (AD) of lipids, while highlighting the most important breakthroughs in the field and identifying the future perspectives. In the past decades, several treatment processes have been developed for lipid- rich wastewaters, moving from the upflow granular sludge based reactor designs to anaerobic membrane bioreactors and in situ flotation based bioreactors all now commercially available. Knowledge on the complexity of microbial communities and microbial interactions has increased greatly, allowing a better interpretation of lipids anaerobic biodegradation. However, there are still knowledge gaps and bottlenecks in lipids AD that need to be overcome to improve industrial applications. A multi-faceted approach with industrial and academic partners will provide a unique strategy for future widespread usage of waste-lipids as valuable resource for AD. Keywords Lipid-rich wastewater; long chain fatty acids; anaerobic bioreactors. INTRODUCTION Lipid-rich wastewater has high energy potential, since 1.0 L methane can be produced from 1 g of lipids, while proteins and carbohydrates generate only 0.6 and 0.4 L CH 4 g -1 , respectively [1]. Examples of industries generating wastewater with high lipid content are slaughterhouses (45 – 300 mg L -1 ) [2], dairies (500 – 9500 mg L -1 ) [3] and edible oils production (2000 – 15000 mg L -1 ) [4]. Hydrolysis of triacylglycerol lipids releases glycerol and long chain fatty acids (LCFA), and subsequent LCFA beta-oxidation is the rate-limiting step in lipids anaerobic digestion (AD) [5].