Waste biorefineries — integrating anaerobic digestion and microalgae cultivation for bioenergy production

[Display omitted] •Microalgae can be grown with liquid (COD, N, P) and gaseous (CO2) effluents of anaerobic digestion.•The organic acids present in the effluent are utilized efficiently in a mixotrophic mode of cultivation.•Simultaneous biogas upgrading with over 90% methane content could be obtained.•Lipid content in microalgae grown on anaerobic digestion effluents can be improved for biofuel applications.•Integration of sewage sludge AD and microalgal cultivation is an interesting waste biorefinery option. Commercialization of microalgal cultivation has been well realized in recent decades with the use of effective strains that can yield the target products, but it is still challenged by the high costs arising from mass production, harvesting, and further processing. Recently, more interest has been directed towards the utilization of waste resources, such as sludge digestate, to enhance the economic feasibility and sustainability of microalgae production. Anaerobic digestion for waste disposal and phototrophic microalgal cultivation are well-characterized technologies in both fields. However, integration of anaerobic digestion and microalgal cultivation to achieve substantial economic and environmental benefits is extremely limited, and thus deserves more attention and research effort. In particular, combining these two makes possible an ideal ‘waste biorefinery’ model, as the C/N/P content in the anaerobic digestate can be used to produce microalgal biomass that serves as feedstock for biofuels, while biogas upgrading can simultaneously be performed by phototrophic CO2 fixation during microalgal growth. This review is thus aimed at elucidating recent advances as well as challenges and future directions with regard to waste biorefineries associated with the integration of anaerobic waste treatment and microalgal cultivation for bioenergy production.