Integrating microalgae tertiary treatment into activated sludge systems for energy and nutrients recovery from wastewater

•The microalgae growth treating secondary effluent and digestate was assessed.•A novel co-pretreatment was evaluated for enhancing microalgae anaerobic digestion.•A steady biomass concentration (1 gTSS/L) dominated by Scenedesmus sp was achieved.•A high organic matter and a complete removal of N-NH4+ and P-PO43−, was observed.•CH4 yield increased by 130% by microalgae and WAS co-pretreatment and codigestion.

In this study, microalgae digestate and secondary effluent were used to grow microalgae in a tertiary wastewater treatment, and then, the biomass was co-digested for biogas generation. A 30 L closed-photobioreactor was used for microalgae cultivation. The biomass, mainly composed by Scenedesmus sp., reached and maintained a concentration of 1.1 gTSS/L during 30 days. A complete removal of N-NH4+ and P-PO43− and high nitrates and organic matter removals were achieved (58% N-NO3− and 70% COD) with 8 d of HRT. The potential biogas production of the cultivated microalgae was determined in batch tests. To improve their biodegradability, a novel method combining their co-digestion with activated sludge after a simultaneous autohydrolysis co-pretreatment was evaluated. After the co-pretreatment, the methane yield increased by 130%. Thus, integrating microalgae tertiary treatment into activated sludge systems is a promising and feasible solution to recover energy and nutrients from waste, improving wastewater treatment plants sustainability.