Biogranulation process facilitates cost-efficient resources recovery from microalgae-based wastewater treatment systems and the creation of a circular bioeconomy

Energy self-sufficient wastewater treatment designs can reduce net energy consumption and achieve resources recovery. Microalgae are regarded as a promising candidate for developing a circular bioeconomy in wastewater treatment plants (WWTPs) due to its potential for simultaneous wastewater remediation and high value-added materials production. Much effort has been made to overcome the high production costs for microalgae; however, biomass harvesting still remains as the bottleneck for its large-scale application. In this study, the novel biogranulation system facilitating easier and faster microalgae harvesting was firstly compared with the conventional suspended culture for energy-efficiency and sustainability assessment on microalgae (Ankistrodesmus falcatus var. acicularis) cultivation using the synthetic anaerobic digestion liquor. Results demonstrated that the biogranulation system enhanced volumetric biomass productivity (223.17 ± 11.82 g/m3/day) by about 4.4 times compared to that from the suspended system (41.57 ± 2.08 g/m3/day) under the same environmental conditions. It was noticed that lipids, carbohydrates and proteins were accumulated in microalgae cells along with nutrients remediation, and the microalgae granules with much higher proteins content (313.28 ± 26.67 mg/g-VSS) could be easily harvested through 2 min gravity sedimentation with little impact on the contents of carbohydrates and lipids. In the whole cultivation and harvesting process, the biomass mass-based electricity consumption and footprint demand by the biogranulation system were reduced by 58% and 76%, respectively. Results from this study provide a cost-effective and sustainable approach for microalgae in the treatment of nutrients rich digestion liquor with simultaneous production of valuable biomaterials. This work was supported by JSPS KAKENHI Grant Number JP18H03403. Ms. Qian Wang would like to thank the financial support from the China Scholarship Council (No. 201808050110) for her PhD study in University of Tsukuba, Japan.