Insight into the rapid biogranulation for suspended single-cell microalgae harvesting in wastewater treatment systems: Focus on the role of extracellular polymeric substances

Biomass harvesting is the bottleneck for microalgae-based technologies in practical application for simultaneous wastewater remediation and resource recovery. Biogranulation has been proven to possess high potentials for cost-effective microalgae cultivation and harvesting, while the long start-up duration (about several months) is challenging its application in real wastewater treatment. This study examined the contribution of intact and crushed microalgae granules, and the solid and liquid (AddL) fractions from crushed microalgae granules to the rapid granulation of single-cell microalgae (Ankistrodesmus falcatus var. acicularis) suspension. In addition to > 90% organics and 70–80% total N removal, microalgae aggregates were obtained with excellent settleability when AddL was used as additives, achieving harvesting efficiency of 90.75 ± 1.23% during the 24-h aggregation tests in comparison to 17.74 ± 0.29% by the control. The continuous increase of polysaccharides in soluble extracellular polymeric substances (EPS) might be responsible for cells capture and adhesion at the early stage of aggregation, while the enhanced secretion of bound EPS (mainly tightly bound EPS (TB-EPS)) from 41.86 ± 5.08 to 264.05 ± 6.41 mg/g-volatile suspended solids (VSS) favored the formation of stable microalgae aggregates with better integrity. The fluorescence intensity of aromatic proteins in loosely bound EPS (LB-EPS) and TB-EPS was dramatically strengthened in response to the AddL addition, most probably due to its crucial role in microalgae granulation and stable structure. Results from this study may propose a novel approach for rapid and high-efficiency harvesting of microalgae via biogranulation, which can promote the practical application of microalgae-based wastewater treatment systems.