Effects of seeding density on photogranulation and the start-up of the oxygenic photogranule process for aeration-free wastewater treatment

The oxygenic photogranule (OPG) process is a new algae-based wastewater treatment method, relying on light-driven granulation of filamentous cyanobacteria, microalgae, and non-phototrophic microorganisms (i.e., photogranulation). Seeding the OPG reactor with hydrostatically formed photogranules has been shown as a new start-up approach, which is reproducible and can significantly decrease the start-up period. However, due to early research stage for the OPG process, this seeding approach has not yet been investigated at good depth and its protocol is yet to be established. The objective of this study is to investigate the impact of seeding density on photogranulation and the start-up of the OPG process. Three sequencing-batch reactors (R1-R3) were seeded with a different number of hydrostatically formed photogranules (thus, started with different seeding densities) and operated to treat primary effluent wastewater. The three reactors showed significantly different photogranulation rates and physicochemical characteristics of the produced biomass. Reactor R2, seeded at 900 mg/L dry solids, produced photogranules showing compact, spherical structures with increased hydrophobicity. R2 exhibited the highest yield of biomass, which was also found to be more enriched with proteins, carbohydrates and lipids than biomass from other reactors. R2 also showed better settleability of biomass than others, while all three reactors enabled effective organic matter and dissolved nitrogen removal without external aeration. These results suggest that seeding density is important for the OPG process and optimized seeding density can promote effective start-up as well as bioenergy recovery with increased biomass yield and biorefinery/bioenergy potential.