Basic principles and effective parameters for microalgae–bacteria granulation in wastewater treatment: a mini review

Microalgae–bacteria granulation is overviewed based on their internal interaction and effective parameters. The effect of the feed component, light, operational parameters (such as HRT and SRT), and the type of photobioreactors are evaluated in this article to determine how they can accelerate granule formation with a stable evolution trend. Extracellular polymeric substance (EPS) secreted in the media through microbial activity causes nucleation and then matures granules of microalgae–bacteria via aggregation. Therefore, the content of EPS has crucial impact on granulation via enhancement of protein-to-carbohydrate ratio. Extracellular proteins can change the surface charge of microalgae and bacteria in favor of increasing their hydrophobicity and zeta potential. Microalgae and bacteria arrangement in the granule structure is dependent to nutrients in wastewater and operational parameters. Nitrate and ammonium as the N-source components of the feed can play significant roles on the diversity of microorganisms in a granule structure. Determination of appropriate HRT can accelerate granule formation in both photo-sequencing batch reactors and up-flow photobioreactors. Some proposed models are also summarized to predict the manner of microalgae and bacteria within granulation. This review is favorable to promoting the development of microalgae–bacteria granule systems by presenting recent progress and future perspectives.