Emerging biological wastewater treatment using microalgal-bacterial granules: A review

•Algae inoculation is not the essential requirement for algal-bacterial granules.•Formation of algal-bacterial granules with various inocula are summarized.•Multi-scale transfer phenomena involved in bioreactors are analyzed.•Advances in algal-bacterial granule-based bioreactors are reviewed.•Future development directions of algal-bacterial granules systems are outlined.