The Oxygenic Photogranules—Current Progress on the Technology and Perspectives in Wastewater Treatment: A Review

Wastewater generation is a worldwide problem, and its treatment is an important practice for maintaining public health and environmental protection. Oxygenic photogranules (OPGs) are a relatively novel type of biogranules that have the potential to substitute the conventional activated sludge (AS) process due to the production of in situ oxygen, better physical properties such as settling velocity and density, as well as carbon and nutrient removal efficiencies. The formation of the granules is attributed to many factors, among which the most influential are light intensity, ammonium nitrogen concentration, and the presence of filamentous cyanobacteria that, along with heterotrophic microorganisms situated in the granule’s core, create a self-sustainable system that combines denitrification, carbon removal, and oxygen production. Hydrostatic and hydrodynamic cultivations are two ways that allow for obtaining OPGs. These two cultivation methods lead to the formation of various types of granules which differ in both structures as well as physical properties. This review article aims to aggregate the available literature information regarding the methods of cultivation of OPGs, their formation mechanisms, and factors that influence the cultivation as well as an overview of studies that were conducted thus far concerning this type of biogranules. Additionally, further research directions are proposed in the article