Phototrophic aggregates for wastewater treatment: identifying key parameters for formation and characterization

Phototrophic granules (or photogranules) are biological aggregates containing phototrophic and heterotrophic microorganisms. These organisms may engage in syntrophic interactions. In contrast to conventional activated sludge, photogranules keep carbon in the biomass through photosynthesis, leading to a higher potential for energy recovery. Photogranules are a candidate biomass for aeration-free wastewater treatment. About 10 years after the first description of photogranules, we review the emerging literature on this promising biomass and propose a unifying nomenclature, attempting to standardize terminology in the field of phototrophic aggregates. These efforts aim at making results in future publications more comparable. We critically discuss methods to assess the main performance indicators for successful photogranulation: settleability assessment, microbial activity and oxygen production. In a second part, characterization and monitoring methods of the physical properties of photogranule such as particle size distributions and microscopy are detailed. The review underscores the need for standardized and adapted methodologies to accurately describe photogranulation. The key factors to produce photogranules are investigated, focusing on the challenges of achieving a critical mass of relevant phototrophic microorganisms and providing the environmental conditions that favour photogranulation. The lack of consensus and data regarding several important parameters influencing photogranulation is highlighted and future research perspectives are indicated. Achieving and maintaining the performance of a photogranule-based process at larger scales will require a deeper understanding of the phenomena leading to photogranulation. Understanding photogranule formation is the first step towards a more sustainable wastewater treatment technology.