The A-stage process to promote bioflocculation and microbial storage for carbon redirection: current perspectives and future research directions

A progressive shift from energy-intensive wastewater treatment plants toward sustainable water resource recovery facilities (WRRFs) has gained traction over the years. The A-stage coupled with the B-stage shortcut biological nitrogen removal is enticing, owing to its efficacy in terms of land and energy conservation. This paper is a critical review of the A-stage process that provides a mechanistic understanding of its performance in terms of removal mechanisms, and the influence of its operational parameters. In accordance, future research directions are suggested to deepen the current understanding of the process, develop alternative technologies, and build more efficient WRRFs. Several factors such as HRT, SRT, DO concentration, OLR, chemical oxygen demand (COD) mass load, reactor VSS, feeding regime (i.e., feast/famine), and feast-to-famine retention time ratio independently affect the A-stage process. These factors alternate the substrate acquisition-based mechanisms from being transitional/preparatory mechanisms and typically overlooked in the conventional activated sludge process to critical removal mechanisms in the A-stage process. Although the influence of SRT on the A-stage process has been widely studied, this study demonstrated that SRT should be determined according to the influent COD fractionation and mass load. Moreover, it was inferred that a high DO concentration allows for high bioflocculation and storage under controlled SRT and HRT. Further research is needed to better understand the influence of HRT and feast-to-famine retention time ratio. Furthermore, there are discrepancies regarding the actual selection pressures that induce the substrate acquisition-based mechanisms which require further investigation and resolution.