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The application of aerobic granular sludge (AGS) technology has increased in popularity, largely due to the smaller physical footprint, enhanced biological nutrient removal performance and ability to perform with a more stable operation when compared to conventional activated sludge (CAS) systems. To date, the ability of AGS to remove microbial pathogens such as; Escherichia coli, Giardia, and Cryptosporidium has not been reported. This study compared the log removal performance of commonly used pathogen surrogates (sulfite-reducing clostridia spores, f-RNA bacteriophage, E. coli and total coliforms) by AGS and CAS during the start-up phase, through to maturation. Results showed that AGS performed as well as CAS for the log removal performance of all microbial surrogates, except for spores which were removed more effectively by AGS most likely due to greater adherence of spores to the AGS biomass compared to CAS mixed liquor. Results suggest that AGS is capable of meeting or exceeding CAS-equivalent health-based targets for pathogen removal in the context of water recycling as well as not adversely affecting the secondary effluent water quality (suspended solids, turbidity and particle size) in terms of ultraviolet light transmissivity (254 nm). These findings confirmed for the first time that the adoption of AGS operation would not adversely impact downstream tertiary disinfection processes from altered water quality, nor would it require further pathogen treatment interventions in addition to what is already required for CAS systems.

The retrofitting of existing wastewater sequencing batch reactors (SBRs) to select for rapid-settling aerobic granular sludge (AGS) over floc-based conventional activated sludge (CAS), could be a viable option to decrease reactor cycle time and increase hydraulic capacity. Successful CAS-to-AGS conversion has previously been shown to be highly dependent on having a dedicated anaerobic feed, which presents additional engineering challenges when retrofitting SBRs. In this study we compared the performance of a split anaerobic-aerobic (An-Aer) feed with that of a traditional dedicated anaerobic feed regarding AGS formation and stability, nitrogen removal performance and microbial ecology. Using pilot trials, we showed that AGS could be established and maintained when using a split An-Aer feed at low organic loading rates analogous to that of a parallel full-scale conventional SBR. Additionally, we showed that AGS start-up time and nitrogen removal performance were comparable under a split An-Aer feed and dedicated anaerobic feed. Microbial ecology characterisations based on whole-of-community 16S rRNA profiles and targeted analysis of functional genes specific for nitrifying and denitrifying microorganisms, showed that the two different feed strategies had only subtle impacts on both the overall community composition and functional ecology. A much greater divergence in microbial ecology was seen when comparing AGS with CAS. Data presented here will be of value to those planning to retrofit existing CAS-based SBRs to operate with AGS and demonstrates the viability of using a more cost-effective split An-Aer feed configuration over a dedicated anaerobic feed.

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