The start-up of red blood cell-like Anammox granular sludge based on substrate volume flux

Enhancing substrate transfer is crucial for promoting the nitrogen removal rate of Anammox granular sludge, as the shape of the granules significantly influences substrate diffusion. In this study, inspired by biomimicry principles, we propose a novel red blood cell-like granular sludge to increase the contact area and improve substrate transfer. The startup and operation of Anammox granular sludge in low-strength ammonia nitrogen wastewater were successfully achieved through an increase in substrate volume flux (SVF). As the SVF increased from 2.38 to 7.43 L²/(g VSS·h), specific Anammox activity (SAA) rose from 0.138 to 0.483 g N/(g VSS·d). The Anammox granular sludge system achieved a total nitrogen removal efficiency of 85.70 % and a nitrogen removal rate (NRR) of 0.792 kg N/(m³·d) after 126 days. Notably, red blood cell-like granular sludge was observed by day 116. The rise in SVF induced a granule structure more conducive to substrate transport, improving the granules' efficiency in substrate uptake and thereby enhancing their SAA. High-throughput pyrosequencing revealed that the dominant bacteria using the SVF-enhancing strategy remained anaerobic ammonium oxidizing bacteria (AnAOB) (29.53 %-29.89 %), with an increased diversity of species, including Candidatus Kuenenia, Candidatus Anammoxoglobus, and Candidatus Brocadia.