Elucidating the factors that impact the abundance of aerobic and denitrifying polyphosphate‐accumulating organisms in an integrated fixed film activated sludge‐enhanced biological phosphorus removal process

Abstract The integrated fixed‐film activated sludge (IFAS)‐enhanced biological phosphorus removal (EBPR) (IFAS‐EBPR) system is significant for its ability to enhance nitrification and phosphorus removal by leveraging fixed‐film carriers for nitrifiers and suspended growth for heterotrophs, but limited understanding of microbial dynamics and variable performance continues to drive interest in optimizing its application. This study investigated the key factors that affect the distribution and abundance of denitrifying (DPAOs) and non‐denitrifying types of Accumulibacter‐ like polyphosphate‐accumulating organisms (non‐DPAOs) in a lab‐scale continuous‐flow IFAS‐EBPR. The increase of nitrate recycles ratios from 1.0, 1.5, to 2.5Q affected both nitrogen (N) removal efficiency and EBPR activities, with P removal profile transitioning from having continuous P release to P uptake in anoxic zone. Population abundance analysis revealed that Clade I Accumulibacter (preconceived DPAO ‐Accumulibacter clade) correlated positively with the nitrate recycle flows ( r = 0.96) and anoxic residual nitrate concentration ( r = 0.99), while Clade II Accumulibacter (non‐DPAO clade) showed negative correlation ( r = −0.93), indicating the response of DPAO to the operational condition, particularly the nitrate recycle ratio, in our system. Furthermore, the relative abundance of DPAOs correlated negatively with the anoxic residual biodegradable COD ( r = −0.99), anoxic hydraulic retention time (HRT) ( r = −0.99), and nitrate and nitrite levels in the anoxic zone ( r = −0.95, −0.99). SUMO model simulations revealed DPAO denitrification rates of 8.2, 7.8, and 4.1 gN/m 3 /d from 1.0, 1.5, to 2.5Q, accounting for only 5.8%, 9.5%, and 4.0% of nitrate removal in the anoxic zone, respectively. This study demonstrates that DPAO and non‐DPAO populations are dynamically affected by operating conditions in a continuous‐flow EBPR system, and the actual contribution of DPAO to the overall denitrification is relatively small (<10%). This study contributed to our better understanding of DPAO population dynamics and providing insights for optimizing DPAO in a EBPR processes for more sustainable wastewater treatment. Practitioner Points Accumulibacter ‐like DPAO subgroups and non‐DPAOs coexist in IFAS‐EBPR systems, with abundances controlled by nitrate recycle flows. DPAOs contribute <10% to overall denitrification, but their enrichment significantly impacts carbon reduction in EBPR processes. Optimizing nitrate recycle ratio (around 1.5) balances nitrate recycling and oxygen inhibition, enhancing DPAO performance in wastewater treatment. Understanding DPAO subpopulation dynamics is crucial for designing efficient BNR systems with reduced external carbon requirements.