Programmable cyclic flow bioprocess enables in-situ anammox bacteria enrichment and advanced nitrogen removal from low C/N wastewater

Abstract

Efficient nitrogen removal from low C/N wastewater remains a major challenge for conventional biological treatment processes. In this study, a modified continuous-flow A/O process with programmable cyclic flow control was established to achieve enhanced nitrogen removal. The system consistently achieved low effluent TIN concentrations of 2.6 ± 0.9 mg-N/L and 6.3 ± 1.1 mg-N/L under influent C/N ratios of 5.5 and 3.1, respectively. Notably, the process promoted in-situ enrichment of anammox bacteria, with their relative abundance reaching 0.8%. Correspondingly, the abundances of key anammox functional genes increased substantially, with hzs and hdh rising from 7.3 and 0.9 TPM to 202.6 and 31.4 TPM, respectively. Mass balance analyses revealed clear functional partitioning of nitrogen transformation pathways. Simultaneous nitrification–denitrification (SND) and partial denitrification–anammox (PDA) predominantly occurred in the oxic and anoxic tanks, respectively, while conventional nitrification–denitrification (CND) acted as a complementary pathway. At the higher C/N of 5.5, SND dominated nitrogen removal, accounting for 67.2% ± 0.9% of the total flux. At the lower C/N of 3.1, PDA played a more prominent role, contributing 39.4% of the overall nitrogen removal. This functional differentiation was supported by microbial community profiling and functional gene analyses. Batch experiments further confirmed the pronounced SND and PDA potentials in the oxic and anoxic tanks, respectively. This study establishes a novel operational strategy that enables in-situ enrichment of anammox bacteria and achieves highly efficient nitrogen removal via PDA, SND, and CND.

Graphical abstract

Keywords

Anammox / Simultaneous nitrification and denitrification / Advanced nitrogen removal / Engineering resilience / Low C/N wastewater

Highlight

	● Programmable cyclic flow A/O process achieves stable nitrogen removal at low C/N.
	● Effluent TIN remained at 2.6 ± 0.9 and 6.3 ± 1.1 mg/L at C/N ratios of 5.5 and 3.1.
	● In-situ enrichment of anammox bacteria (0.8%) was achieved in the system.
	● Anammox genes ( hzs , hdh ) increased markedly from 7.3 and 0.9 to 202.6 and 31.4 TPM.
	● Batch experiments confirmed strong SND and PDA potentials in oxic and anoxic tanks.

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