When Algammox facing low C/N wastewater: role of microalgae in promoting denitrification to synergically achieve effective water treatment

The partial nitritation-anammox (PNA) process significantly reduces oxygen and organic carbon demands for nitrogen removal, but its nitritation step still relies on mechanical aeration. To advance wastewater treatment decarbonization, we integrated microalgae into PNA process to establish a symbiotic microalgal-PNA (Algammox) biofilm system. Microalgae provide in-situ dissolved oxygen via photosynthesis, reducing aeration dependency. Meanwhile, a biofilm carrier strategy was adopted to protect anaerobic ammonium-oxidizing bacteria from light inhibition while enhancing biomass retention. Results demonstrated that Algammox achieved a peak total nitrogen removal efficiency of 98.8%, with an average of 92.6% during stable operation. Microalgae incorporation significantly improved nitrate removal, indicating diversified nitrogen metabolic pathways. Microbial community analysis confirmed that the symbiotic relationship effectively enhanced denitrifying bacteria activity. This work systematically elucidated the complex nitrogen metabolic network of Algammox, including anaerobic ammonium oxidation, partial nitritation, denitrification, and nitrogen assimilation, with the dominant pathway shifting according to community structure. Molecular biological analyses revealed that microalgae significantly upregulated key nitrogen-metabolism genes and enzymes, alongside regulating ATP production, thus successfully resolving the nitrate accumulation in PNA. Algammox is therefore able to achieve efficient nitrogen removal while promoting denitrification, providing an insightfully theoretical and practical foundation for the development of low-carbon, energy-efficient, and nitrate-free wastewater treatment technologies.