Photosynthetic oxygenation in algal-bacterial symbiotic (ABS) system was mainly concerned to enhance contaminant biodegradation by developing an aerobic environment, while the role of nitrification-denitrification involved is often neglected. In this study, an algal-bacterial aggregates (ABA) system was developed with algae and activated sludge (PBR-1) to achieve simultaneous pyridine and nitrogen removal. In PBR-1, as high as 150 mg·L^-1 pyridine could be completely removed at hydraulic residence time of 48 h. Besides, total nitrogen (TN) removal efficiency could be maintained above 80%. Nitrification-denitrification was verified as the crucial process for nitrogen removal, accounting for 79.3% of TN removal at 180 μmol·m^-2·s^-1. Moreover, simultaneous pyridine and nitrogen removal was enhanced through nitrification-denitrification co-metabolism in the ABA system. Integrated bioprocesses in PBR-1 including photosynthesis, pyridine biodegradation, carbon and nitrogen assimilation, and nitrification-denitrification, were revealed at metabolic and transcriptional levels. Fluorescence in situ hybridization analysis indicated that algae and aerobic species were located in the surface layer, while denitrifiers were situated in the inner layer. Microelectrode analysis confirmed the microenvironment of ABA with dissolved oxygen and pH gradients, which was beneficial for simultaneous pyridine and nitrogen removal. Mechanism of nitrification-denitrification involved in pyridine and nitrogen removal was finally elucidated under the scale of ABA.

Environmental Implication

This study shed new light on the algal-bacterial aggregates (ABA) system for simultaneously removing nitrogenous organic compounds and nitrogen. The ABA system was constructed by inoculation of pre-cultured algal-bacterial consortia to activated sludge. Simultaneous pyridine and nitrogen removal were enhanced by nitrification-denitrification as a co-metabolism process. The coexistence of aerobic and anoxic/anaerobic bioprocesses in ABA was confirmed at microbial metabolism and transcriptional level. The ABA system primarily emitted algal-bacterial biomass, O2 and N2, resulting in nearly zero greenhouse gas emissions and extremely low energy consumption, showing the great potential in the multiple functionality of synergetic wastewater reclamation and carbon neutral.