Exploring denitrification and anammox processes in the saturated zone of passively operated vegetated biofiltration systems

Vegetated biofiltration systems (VBS) are common nature-based solutions (NBS) for urban water treatment, but their performance in treating nitrogen-rich wastewater is yet to be explored. This study investigated the submerged zone (SZ) component of VBS, for its ability to treat nitrogen through traditional denitrification, as well as the anaerobic ammonium oxidation (anammox) process that was rarely studied in NBS. A two-phase column-based experiment was conducted: (1) denitrification phase, to explore the impact of different carbon sources (hardwood chips, softwood chips, walnut shells), retention time and dissolved oxygen (DO) on denitrification; and (2) anammox phase, to identify whether the anammox process could occur in passively operated VBS. The results show that the VBS was able to treat nitrogen rich wastewater (with 80-100 mg L⁻¹ total nitrogen - TN) in both phases (average TN removal rates of 58.0 ± 16.4%). Carbon sources had significant influences on TN and NO₃-N removal in the denitrification phase (p < 0.05), with hardwood chips being the most effective (76.3 ± 16.1%, standard conditions). Noticeable improvement was observed when increasing the retention time from 12 h (15.5 ± 4.5% TN removal) to 48 h (81.0 ± 2.9%). DO levels (<0.5 mg L⁻¹ to 3 mg L⁻¹) did not have a significant impact on nitrogen removal in all tested systems. Microbial analysis showed significant differences between hardwood chips/softwood chips and walnut shells. Various types of denitrifying bacteria dominated the microbial community (total relative abundance >10%), with Comamonadaceae_unclassified being the most abundant (average 8.3%) across all media types. A suspected anammox genus SM1A02 was found in the systems, but at very low abundance levels (up to 0.137%), which may indicate the presence of the anammox process in the passively operated VBS.