Nitrogen removal mechanisms in duckweed-based stabilization tanks with and without aerobic zones and intermediate feeding stream

This study evaluated the performance of duckweed-based systems ( Spirodela polyrhiza ) for enhancing nitrogen removal from anaerobically pretreated domestic wastewater. Three pilot-scale treatment lines, each consisting of seven tanks in series, were operated under tropical conditions to assess the effects of plant coverage and intermediate feeding. To stimulate nitrification, specific tanks 4 and 5 in Lines 2 and 3 were maintained without duckweed, creating aerobic zones with dissolved oxygen (DO) levels up to 6.4 mg O 2 L −1 . Line 3 also incorporated an intermediate feeding of 10% raw UASB reactor effluent to mitigate carbon limitation for denitrification. Results showed high system stability, with average TSS removal efficiencies reaching 89.9%. While TKN and NH 4 + -N removal showed no significant differences between lines, NO 3 − - N concentrations were significantly higher in Line 1, indicating that duckweed-covered units primarily facilitate balanced nitrogen transformation through microenvironment regulation rather than simple uptake. Nitrogen mass balance identified nitrification-denitrification and duckweed uptake as the dominant removal pathways. Interestingly, the intermediate feeding strategy did not significantly enhance overall nitrogen removal, suggesting that supplemental carbon must be more precisely synchronized with active redox zones and microbial activity. These findings highlight that the strategic spatial configuration of vegetated and non-vegetated zones is a critical management tool. This research provides a baseline for transitioning from passive lagoons to precision-engineered nature-based solutions, emphasizing the importance of aligning carbon availability with oxygen-driven redox gradients to maximize nutrient mitigation in decentralized wastewater treatment. • Spatial configuration of macrophyte cover optimizes redox zones for nitrification. • Intermediate feeding requires precise synchronization with active oxic-anoxic zones. • Duckweed suppresses algal proliferation ensuring tertiary-quality effluent standards.