• Fully Fe-C packed UBAF, with simple G/W control nitrogen conversions. • Moderate G/W (3:1) adjusted aerobic-anoxic niches, enabling stable SND. • G/W tuning changed biological and Fe-mediated routes for low-C/N saline effluent. Nitrogen pollution from mariculture tailwater threatens coastal ecosystems, calling for more effective processes to conduct wastewater biotreatment. In this study, we evaluated how gas-to-water ratio (G/W) regulates nitrogen conversion in a Fe-C upflow biological aerated filter. Three G/W levels (6:1, 3:1, 0:1) were tested in wastewater biotreatment while coupling performance monitoring with 16S rRNA profiling, functional prediction, and enzyme assays. Increasing the G/W enhanced ammonium oxidation, with NH 4 + -N removal reaching 92.1% at G/W = 6:1, but nitrate accumulated to 4.01 mg L -1 . At G/W = 0:1, nitrification was suppressed, and NH 4 + -N removal fell to 75.2%, yet nitrate reduction was maintained and effluent NO 3 - -N decreased to 1.14 mg L -1 . The moderate G/W = 3:1 established complementary aerobic-anoxic niches that supported simultaneous nitrification-denitrification and achieved the most balanced total nitrogen removal of 87.1%. Community analysis showed oxygen-favored taxa dominated at high G/W, while anoxic and fermentative groups increased under oxygen limitation. Enzyme assays corroborated these patterns, showing the highest Ammonia monooxygenase activity at G/W = 6:1, nitrite reductase peaking at G/W = 3:1, nitrate reductase remaining stronger at G/W = 0:1 and 3:1, and nitric oxide reductase increasing with aeration. These results suggest that tuning G/W restructures redox micro-niches, potentially driving biological and putative Fe-mediated pathways, providing a practical route to robust nitrogen removal from mariculture tailwater.
Tian et al. (Wed,) studied this question.