Positive impact of a reducing agent on autotrophic nitrogen removal process and nexus of nitrous oxide emission in an anaerobic downflow hanging sponge reactor.

The adjustment of hydraulic retention time (HRT) and the supplement of titanium(III) nitrilotriacetate (Ti(III)-NTA) as a reducing agent were implemented in an anaerobic downflow hanging sponge (DHS) reactor with the aims to (i) improve nitrogen removal performance and to (ii) eliminate N2O emission. A laboratory-scale DHS reactor was operated at 35 °C, under autotrophic denitrification conditions with methane gas (14.2 L d-1) as the main carbon source, NaNO3 and NaNO2 (20 mg N L-1 per compound) as nitrogen sources. The sufficient HRT for simultaneously removing nitrate and nitrite in this reactor was found at 12 h when HRT was reduced from 24 to 6 h. Then at the HRT of 12 h, the addition of Ti(III)-NTA at a final concentration of 25 μM Ti(III) boosted the reactor's nitrogen removal rates from 1.4 ± 0.6 to 4.1 ± 1.9 g NO3--N m-3 d-1 and 3.2 ± 2.8 to 6.6 ± 3.3 g NO2--N m-3 d-1. Furthermore, this study is the first to consider the N2O emission in a continuous reactor applying denitrification coupled to anaerobic methane oxidation (DAMO) process. Produced N2O in this DHS reactor was from 10.6 × 10-4% to 89.0 × 10-4% of removed NOx- without Ti(III)-NTA and from 0.7 × 10-4% to 61.4 × 10-4% of removed NOx- with Ti(III)-NTA. Overall, these findings suggested the advantage of Ti(III)-NTA as an oxygen scavenger for denitrification processes and the potential of the anaerobic DHS reactor for facilitating the DAMO processes and mitigating N2O gas.