Transport and Fate of Nitrogen-Containing Compounds in Gas-Phase Biofilters: A Swing Test to Mitigate Ammonia

Abstract

This work carried out a three-month experiment to study the transport, transformation, and fate of nitrogen (N) in two gas-phase ammonia mitigation biofilters using agricultural products (a mixture of wood chips and compost) as the packing media. Two nitrogen enriching steps and one nitrogen depleting step were included as a swing test. In the first nitrogen enriching step, a high ammonia inlet concentration (~70 ppm) was applied. The ammonia removal efficiencies reached 94%, and 73.5% to 86.6% of N-NH3 was transformed into NH4+-N, NO2--N, or NO3--N. The nitrogen depleting step partially cleaned nitrogen compounds with limited disturbance to the media and restored ammonia removal efficiency for a short time. The second nitrogen enriching step used a lower (40 ppm) ammonia inlet concentration. Only 33.4% to 46.2% of NH3-N was transformed into NH4+-N, NO2--N, or NO3--N in the media, and the ammonia removal efficiency fell quickly after ten days. The pH values in the biofilter media were high and changed only slightly during the test (8.5 to 8.2). The free ammonia concentration reached 784 mg L-1 in the media moisture. The nitrogen mass balance analysis showed that the NH4+-N, NO2--N, and NO3--N accumulated in the media accounted for 50% to 100% of the nitrogen captured from the inlet gas. The nitrification process was modeled as consecutive first-order reactions. The kinetic time constants of the two first-order reactions decreased with time and with accumulated nitrogen in the media, suggesting a decline of the nitrogen transformation rates. One very likely reason that caused the decrease of ammonia removal efficiency and kinetic time constants is the accumulated free ammonium in the biofilter media.