(122c) Greenhouse-Gas (GHG) Emissions From Denitrifying Fluidized Bed Bioreactors (FBBR)

Anthropogenic greenhouse gases (AnGHGs) emissions are globally recognized by the United Nation framework Convention on Climate Change (UNFCCC) (UNFCCC, 2007). GHGs include carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O). N₂O, the dominant ozone-depleting substance, is the third most important GHGs with a global warming potential (GWP) of 310 times that of CO₂. Nitrous oxide (N₂O) is a significant anthropogenic greenhouse gases (AnGHGs) emitted from biological nutrient removal (BNR) processes. This study tries to fill the gap of knowledge and get a deeper insight into N₂O emissions from denitrifying fluidized bed bioreactors (DFBBRs). The DFBBR was operated on a synthetic municipal wastewater at organic and nitrogen loading rates of 5.9 kg COD/(m³.d) and 1.2 kg N/(m³.d), respectively in phase I for 150 days. In phase II, the DFBBR columns were tested for 50 days at higher organic and nitrogen loading rates of 7 kg COD/(m³.d) and 2 kg N/(m³.d), respectively. The aforementioned organic and nitrogen loading rates represent two different chemical oxygen demand (COD)-to-nitrogen (COD/N) ratios of 5 and 3.5 for phases I and II, respectively.

After the limited carbon phase, the DFBBR was tested at COD/N of 5 again for 50 days to investigate the dynamics of N₂O emissions for the particulate biofilm. Results showed that the N₂O conversion rate from the DFBBR was 0.53% of the total nitrogen loading in phase I while in phase II at the low COD/N ratio of 3.5, the N₂O conversion rate increased by 196% to 1.57% of the influent nitrogen loading with a 7 fold increase in liquid nitrite concentration. Subsequent increase to a COD/N ratio of 5, affected a 78% increase in emissions of the influent nitrogen compared to the first phase.  Comparing the N₂O emission of the DFBBR system with other systems (up to 14.6%), showed that the N₂O emission was in a lower range (0.53% to 1.57%).