Characteristics of N₂O release from fluidized media type BNR processes and identification of N₂O sources

The effects of C/N ratios and fluidized media on N₂O emissions were investigated during the conventional biological nitrogen removal (BNR) process for wastewater treatment containing high levels of nitrogen. Low concentration of N₂O emissions was maintained at around 50-100 ppm when the C/N ratio was 3. However higher amounts of N₂O (maximum rate 800 ppm) were released from denitrification, when the C/N ratio was 1.5 and 1, compared to when the ratio was 3. The influence of oxygen concentration on N₂O release as well as its sources was investigated using a nitrogen isotope (¹⁵N) tracer method. Our results indicated that around 50% of the released N₂O was originated from denitrification under aerobic conditions. Under aerobic conditions, the proportion and amount of N₂O emissions from denitrification increased with a decrease in dissolved oxygen (DO). In micro-aerobic conditions, 98.6% of N₂O emission was originated from denitrification. This result implies that an anoxic zone was present even in aerobic reactors and that reactors with low DO tended to have larger anaerobic zones.