Nitrous oxide (N2O), a significant contributor to the greenhouse effect, is generated during the biological nutrient removal in wastewater treatment plants (WWTPs). Developing mathematical models estimating the N2O Dynamics under changing operational conditions (e.g. dissolved oxygen, DO) is essential to design mitigation strategies.
Based on the activated sludge models (ASM) structure, this work presents an ASM2d- N2O model including all the biological N2O production pathways for a municipal WWTP under an anaerobic/anoxic/oxic (A2/O) configuration with biological removal of organic matter, nitrogen and phosphorus, and its application in diferent dynamic scenarios. Three microbial N2O production pathways were considered: nitrifier denitrification, hydroxylamine oxidation, and heterotrophic denitrification, with the first two being activated by ammonia oxidizing bacteria (AOB). A stripping effectivity (SE) coefficient was added to reflect the non-ideality of the stripping modeling.
With the DO in the aerobic compartment ranging from 1.8 to 2.5mgL−1, partial nitrification and high N2O production via nitrifier denitrification were noted, indicating that low aeration strategies lead to a low overall carbon footprint only if complete nitrification is not hindered. High N2O emissions were predicted as a combination of low DO (∼1.1mgL−1) with high ammonium concentration. With the AOB prevailing over the nitrite oxidizing bacteria (NOB), nitrite was accumulated, thus activating the nitrifier denitrification pathway.
After suddenly increasing the influent ammonium load, the AOB had a greater growth compared to the NOB and the same pathway was considered as N2O hotspot. Especially under conditions promoting partial nitrification (i.e. low DO) and raising the stripping effect importance (i.e. high SEs), the highest N2O emission factors were predicted