Development of an ASM2d-N<inf>2</inf>O model to describe nitrous oxide emissions in municipal WWTPs under dynamic conditions

Abstract

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 anaerobic/anoxic/oxic (A2/O) WWTP with biological removal of organic matter, nitrogen and phosphorus, and its application in different 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 model. Partial nitrification and high N2O production via nitrifier denitrification were observed when the range of DO in the aerobic compartment was 1.8 to 2.5 mg·L−1. It could imply that low aeration strategies lead to low overall carbon footprint provided complete nitrification is not hindered. The model predicted high N2O emissions when low DO (∼1.1 mg L−1) and high ammonium concentration concurred. With the AOB prevailing over the nitrite oxidizing bacteria (NOB), nitrite was accumulated, triggering the activation of 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.