Decoupling the simultaneous effects of NO2−, pH and free nitrous acid on N2O and NO production from enriched nitrifying activated sludge

Abstract

In the pursuit of energy and carbon neutrality, nitrogen removal technologies have been developed featuring nitrite (NO<inf>2</inf>⁻) accumulation. However, high NO<inf>2</inf>⁻ accumulations are often associated with stimulated greenhouse gas (i.e., nitrous oxide, N<inf>2</inf>O) emissions. Furthermore, the coexistence of free nitrous acid (FNA) formed by NO<inf>2</inf>⁻ and proton (pH) makes the consequence of NO<inf>2</inf>⁻ accumulation on N<inf>2</inf>O emissions complicated. The concurrent three factors, NO<inf>2</inf>⁻, pH and FNA may play different roles on N<inf>2</inf>O and nitric oxide (NO) emissions simultaneously, which has not been systematically studied. This study aims to decouple the effects of NO<inf>2</inf>⁻ (0-200 mg N/L), pH (6.5-8) and FNA (0-0.15 mg N/L) on the N<inf>2</inf>O and NO production rates and the production pathways by ammonia oxidizing bacteria (AOB), with the use of a series of precisely executed batch tests and isotope site-preference analysis. Results suggested the dominant factors affecting the N<inf>2</inf>O production rate were NO<inf>2</inf>⁻ and FNA concentrations, while pH alone played a relatively insignificant role. The most influential factor shifted from NO<inf>2</inf>⁻ to FNA as FNA concentrations increased from 0 to 0.15 mg N/L. At concentrations below 0.0045 mg HNO<inf>2</inf>-N/L, nitrite rather than FNA played a significant role stimulating N<inf>2</inf>O production at elevated nitrite concentrations. The inhibition effect of FNA emerged with further increase of FNA between 0.0045-0.015 mg HNO<inf>2</inf>-N/L, weakening the promoting effect of increased nitrite. While at concentrations above 0.015 mg HNO<inf>2</inf>-N/L, FNA inhibited N<inf>2</inf>O production especially from nitrifier denitrification pathway with the level of inhibition linearly correlated with the FNA concentration. pH and the nitrite concentration regulated the production pathways, with elevated pH promoting the nitrifier nitrification pathway, while elevated NO<inf>2</inf>⁻ concentrations promoting the nitrifier denitrification pathway. In contrast to N<inf>2</inf>O, NO emission was less susceptible to FNA at concentrations up to 0.015 mg N/L but was stimulated by increasing NO<inf>2</inf>⁻ concentrations. This study, for the first time, distinguished the effects of pH, NO<inf>2</inf>⁻ and FNA on N<inf>2</inf>O and NO production, thereby providing support to the design and operation of novel nitrogen removal systems with NO<inf>2</inf>⁻ accumulation.