Partial Denitrification and Nitrous Oxide Dynamics Across Carbon Sources

[b]Introduction[/b] Nitrous oxide (N[sub]2[/sub]O) is a potent greenhouse gas (GHG), making its mitigation a critical priority in efforts to combat climate change. Water Reclamation Facilities are recognized as a major source of N[sub]2[/sub]O emissions, particularly during biological nutrient removal processes (Daelman et al., 2013). Effective mitigation strategies focus on reducing N[sub]2[/sub]O carryover from anoxic denitrification zones into highly aerated zones where stripping occurs. Since nitrite is a key precursor for N[sub]2[/sub]O formation, understanding the mechanisms of N[sub]2[/sub]O production during denitrification, especially partial denitrification, is essential as the industry advances toward more resource-efficient treatment processes. [b]Objective and Novelty[/b] Research on N[sub]2[/sub]O dynamics during partial denitrification, especially the role of different carbon sources, is limited. This study addresses that gap through the following objectives: •[i]N₂O production during partial denitrification[/i]: Investigate the pattern of N₂O production under partial denitrification using various carbon types. •[i]Investigation N₂O mitigation strategy[/i]: Assess the feasibility of employing alternative carbon sources to sustain denitrification while minimizing N₂O emissions. [b]Methods[/b] A 100 gallon sequencing batch reactor (SBR) was operated at the Hayward WPCF using low strength trickling filter effluent. The reactor ran with a 12 hour HRT under intermittent aeration. The aerobic SRT was gradually reduced from 8.3 to 6 to 4 days. Carbon was added at the start of each anoxic phase to support denitrification. At 4 day aeSRT, two carbon sources were tested: MicroC®2000 and a synthetic fermentate (SF) containing 50% acetate, 30% propionate, and 20% butyrate. With SF, both low and high COD:NOx conditions were evaluated. After ~3 weeks of acclimation for each carbon source, in situ denitrification batch tests were performed to assess denitrification rates and N₂O dynamics. Under high COD:NOx with SF, individual VFAs were also tested. Dissolved N₂O was measured using a Unisense sensor. [b]Results and discussion[/b] [i]Higher N₂O with MicroC[/i] Batch denitrification assays indicated partial denitrification with both MicroC and SF as carbon sources showed that nitrite accumulation increased with higher COD:NOx dosing ratios (Figure 1). It was observed that the ecology acclimated to higher COD:NOx dosing exhibited lower nitrite accumulation. However, parallel bench-scale experiments during the same period of reactor operation showed nitrite accumulation as high as 65 %, highlighting variability in the efficiency of nitrite accumulation. Figure 2 shows typical nitrate, nitrite, and N₂O profiles during denitrification testing at 4 day aeSRT with both carbon sources dosed at a COD:NOx of 6:1. While both carbon types produced partial denitrification, their N₂O patterns differed: MicroC led to continued N₂O buildup until NOx was fully depleted, whereas SF resulted in minimal, short lived N₂O accumulation, indicating simultaneous NOx and N₂O reduction. [i]VFA fractionation in fermentate impacts nitrite accumulation during denitrification[/i] Denitrification assays using SF acclimated biomass were conducted to evaluate partial denitrification efficiency with individual VFAs. NO2 accumulation differed across carbon sources (Figure 3). Propionate exhibited the least NO₂ buildup, consistent with previous findings that link PHV synthesis to reduced NO₂ compared to PHA derived from sodium acetate (Ji et al., 2025). Despite these differences in nitrite accumulation, N₂O concentrations remained largely unchanged, indicating that N[sub]2[/sub]O dynamics were not significantly influenced by the choice of VFA. [i]Switch between carbon sources[/i] Denitrification and nitrite accumulation were evaluated using MicroC and SF after biomass acclimation to the alternate carbon source. When MicroC was added to SF acclimated biomass, denitrification rates were similar, but nitrite accumulation decreased. Conversely, poor denitrification rates were observed when MicroC was added to SF acclimated biomass. These bench scale tests did not measure N[sub]2[/sub]O, but given earlier evidence of simultaneous N₂O reduction with SF, co dosing SF may help facilities relying on MicroC reduce N[sub]2[/sub]O emissions. [b]Benefits and Significance[/b] This research provides a comprehensive assessment of N[sub]2[/sub]O dynamics during anoxic denitrification, offering valuable insights for process optimization and greenhouse gas mitigation in biological nitrogen removal systems. The findings enhance understanding of N[sub]2[/sub]O behavior and inform strategies for sustainable wastewater treatment. Key contributions include: •Comparable partial denitrification efficiency was achieved using both MicroC and synthetic fermentate as carbon sources. •Carbon source selection is critical for N[sub]2[/sub]O mitigation; glycerol-based sources tend to generate higher N2O emissions compared to acetate-rich fermentate. •Partial denitrification depends on VFA composition, with propionate exhibiting lower nitrite accumulation than acetate and butyrate. •Synthetic fermentate demonstrates strong affinity for N[sub]2[/sub]O reduction, and its comparable performance in MicroC-acclimated systems highlights its potential as a co-dosing strategy to reduce N[sub]2[/sub]O emissions.