Impact of carbon sources and dosing ratios on denitrification and N2O dynamics in a low DO BNR process

Introduction

Nitrous oxide (N2O) is a potent greenhouse gas (GHG), making its emission a critical concern. Wastewater treatment plants (WWTPs) are recognized as notable sources of N2O, particularly in a biological nutrient removal (BNR) process (Daelman et al., 2013).

Low dissolved oxygen (DO) BNR is gaining popularity due to its potential for significant energy savings with improved BNR efficiency. There exists conflicting evidence on N2O emissions in low DO BNR, which undermines its energy saving advantages. Identifying N2O production and developing mitigation strategies are crucial to fully leverage the advantages of low DO BNR.

Objective and Novelty
There is limited research on N2O production pathway across aerobic and anoxic conditions in a low-DO BNR system. This study aims to fill this gap by focusing on the following test objectives:
- N2O production in low DO aerobic zones: Investigate the trends of N2O production under low DO conditions in aerobic zones.
- Investing N2O mitigation strategies in anoxic zones: Identify impact of (a) carbon sources and (b) dosing ratios on N2O production and mitigation.

Methods
A 100 gallons sequencing batch reactor (SBR) fed with trickling filter effluent (TFE) (low C:N, ~2.5) was operated at the Hayward WPCF in Hayward, CA. The SBR cycle was 4.8 hrs long with alternating aerobic and anoxic phases. Carbon was dosed at the start of the anoxic phases. The reactor was commissioned with a an aerobic SRT of 8.3d, and then gradually dropped to 6 days and 4 days. The SBR at 4d aeSRT was operated under two distinct phases with variation in the carbon type (a) using MicroC®2000 (EOSi), and (b) synthetic fermentate (SF). The SF comprised 50% acetate, 30% propionate, and 20% butyrate, based on our previous research (Cecconi et al., 2023).

Comprehensive SBR cycle characterization and batch tests were conducted at each phase of operation. Batch tests included (a) N2O dynamics at different COD:NOx, (b) comparison of denitrification rates with different carbon sources.

Results and discussion
Anoxic N2O detected, not in low DO aerobic zones.

In a representative SBR cycle (Fig 1), we observed an increase in N2O concentration through the anoxic phase, while the N2O concentration dropped through the low DO (0.3 mg/L) aerobic phase. This suggests that anoxic N2O production was significant in our alternating anoxic-aerobic configuration. This is an important note, as it focuses our N2O mitigation efforts on anoxic operation and allows for continued optimization of aerobic operation for low DO setpoints. This low DO benefit with lack of N2O has been highlighted elsewhere (Wen et al., 2020), but this is the first detailed exploration of the interaction in anoxic and aerobic conditions. The magnitude of anoxic N2O varied with the type of carbon used- MicroC®2000 showed higher N2O (maximum 1.1 mg-N/L) compared to SF (maximum 0.27 mg-N/L) (Fig 1).

Carbon choice makes a large difference in N2O production dynamics
During our testing periods, we observed simultaneous reduction of NO3 and accumulation of NO2 at the start, followed by reduction of NO2 during denitrification (Fig 2). However, a distinct difference in the N2O dynamics were observed with different carbon sources. The denitrification with MicroC®2000 showed higher N2O during the experiment compared to denitrification with SF. More importantly, a drop in N2O concentration was only observed after the depletion of inorganic NOx when MicroC was used.

Additionally, the production, reduction, and accumulation of N2O increased with increasing COD:NOx (Fig 3). However, it should be noted that the N2O accumulation lasted only for a brief period with SF, unlike MicroC (Figure 2b). This is a crucial point to consider when using NOx-based control strategies with MicroC as carbon source. It appears that with MicroC, a complete NOx reduction, and beyond, is critical to ensure that the dissolved N2O isn't carried over to the aeration basins where they are likely to get stripped. Unlike MicroC, SF showed less risk of N2O carryover at lower NOx concentrations (Fig 2b).

Further, we observed similar denitrification rates when SF and MicroC were used with the MicroC acclimated biomass (Fig 4). However, the denitrification rate using MicroC on SF acclimated biomass was significantly lower. Our results were similar to those observed in Cherchi et al 2009 that compared MicroC and acetate as carbon sources for denitrification. Nitrite accumulation was notably significant when carbon was introduced to the ecology that had already acclimated to the respective caron type.

Benefits and Significance
This research provides a comprehensive assessment of N2O dynamics across the low DO BNR system. The data and findings from this study will enrich the audience's understanding of N2O in low DO BNR system. Significant findings include:
- Evidence that low DO does not increase N2O production.
- Anoxic zones are the highest risk for N2O production.
- Higher COD:NOx results in increased rates of production, reduction, and accumulation of N2O. Sufficient anoxic HRT is critical to prevent N2O carryover to aeration basins.
- Carbon source selection is critical for N2O mitigation strategies. Glycerol-based carbon source tend to produce more N2O than acetate-rich fermentate.
- MicroC-acclimated sludge readily utilized fermentate, while fermentate-fed sludge showed limited ability to use MicroC, highlighting the significance