Temporal triggers of N₂O emissions during cyclical and seasonal variations of a full-scale sequencing batch reactor treating municipal wastewater

Highlights

• N₂O emissions from a full-scale SBR in WWTP were monitored.

• N₂O emissions fluctuated with cycling in a SBR and the seasons.

• RF model identified the sOUR-ratio as the most influential trigger of N₂O emissions.

• N₂O emissions may be independent of the microbial communities of AOB and NOB.

Abstract

To investigate the major triggers of nitrous oxide (N₂O) production in a full-scale wastewater treatment plant, N₂O emissions and wastewater characteristics (ammonia, nitrite, nitrate, total nitrogen, dissolved inorganic carbon, dissolved organic carbon, pH, temperature, dissolved oxygen and specific oxygen uptake rate), the results of variations in the cycling of a sequential batch reactor (SBR, where only full nitrification was performed), were monitored seasonally for 16 months. Major triggers of N₂O production were investigated based on a seasonal measured database using a random forest (RF) model and sensitivity analysis, which was applied to identify important input variables. As the result of seasonal monitoring in the full-scale SBR, the N₂O emission factor relative to daily total nitrogen removal ranged from 0.05 to 2.68%, corresponding to a range of N₂O production rate from 0.02 to 0.70 kg-N/day. Results from the RF model and sensitivity analysis revealed that emissions during nitrification were directly or indirectly related to nitrite accumulation, temperature, ammonia loading rate and the specific oxygen uptake rate ratio between ammonia oxidizing bacteria and nitrite oxidizing bacteria (sOUR-ratio). However, changes in the microbial community did not significantly impact N₂O emissions. Based on these results, the sOUR-ratio could represent the major trigger for N₂O emission in a full-scale BNR system: a higher sOUR-ratio value with an average of 3.13 ± 0.23 was linked to a higher N₂O production rate with an average value of 1.27 ± 0.12 kg-N/day (corresponding to 3.96 ± 1.20% of N₂O emission factor relative to daily TN removal), while a lower sOUR-ratio with an average value of 2.39 ± 0.27 was correlated with a lower N₂O production average rate of 0.17 ± 0.11 kg-N/day (corresponding to 0.74 ± 0.69% of N₂O emission factor) (p-value = 0.00001, Mann-Whitney test).

Introduction

Although emissions of total N₂O, one of the principal greenhouse gases, have decreased by 15.3 MMT CO₂ Eq. from 2005 to 2017 in the U.S. (Hockstad and Hanel, 2018), N₂O emissions from WWTPs have increased by 0.6 MMT CO₂ Eq. and are responsible for the higher inflow of nitrogen species to wastewaters, reflecting an increase in both population and protein consumption (Yan et al., 2019). To minimize N₂O emissions from WWTPs, studies about the mechanisms and influential factors contributing to N₂O emissions are ongoing, targeting either nitrification or denitrification or both in BNR processes known to be N₂O producers (Wunderlin et al., 2012).

In a BNR process, N₂O as the major end product during denitrification is produced through an intermediate metabolite by inactivation of N₂O reductase in heterotrophic denitrifiers (Schreiber et al., 2012). During oxidation of NH₄⁺ by AOB, it has been shown that more N₂O can be produced from nitrification of WWTPs than from denitrification via two main pathways (Rassamee et al., 2011; Wunderlin et al., 2012): oxidation of NH₂OH or denitrification of nitrifiers or both. Based on the current knowledge of N₂O production pathways during nitrification, influential factors triggering N₂O production have been identified through lab-scale experiments using pure or mixed culture biomass under limited conditions (Wunderlin et al., 2013; Yu et al., 2018). High ammonia loading (substrate of AOB) or nitrite accumulation (byproduct of AOB) or both in the nitrifying zones have shown positive correlations with higher N₂O production (Castro-Barros et al., 2016; Wunderlin et al., 2012). Abrupt changes in some variables − DO and pH responsible for nitrification − can also significantly affect N₂O formation (He et al., 2017; Su et al., 2019). Effects of these key variables on N₂O production in the metabolism of AOB are closely linked with the AOR of AOB: AOR under optimal DO and pH conditions can be promoted, leading to NO₂⁻ accumulation causing higher N₂O production (Ribera-Guardia and Pijuan, 2017). Also, higher NH₄⁺ loading can accelerate AOR, resulting in increased N₂O production by promotion of NH₂OH oxidation with accumulation of NO₂⁻ (Law et al., 2012a; Yu et al., 2010). Furthermore, NH₄⁺ loading on AOB with a high metabolic rate increases N₂O emissions during nitrification due to the differential recovery of AOB and NOB via inhibition (such as shock by hexavalent chromium) (Kim et al., 2016).

Promotion of N₂O production during nitrification can be attributed to various influential factors as well as enzymatic reactions of AOB. Compared to lab-scale experiments under limited conditions, N₂O production from full-scale WWTPs is temporally and spatially variable since the following factors can occur simultaneously, dynamically and beyond the operators' control: aeration rate, DO, pH, NH₄⁺ loading, etc. (Vasilaki et al., 2019). Moreover, different methods for real-time monitoring of N₂O, diverse configurations of BNR systems, or differences in microbial communities configured in systems under specific operational/environmental conditions can also affect the amounts, characteristics or factors of N₂O production of full-scale WWTPs (Ahn et al., 2010a; Vieira et al., 2019). Thus, identifying the causes of N₂O production in full-scale WWTPs is challenging, requiring diverse full-scale studies. In particular, there has been little research about the seasonal and diurnal dynamics of N₂O emissions from full-scale WWTPs (Brotto et al., 2015; Daelman et al., 2015). The current full-scale study attempted to identify the primary factors influencing seasonal variations in N₂O emissions. In addition, even when long-term monitoring has been performed, deeper data analysis showing the correlation between N₂O emissions based on process operating conditions and triggers has been lacking and could be utilized to minimize N₂O emissions through improving water quality or achieving energy-efficiency in wastewater treatment. A full-scale SBR treating domestic wastewater was selected for seasonal monitoring. In this targeted system, only nitrification without coupling with denitrification occurs by maintaining high levels of DO in the reactors. The seasonal database of N₂O emissions from variations in cycling of the SBR was investigated and correlated with measured operational factors using a RF model. Microbial communities in the SBR were analyzed to identify their putative role linked with N₂O emissions via 16S high-throughput sequencing.