Characterizing N₂O emissions from WWTPs

The impact of plant size, reactor operation and aeration systems

Shared with permission from DHI and Fors.
Authors: Trine Dalkvist, DHI, Rune Popp, DHI, Niels Eisum, Hugin Consult, Fabio Polesel, DHI, Mette Guldborg Hansen, Fors, Henrik Correll, Fors, Peter Andreasen, DHI

Introduction

Background

Nitrous oxide (N₂O) is a well known greenhouse gas produced and released in the biological sections of wastewater treatment plants (WWTPs)
Due to the significant contribution to the carbon footprint of WWTPs, various attempts are currently being made to monitor and minimize N₂O emissions, also through dedicated regulation
Multiple studies have addressed N₂O emissions from full scale WWTPs employing different treatment technologies and operational modes focusing on medium and large sized WWTPs
Considerably less information is available for small sized WWTPs (< 20,000 PE), which in Denmark represent 16% to the overall treatment capacity

Objectives

The objective of the study was to evaluate the importance on N₂O emissions from both main and sidestream treatment processes of

WWTP size and/or capacity
Operational mode of bioreactors (continuous flow, sequencing batch)
Aeration systems (bottom and surface aerators)
Temporary changes and transient conditions in the operation of WWTPs

Methods

A measuring campaign was conducted to monitor N₂O emissions from three different WWTPs managed by the same utility (Fors A/S, Denmark)

When: June to September 2021 for a period of 14 days in each WWTPs

How: Two N₂O Wastewater Sensors from Unisense Environment were used to measure N₂O concentrations in the water phase and emissions to the gas phase were estimated based on the supplier’s recommendations. A transportable sensor setup was employed, with a monitoring suitcase containing a mini pc that could be accessed remotely (Figure 1). Data was collected and stored in DIMS CORE (DHI A/S, Denmark) installed on the mini pc to avoid setup in SCADA. N₂O emissions were monitored in three municipal WWTPs.

Characterizing N2O emissions from WWTPs figure 1

Where (Figure 2):

Bjergmarken WWTP (125,000 PE) includes biological treatment with BioDenipho^TM configuration. N₂O sensors were placed in two aerated tanks (LT2 and LT3) of parallel lines.
Holbæk WWTP (60,000 PE) includes five parallel treatment lines operated in sequencing-batch reactor (SBR) mode with alternating anoxic and aerated phases and sidestream treatment of reject water with ANITA^TM Mox. N₂O sensors were placed in two parallel sequencing batch reactors (SBR4 and SBR5) and in the ANITA^TM Mox reactor.
Hvalsø WWTP (11,570 PE) and includes biological treatment with pre-denitrification and nitrification, whereby oxygen is supplied through surface aerators. N₂O sensors were placed before and after surface aerators.

Characterizing N2O emissions from WWTPs figure 2

Results & Discussion

Bjergmarken WWTP

High variability in N₂O emissions was observed during the monitoring campaign (Figure 3). Very high N₂O emissions were measured in the first part of the monitoring campaign and were associated to temporary changes in process operation (namely inlet pumping, and aeration set points). After stable operation was achieved, short periods of elevated N₂O emissions could still be detected. Overall, elevated emissions were observed in less than 10 of the monitoring time, leading to significant differences in emission factors calculated by considering (0.8% N₂O-N/N removed) and neglecting (0.4%) unusually high emissions.

Characterizing N2O emissions from WWTPs_fig3_1400x548

Holbæk WWTP

N₂O emissions in the two SBR tanks differed by a factor 4. Phase length can result in uneven load concentrations in the tanks and can thus lead to diverting N₂O emissions. Strategies of load equalization could minimize emissions. Interestingly, low N₂O emissions from ANITA^TM Mox were observed (0.7-0.8%) as compared to other reject water treatment systems (e.g., 5.5% for DEMON). Continuous aeration and inflow, together with the use of biofilm systems, can be thus hypothesized as strategies for emission reduction in sidestream treatment.

Hvalsø WWTP

Monitoring results indicated considerably low N₂O emissions (0.00005% N₂O-N/N removed). This observation is of relevance to assess strategies supporting centralized treatment in medium and large sized WWTPs.

The overall results from the monitoring campaign, including calculated N₂O emission factors for main and sidestream processes in the three WWTPs, are summarized in Table 1.

Characterizing N2O emissions from WWTPs table 1

Conclusions

The small sized WWTP showed very low N₂O emissions as compared to the other WWTPs investigated. While it may not be sufficient to draw definitive conclusions, this finding seems to indicate that small sized WWTPs are overall low contributors to greenhouse gas emissions.

N₂O emissions from WWTPs showed considerable temporal and spatial variability highlighting the need for detailed monitoring and supporting the refinement of emission factor calculation methods.

Transient periods with anomalies in influent loading and changes in WWTP operation (including equipment malfunctioning) may lead to increased N₂O emissions and should not be neglected in the carbon footprint evaluation of a WWTP.

While long term measurements are certainly beneficial, target monitoring during shorter periods can be a cost effective strategy to evaluate emissions in multiple locations and identify underlying critical factors.