Lessons Learned from Nitrous Oxide Monitoring at 15 WRRFs

Objectives
No standard methodologies exist for N2O measurement at water resource recovery facilities (WRRFs) and multiple approaches are being taken by utilities. Sharing experiences, approaches and findings is critical in order to support progressive N2O measurement and mitigation. The objective of this study is to analyse and compare insights from N2O measurement campaigns at 15 WRRFs which have been ongoing since 2022/2023 and to summarise key lessons learned.

Methodology
Nitrous oxide monitoring campaigns were designed for 14 WRRFs (a total of 15 campaigns with full scale and pilot facility monitored at one site; refer Table 1). Methodology considerations which will be discussed in the paper include methodology applied in design of the campaign, specification of measuring methods, the number and location of N2O measurements and methodologies for data analysis and visualisation to obtain meaningful insights into N2O production and opportunities for mitigation.

Findings
Key findings from the 15 campaigns to date with examples will be presented in the paper as ten key learnings with key examples from the WRRF campaigns.
(1) The seasonality of N2O emissions is clearly observed at some facilities but in plug flow systems it is most useful if all tapered aeration zones are monitored as the system nitrification point is known to shift. Figures 1 and 2 show this summer and winter variation in N2O concentrations in location along reactor as well as the difference in magnitude between summer and what appears to be a peak winter period. This highlights the importance of multi-zone N2O measurement.

(2) Short term emissions calculations can yield very different conclusions because of this seasonality. Four discrete periods and EFs were calculated for the WRRF in Figure 2 from available data of sufficient volume/quality.

(3) Nitrous oxide is typically correlated with diurnal ammonia concentration (and load) profiles in settled sewage. Figure 3 shows a moderate correlation of winter time zone 2 N2O (nitrifying ASP, 19MGD) with in-zone ammonia (R = 0.67), previous zone 1 ammonia (R = 0.55) and in-zone dissolved oxygen (DO. R = 0.72) which is often below 1.0mg/L. Sufficient DO and improved DO control may support reduced N2O at this WRRF.

(4) More complex data analysis can offer additional insights and mitigation tools. An innovative data model development and application as described in Figure 4 has been developed and applied at 2 WRRFs (nitrifying ASP, 166MGD and nitrifying ASP + MABR, 2MGD). This includes use of measured and simulated model inputs. It identifies key operational parameters that have the highest importance on the measured N2O peaks within the given dataset which may allow meaningful mitigation interventions and future online effluent energy and N2O optimisation.

(5) To quantify N2O mass emissions, accurate air flow data is key — including accuracy of installed mass flow meters and assumptions about individual aeration zone distribution, particularly where flow data is lacking. Calibration of air flow meters is recommended as significant over or underestimates of N2O can otherwise result (Figure 5).

(6) Additional measurement sensors may be required to draw useful N2O insights — for example when monitoring carbonaceous facilities which may unintentionally nitrify in which N is not measured or to understand in-zone nitrogen species variation with N2O. In lane profiling is an innovative approach which may support production and emission understanding in plug flow systems (Figure 6; pilot concept this is a completely mixed reactor).

(7) Quantification at some WRRF typologies is challenging — these include coarse air bubble systems such as biological aerated filters (BAFs) (Figure 7) and trickling filters. For MABRs both liquid phase and off-gas measurement (both in lumen and from bulk reactor during MABR scouring and in any downstream aerated tanks) is key to provide whole-system N2O quantification.

(8) Site-level N2O campaigns may also support WRRFs and utilities. Whilst also offering a short term snap shot only, site-wide N2O quantification is useful for typologies which cannot be measured with in-process sensors and to allow an estimate of site-wide emissions across all bioreactors/lanes as well as prioritisation across multiple WRRFs in the event of limited resources (Figure 8).

(9) Sidestream treatment processes can be extremely high N2O sources due to their designed operational conditions which have been shown to pose risks for N2O. These include high ammonia loadings, intentional nitrite production, reliance on external carbon dosing and high process rates and conditions. EF of 8% N2O_N/TNin was calculated during initial 3 month campaign at a liquor treatment plant (Figure 9) which is now considering N stripping and N2O destruction options in funded mitigation work.

(10) Regular calibration of liquid phase sensors at recommended 2 monthly or +-/ 3 degrees Celsius is important for quality data. Calibration records provide an important audit trail, as utilities move towards facility level quantification and reporting of N2O.

Conclusions and significance
Ten key lessons learned from 15 WRRF ongoing N2O measurement campaigns illustrate the challenges and opportunities for utilities to collaborate for shared learning, development of standard measurement approaches and most effective mitigation.