Unravelling the spatial variation of nitrous oxide emissions from a step-feed plug-flow full scale wastewater treatment plant

Abstract

Plug-flow activated sludge reactors (ASR) that are step-feed with wastewater are widely adopted in wastewater treatment plants (WWTPs) due to their ability to maximise the use of the organic carbon in wastewater for denitrification. Nitrous oxide (N2O) emissions are expected to vary along these reactors due to pronounced spatial variations in both biomass and substrate concentrations. However, to date, no detailed studies have characterised the impact of the step-feed configuration on emission variability. Here we report on the results from a comprehensive online N2O monitoring campaign, which used multiple gas collection hoods to simultaneously measure emission along the length of a full-scale, step-fed, plug-flow ASR in Australia. The measured N2O fluxes exhibited strong spatial-temporal variation along the reactor path. The step-feed configuration had a substantial influence on the N2O emissions, where the N2O emission factors in sections following the first and second step feed were 0.68% ± 0.09% and 3.5% ± 0.49% of the nitrogen load applied to each section. The relatively high biomass-specific nitrogen loading rate in the second section of the reactor was most likely cause of the high emissions from this section.

A decade of nitrous oxide (N2O) monitoring in full-scale wastewater treatment processes: A critical review

“…The majority of the full-scale N 2 O monitoring campaigns were driven by the need to expand the knowledge on the N 2 O generation in wastewater treatment systems. Several studies have highlighted that it is important to understand the dominant N 2 O generation pathways in full-scale processes and investigate the effect of specific operational conditions triggering specific enzymatic reactions linked with elevated EFs in full-scale biological systems (Bollon et al, 2016a;Pan et al, 2016;Wang et al, 2016b).…”

N2O emission in full-scale wastewater treatment: Proposing a refined monitoring strategy

“…Aboobakar et al (2013) found high spatial variation (0.01-0.1%) in a CAS process; however, this was paired with a low overall emission factor of 0.1% and a short-term emission period (Aboobakar et al, 2013). For a two-step activated sludge process, the spatial distribution was shown to be substantial (0.6-3.5%), with significantly higher emissions in the second step (Pan et al, 2016). This is to be expected, since nitrification occurs predominantly in the second step.…”

“…Measurements were spatially resolved with an automated valve system preceding the dehumidifier and the analyzer. This enables the measurement of the off-gas composition sequentially at several spots within the WWTP, as in the procedure presented by Pan et al (2016). The off-gas sampling location was changed every 4 to 6 minutes.…”

“…Hence, guiding principles for long-term monitoring on open tanks are not yet available. Since many WWTPs in industrialized countries are equipped with open tanks, long-term online monitoring campaigns on uncovered WWTPs are crucial to establish a representative N2O emission factor for greenhouse gas reporting (Pan et al, 2016).…”

“…Another possible approach to monitor the emissions are the application of a measurement for dissolved N2O via i) Clark-Type electrodes (Marques et al, 2014) or ii) stripping of N2O from a sludge sample (Mampaey et al, 2015) combined with an air stripping model (Marques et al, 2014). Due to the potentially high spatial heterogeneity of N2O emissions within a treatment process, measurements at various spots on a WWTP are required to arrive at a representative measurement of the N2O emission factor for an entire WWTP (Aboobakar et al, 2013;Pan et al, 2016). For this purpose, a flux chamber based monitoring approach is more appropriate than a dissolved N2O measurement based monitoring, due to i) significantly higher costs for additional point measurements and ii) that quantifying stripping efficiency with sufficient accuracy is not a simple task at full-scale.…”