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.
Highlights
Continuous online monitoring has revealed that N2O emissions from wastewater treatment systems are highly dynamic[8,9]
It is possible that this gradient in biomass concentration would cause further spatial variations in the N2O fluxes, given previous studies have shown that the biomass-specific nitrogen loading rate has a strong influence on N2O production[4,13]
Investigations relied on the grab sampling method, which yielded highly variable N2O emission factors with values ranging between 0.6% to 25% of the influent nitrogen load[3]
Summary
Continuous online monitoring has revealed that N2O emissions from wastewater treatment systems are highly dynamic[8,9]. The overall aim of this study was twofold: 1) to characterise the spatial variation of N2O emissions from a full-scale plug-flow WWTP by developing a novel online method that can sequentially measure N2O concentrations in the off-gas from multiple locations; and 2) to investigate the effect that a step-feed configuration ASR has on N2O emissions. To this end, a comprehensive online monitoring program was undertaken to quantify N2O emissions at multiple sampling locations positioned along the aerobic zones of a plug-flow step-fed reactor. The ammonium and Total Kjeldahl Nitrogen (TKN) concentrations in the influent were measured to calculate the N2O emission factors for the different steps
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