Assessing the spatial and temporal variability of greenhouse gas emissions from different configurations of on-site wastewater treatment system using discrete and continuous gas flux measurement
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Published:2022-02-18
Issue:4
Volume:19
Page:1067-1085
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ISSN:1726-4189
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Container-title:Biogeosciences
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language:en
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Short-container-title:Biogeosciences
Author:
Knappe JanORCID, Somlai Celia, Gill Laurence W.ORCID
Abstract
Abstract. Global emissions linked to wastewater treatment are estimated to account for up to 1.5 % of total greenhouse gas (GHG) emissions
globally. However, few studies have measured GHG emissions from domestic on-site treatment systems (DWWTSs) directly. In this study, two DWWTSs were
monitored for 446 d and > 42 000 gas flux measurements were conducted using both discrete spot measurements and continuous flux chamber
deployments. The observed GHG fluxes from biological activity in the soil and water phase were found to be highly spatially and temporally variable
and correlated to environmental factors, water usage patterns and system design. In total, the results show that a septic tank discharging effluent
into a well-designed soil treatment unit is estimated to emit a net 9.99 kg-CO2eq.cap-1yr-1, with approximately 63 %,
27 % and 10 % of the total CO2-equivalent net emissions in the form of CO2, CH4 and N2O,
respectively. Emissions from the septic tank surface contributed over 50 % of total emissions and tended to be strongly underestimated by
one-off discrete measurements, especially when episodic ebullitive events are to be considered. Fluxes from the soil treatment unit (STU) stemmed
from both the soil surface and the vent system. Soil fluxes were mostly influenced by temperature but peaked regularly under conditions of rapidly
changing soil water content. Vent fluxes were mostly governed by effluent, quality and a low number of high-emission events were responsible for the
majority of total observed vent emissions. Owing to the strong overall spatial and temporal heterogeneity of observed fluxes from DWWTSs across all
modules, future studies should focus on continuous deployments of a number of flux chambers over discrete measurements to accurately assess GHG
emissions from on-site systems. This study also provided insights into managing GHG emissions from DWWTSs by different system configuration design,
as well as indicating that the current IPCC emission factors for CH4 and N2O significantly overestimate emissions for
on-site wastewater treatment systems.
Funder
Science Foundation Ireland
Publisher
Copernicus GmbH
Subject
Earth-Surface Processes,Ecology, Evolution, Behavior and Systematics
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