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.
Highlights
Overall greenhouse gas (GHG) emissions from the waste and wastewater sector contribute an estimated 2 % to the total national emissions in Ireland (EPA, 2018)
2.5 % of the observations were recorded at the septic tank (ST) surface, 0.5 % at the soil treatment unit (STU) vent system and the remainder over the STU from the soil gas flux measurements, mainly during the automated long-term deployments
The highest measured CO2 flux rates were observed from the STUs at both sites; when these rates were adjusted to account for the background soil emissions, one STU was a relatively high net emitter of CO2 emissions compared to the other STU, which was apparently emitting less CO2 emissions in comparison to the background soil
Summary
Overall greenhouse gas (GHG) emissions from the waste and wastewater sector contribute an estimated 2 % to the total national emissions in Ireland (EPA, 2018). Global emissions linked to wastewater treatment are estimated to account for up to 1.5 % of total GHG and 5 % of non-CO2 GHG emissions and are expected to contribute 42 % to all waste–related GHG emissions by 2030, compared to 36 % in 1990 (Bogner et al, 2008; US EPA, 2012). The quantification of direct GHG emissions from wastewater treatment systems is currently based on the application of estimation methodologies that have been published by the Intergovernmental Panel on Climate Change (IPCC, 2013). National and global estimations are considered highly uncertain as they are based on a limited number of case studies and rely heavily on secondary assumptions such as load-based calculations or emission factors rather than primary data. Knappe et al.: Variability of GHG emissions from on-site treatment systems
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