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

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.

Field-based methods for measuring greenhouse gases emissions from on-site sanitation systems: A systematic review of published literature

“…One out of eight studies measured GHG emissions from the septic tank (ST) and soil treatment unit (STU) [ 25 ]. Three studies have measured GHG emissions from STU [ [25] , [26] , [27] ]. Two studies have evaluated the emission from STs [ 28 , 29 ].…”

“…Indeed, the FC methods were found to be the most prominently field-based method used. FCs can also be an automated static cylindrical chamber used to determine the emissions from the ST and STU [ 20 , [25] , [26] , [27] ]. Most of the studies have preferred static chambers.…”

“…For the vent and the clean-out port, different modified and fitted equipment are connected to anemometers and thermometers to also assess gas flow rates. The body of the typical flux chamber, used for ST or STU, is constructed from Polyvinyl Chloride (PVC) with a cylindrical shape – i.e., diameter of 0.15–0.3 m [ 18 , 20 , 25 , 29 ]. The cap used to cover the chamber is also made from PVC and fitted with 3/18 inches brass Swagelok® connectors for the connection of the sampling tube with rubber septum [ 18 , 20 , 30 ].…”

“…The insert was permanently fixed to 5 cm on the soil to stabilize the FC for STU. Inserts used in the tanks were 10 inches in diameter with PVC legs to support the insert [ 18 , 25 , 30 ]. Unlike static chambers, only one study has shown the use of floating chambers connected to the sample tube and pressure equilibrium pipes.…”

“…The samples were stored either in pre-evacuated vials, a Teflon bag or a large foil sampling bag to safely transport the gas samples to the laboratory for further analysis. In three among eight studies that used the automated chambers, a similar system was found to be deployed for hourly measurement and a 3-min measurement period using portable gas samplers [ [25] , [26] , [27] ].…”