Greenhouse gas emissions from on-site wastewater treatment systems

Abstract

Domestic wastewater is rich in carbon and nitrogen and given the biogeochemical transformations that these compounds undergo during different treatment processes can be significant sources of greenhouse gas (GHG) emissions. However, most research focus in relation to GHG emissions and wastewater treatment has been on large-scale treatment units with only a small number of studies carried out directly on domestic on-site wastewater treatment systems (DWWTSs) in the field. In the Republic of Ireland, around 500,000 dwellings rely on on-site treatment and disposal of their wastewater, 87 % of which use septic tanks (STs) followed by some form of soil treatment unit (STU). Considering the large number of DWWTSs in Ireland, as well more internationally, septic systems are potential significant sources of GHG emissions.

The main aim of this research, therefore, was to develop a better understanding of the contribution of the DWWTSs to the GHG inventories. Four DWWTSs were investigated during this study. One site had a simple one chamber ST with a soakaway, which is common of many older systems in existence, Site 1. Two sites were recently constructed following the latest Irish EPA standards including a two-chamber ST, and/or a packaged secondary treatment unit discharging effluent into an engineered STU, Site 2 and 3. The fourth site had a two-chamber ST followed by a willow-based evapotranspiration system, Site 4. In the STs as well as over the STUs and evapotranspiration systems, discrete and long-term CO 2 and CH 4 flux measurements were carried out using automated flux chambers and a closed-transient measurement approach. In addition discrete samples were taken for laboratory analysis of N 2 O.

The soil of the soakaway (Site 1) consumed overall 0.03 kg CO 2Eq. yr −1 less CH 4 and emitted 7.3 kg CO 2 yr −1 more CO 2 than a similarly sized area of control soil. The net GHG emissions were compared from different stages (i.e. ST and STU) of two recently constructed DWWTSs as well as how inclusion of up-front packaged secondary treatment units impact on the net emissions from STU (Site 2 and 3). The total net emissions from the systems were 17.0 and 21.9 kg CO 2Eq. cap −1 yr −1 at the two different sites, respectively. Over 80 % of the total net emissions was in the form of CO 2 , around 15 % in CH 4 and less than 2 % in N 2 O. GHG fluxes had strong spatial and seasonal variation from the willow bed, Site 4.