Assessment of CH4 and CO2 Emissions from a Gas Collection System of a Regional Non-Hazardous Waste Landfill, Harmanli, Bulgaria, Using the Interrupted Time Series ARMA Model

Abstract

Municipal solid waste (MSW) landfills are among the major sources of greenhouse gas (GHG) emissions affecting global warming and the Earth’s climate. In Bulgaria, 53 regional non-hazardous waste landfills (RNHWL) are in operation, which necessitates conducting studies to determine the environmental risk from the emitted GHGs. This study attempted to assess the CH4 and CO2 emissions from three gas wells of a cell (in active and closed phases, each of 2.5 years duration) in an RNHWL, Harmanli (41°54′24.29″ N; 25°53′45.17″ E), based on monthly in situ measurements by portable equipment, using the Interrupted Time Series (ITS) ARMA model. The obtained results showed a significant variation of the CH4 and CO2 concentrations (2.06–15.1% v/v) and of the CH4 and CO2 emission rates (172.81–1762.76 kg/y) by gas wells (GWs), months and years, indicating the dynamics of the biodegradation of the deposited waste in the areas of the three GWs. Throughout most of the monitoring period (2018–2022), the CH4 concentrations were higher than the CO2 concentrations (% v/v), while CO2 emissions were lower than CH4 emissions (kg/y), a fact that could be explained by the differences in the mass of the two gases. The emissions rates of both gases from GW2 dominated over those from GW1 and GW3, giving a reason to determine the zone of GW2 as a hotspot of Cell-1. On the whole, CH4 and CO2 emission rates were higher in the winter (December–February) and partly in the spring (March–May) compared to summer–autumn (June–November). However, the CH4 and CO2 concentrations and emissions decreased drastically after the Cell-1 closure. The CH4/CO2 ratio (0.68–2.01) by months and gas wells demonstrated a great sensitivity, making it a suitable indicator for the assessment of organic waste biodegradation level in the landfills. The ITS ARMA model confirmed the negative and significant effect of the cell closure on CH4 and CO2 emissions; the correlations found between predicted and observed values were strong and positive (0.739–0.896).