Carbon footprint of Finnish wastewater treatment plants

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Insinööritieteiden korkeakoulu | Master's thesis
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Master's Programme in Water and Environmental Engineering (WAT)
84 + 8
Waste and wastewater treatment currently constitute approximately five percent of global greenhouse gas (GHG) emissions. Mitigating nation-wide GHG emissions requires collaboration between municipalities in reducing their carbon footprints. Since wastewater treatment plants (WWTPs) are essential municipal infrastructure, knowledge of their GHG emissions is important for assessing part of the total carbon footprint of municipalities. The aim of this thesis was to evaluate the carbon footprint of four WWTPs in Finland and provide recommendations for the sites in this study to reduce their GHG emissions. The processes included in the carbon footprint boundary were the treatment of wastewater, treatment of sludge and screenings, transport of chemicals, transport of screenings and sludge, as well as the energy and chemical consumption of the plant. The emissions from the wastewater network and disposal of sludge were excluded from the boundaries in this study. Carbon footprint results ranged from 45 – 82 kg CO2eq PE-1 year-1. Total emissions were dominated by fugitive emissions from wastewater and sludge treatment, with an average of 72 % of total carbon dioxide equivalent emissions (CO2eq). Energy consumption emissions were the second highest with an average of 19 %, followed by chemical consumption emissions with 7 % of total CO2eq emissions. Transport emissions constituted a minor share of 1 % of total CO2eq emissions. N2O emissions from wastewater treatment constituted an average of 59 % of total GHG emissions. Due to the high global warming potential of N2O, results showed high sensitivity of the carbon footprint to the N2O emission factor used. Based on the results, it was concluded that WWTPs with higher energy efficiencies or low indirect emissions would have higher shares and sensitivities to N2O emission factors. Due to the high share of direct emissions (particularly N2O), it is recommended that WWTPs in Finland evaluating their carbon footprint should invest in on-site measurement campaigns of fugitive N2O emissions of at least one year. This would reduce the under/overestimation of on-site emissions. In addition, it is also recommended that WWTPs monitor process-specific electricity consumptions which would help them identify energy-intensive processes on-site.
Mikola, Anna
Thesis advisor
Haimi, Henri
Merta, Elina
carbon footprint, wastewater treatment, greenhouse gases, climate change
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