Evaluation of greenhouse gas emissions from the European urban wastewater sector, and options for their reduction

We present an assessment of greenhouse gas emissions from urban wastewater treatment plants in Europe. We propose a quantification in terms of emission factors (kilograms of carbon dioxide equivalents (CO(2)e) per population equivalent (PE) and year) taking into account all phases of wastewater treatment, from the construction of the infrastructure to the discharge of treated effluents. The assessment includes (1) life-cycle emissions of infrastructure; (2) emissions of dissolved methane in the sewer networks; (3) direct emissions of nitrous oxide and methane from the treatment processes; (4) emissions due to COD and nitrogen in the effluents; (5) indirect emissions due to the generation of electricity and the production of reagents; and (6) emission credits due to energy recovery or biomethane export associated with the anaerobic digestion of sludge. Our estimated emissions range between approximately 50 and 125 kg CO(2)e/PE/y depending on the type of treatment plant, of which about 20 to 40 are embedded in the infrastructure. We estimate that direct nitrous oxide emissions and indirect electricity emissions are the main contributors in the operation phase, followed by direct methane emissions. By extrapolating these emissions to the ensemble of the European Union's wastewater treatment plants, we estimate a cumulative emission of about 35 million tonnes CO(2)e/year, of which ca. 14 are due to the infrastructure. We analyse various scenarios to reduce emissions, showing that the efficient use of electricity at the plant and the decarbonisation of electricity would significantly help to improve the CO(2)e footprint of the WWTPs. In particular, the recovery of methane from biogas and the decarbonisation of electricity may reduce emissions below 27 million tonnes CO(2)e/year. Extending N removal to the whole territory for all plants above 10,000 PE may contribute to decrease direct nitrous oxide emissions.