Site-specific carbon footprints of Scandinavian wastewater treatment plants, using the life cycle assessment approach

Abstract

The carbon footprints of seven wastewater treatment plants using different technologies in Denmark and Sweden were evaluated. The life cycle assessment approach was applied by using site-specific data including measured plant-integrated methane and nitrous oxide emissions. Four different functional units were adopted: 1 Mg of input material entering the wastewater treatment plant in 2015, and the removal of 1 kg of carbon, total nitrogen and phosphorus. The net carbon footprint values found in this study were between 0.15 and 0.66 kg CO2 eq (Mg of input material)−1 depending on the treatment facility. Direct greenhouse gas emissions were the main contributors to the carbon footprint, accounting for between 44 and 71% of the total burden. The remaining share of the total burden (66 and 29%) was mainly made by energy consumption, chemicals used, and emissions from effluent and land application of biosolids. Direct greenhouse gas emissions were very sensitive model parameters driving result uncertainties. When default values from emission reporting guidelines were applied instead of measured greenhouse gas emission rates, the net carbon footprint was up to four times smaller or seven times larger. The consumption of electricity from the energy grid for plant operation had a great impact on the carbon footprint, due to differences between the energy systems. The share of electricity consumption to the total carbon footprint burden for the Swedish plants was only 2%, whereas it was between 16 and 28% for the Danish plants. This difference was due to the smaller carbon footprint potential of the electricity mix in Sweden than in Denmark. Normalisation of the carbon footprint to the plant pollution load, and the calculation of model uncertainty, allowed for a reliable comparison of plants operating within the same energy system. Finally, suggestions were provided for performing a sound carbon footprint evaluation of wastewater treatment plants.