Carbon emission calculation of three biological nitrogen removal processes: A case of rare earth element tailings wastewater in Southern Jiangxi, China

Global warming has been caused by greenhouse gases (GHGs) released by human activity. Traditional sewage treatment uses much energy to achieve discharge limits in the water treatment industry, producing many GHG emissions. In this study, the emission factor approach and mass balance method are used to calculate the carbon emission equivalent of nitrification and denitrification, anaerobic ammonia oxidation, and microalgae assimilation using a rare earth tailings (REEs) wastewater example. The results show that, among the three denitrification processes, the traditional nitrification and denitrification method has the greatest carbon emission equivalent (16.23 kgCO2/kgN). Direct causes include N2O emissions, which account for 48 % of the total carbon emission equivalent. The anaerobic ammonia oxidation process produces the carbon dioxide equivalent of 0.02 kgCO2/kgN. Carbon emission comparable to microalgal assimilation is −3.23 kgCO2/kgN. The anaerobic ammonia oxidation process can thus greatly reduce carbon emissions when compared to the conventional nitrification and denitrification processes, whilst microalgae assimilation can result in negative carbon emissions.