New model for evaluating greenhouse gas emissions from sludge treatment based on fossil and biogenic carbon migration

The treatment and disposal of massive amounts of sewage sludge are potential emission sources of greenhouse gases (GHGs) which strongly depend on the migration of fossil and biogenic carbon into GHGs. Owing to the lack of fossil carbon data and suitable estimation models, CO2 emissions generated from sludge combustion and biodegradation are considered carbon-neutral according to the Intergovernmental Panel on Climate Change (IPCC), increasing the inaccuracy of GHG emissions accounting. Therefore, this study aimed to build a new comprehensive assessment model to accurately evaluate GHG emissions from four typical sludge treatment and disposal approaches. Notably, fossil and biogenic carbon migration were considered, rather than the emission factor method when measuring direct GHG emissions. The results of 14C analysis, which quantified the contributions of fossil and biogenic carbon to the sludge samples, showed significant differences in the average fossil carbon fractions among the four types of sludge: 84.11% for chemical sludge, 25.99% for tannery sludge, 79.19% for textile dyeing sludge, and 45.97% for municipal sludge. According to our model, the route of safely landfilling deep-dewatered sludge possessed the highest GHG emission factors of 1.18–3.75 tCO2eq/t dry sludge. The optimal sludge treatment option was anaerobic digestion followed by land application. This treatment maintained a low and stable level of GHG emissions (0.48–0.54 tCO2eq/t dry sludge) and exhibited low sensitivity to changes in fossil carbon fraction and carbon content according to the sensitivity analysis results. For sludge with a high calorific value and a low fossil carbon fraction, the route of dry incineration + utilization of ash and slag as building materials was carbon-negative and more suitable than anaerobic digestion to achieve carbon neutrality. This study complements the fossil carbon data gaps of sludge in the IPCC Guidelines and provides a more accurate GHG emissions accounting model, which will support the sludge treatment industry in developing effective strategies to reduce GHG emissions.