Abstract
The newly revised guidance from the 2019 Intergovernmental Panel on Climate Change (IPCC) encourages assessment of fossil carbon in wastewater treatment plants (WWTPs). In this study, liquid scintillation counting was used to measure the fossil and biogenic carbon in solid wastewater samples obtained from two WWTPs, one with (WWTP A) and one without (WWTP B) a digestion process. The results were compared with those obtained from accelerator mass spectrometry measurements. Greenhouse gas (GHG) emissions under two IPCC scenarios were also compared, an earlier version and a revised version without and with consideration of fossil CO2. The results showed that fossil carbon accounted for 3 – 10 % of the total carbon in mixed sludge. Mixed sludge contributed 50 – 75 % of the fossil carbon total input, with 100 % of the biogenic carbon total input in sewage sludge treatment processes. In the digestion process of WWTP A, the fossil carbon contribution from biogas was nearly 0 %, about 15 % in digested sludge. Only a small amount of fossil carbon decomposes during the anaerobic digestion process. The calculated GHG emissions based on the IPCC’s earlier and revised scenarios differed by 1,100 ~ 1,800 kg CO2-eq/d, accounting for 36 – 65 % of the GHG emissions in sewage sludge treatment processes. The effect of fossil carbon on GHG emissions in sewage sludge treatment cannot be ignored and should be studied further.
