Underestimated Greenhouse Gas Emissions from Sludge Treatment Processes in WWTPs

The carbon footprint of sludge treatment and disposal is typically studied concerning indirect emission sources and fugitive emissions from anaerobic digestion. Whereas greenhouse gas (GHG) emissions from other sludge treatment units in wastewater treatment plants are often overlooked. In this study, we employed an integrated approach using high-resolution trace gas spectroscopy and a steady-state mass balance method to quantify the GHG emission intensities of the frequently neglected mechanical sludge treatment processes. The results demonstrated that these previously underreported GHG emissions are substantial, with their carbon equivalence being comparable to that of known sources in sludge treatment. Notably, centrifugal dewatering was identified as the predominant source of GHG emissions. Moreover, overall GHG emissions are higher in winter (1.80 × 10^-2 kgCO₂eq/kg_TSS) than in summer (1.33 × 10^-2 kgCO₂eq/kg_TSS). The GHG emissions during sludge treatment exhibited distinct pathways: methane was released both in situ (driven by biological activity) and ex situ (induced by mechanical perturbation), while nitrous oxide production primarily originated from in situ biological processes. This study enhances the understanding of GHG emissions during sludge treatment and establishes a tiered accounting framework for sludge carbon emission, thereby facilitating low-carbon sludge treatment.