From lime stabilization to CO₂ sequestration: spontaneous recarbonation in municipal sewage sludge

Lime treatment is widely applied for stabilizing and conditioning municipal sewage sludge to improve handling and hygienic properties. It also offers the potential for carbon dioxide (CO₂) sequestration through lime recarbonation. Thereby, atmospheric CO₂ reacts with calcium hydroxide (Ca(OH)₂) to form calcium carbonate (CaCO₃), partly or fully offsetting process CO₂ emissions generated during lime production. However, its extent and rate are largely unexplored. We investigated the spontaneous recarbonation of lime-treated sewage sludge using samples from two full-scale wastewater treatment plants (WWTPs). WWTP1 applies hydrated lime prior to dewatering, while WWTP2 adds quicklime after dewatering. Samples were incubated under controlled conditions for six months and analyzed using thermogravimetry-mass spectrometry (TGA-MS), X-ray diffraction (XRD), and bulk element determinations. Progressive transformation of Ca(OH)₂ into CaCO₃ confirmed spontaneous recarbonation. Within four months, WWTP1 samples reached near-complete carbonation with rates approaching 100 % of the calcination-related process CO₂ emissions. By contrast, WWTP2 samples exhibited lower rates of recarbonation, with a maximum of ∼94 %, likely due to heterogeneous lime distribution, and encapsulation of unreacted lime along with differences in lime quality. These findings provide the first conclusive evidence for substantial spontaneous recarbonation of lime-treated sewage sludge under ambient conditions. They demonstrate that CO₂ uptake is strongly influenced by lime type, dosing strategy, and sludge composition. This establishes a robust basis for integrating lime recarbonation into carbon accounting frameworks and for evaluating the long-term carbon sink potential of lime-treated sludge, while fuel-related emissions from lime production remain outside the scope of this study.