Improving structural stability and maturity of sludge–MSW compost via controlled lime amendment in in-vessel systems

• Lime addition improved compost structure in all sludge–MSW mixtures. • 2 % CaO was generally most effective in enhancing compost performance. • Moisture, WHC, BD, porosity, and FAS improved with lime treatment. • Humic acid content increased across all lime-amended composts. • Lime enhanced compost maturity, including odor and color stabilization. Co-composting of sewage sludge and the organic fraction of municipal solid waste (MSW) offers a sustainable solution for biowaste management. This study assessed the effects of quicklime (CaO) addition on the physical properties of compost produced from MSW and primary sludge (MSW-PS), secondary sludge (MSW-SS), and a 1:1 blend (MSW-MS) in an in-vessel system over 40 days. Lime was applied at 0 %, 1 %, 2 %, and 3 % (w/w). Lime amendment, especially at 2 %, significantly improved compost properties. Final moisture content decreased to 45.3 % (MSW-PS), 47.0 % (MSW-SS), and 45.1 % (MSW-MS), compared to >55 % in controls. Wet bulk density reached 595.8 kg/m³ (MSW-PS), 592.7 kg/m³ (MSW-SS), and 609.3 kg/m³ (MSW-MS), while dry density exceeded 410 kg/m³ in all lime-treated groups. Porosity and free air space stabilized at ∼50 % and ∼30 %, respectively, in 1–2 % treatments. Water holding capacity improved to 4.2 (MSW-PS), 3.8 (MSW-SS), and 4.05 g water/g ds (MSW-MS) with 2 % lime. Humic acid content, indicating humification and maturity, increased to 6.0 % (MSW-PS), 5.17 % (MSW-SS), and 5.1 % (MSW-MS). Additional chemical parameters (pH, C/N ratio, electrical conductivity, ash, and cation exchange capacity also improved under lime treatments, confirming enhanced compost stability. These findings demonstrate that moderate lime addition improves both physical structure and chemical maturity, supporting the production of a high-quality, stable compost suitable for land application.