Biowaste Moisture as a Regulator of Carbon Monoxide Formation During Composting: Analytical and Microstructural Insights Toward Sustainable Waste Valorization

Rising industrial demand for carbon monoxide (CO) motivates the development of sustainable pathways for its production. Composting has recently emerged as a potential biogenic CO source, yet the role of biowaste moisture in CO production has remained unquantified. In this study, the moisture dependence of CO generation during composting was assessed to address this knowledge gap. Laboratory-scale biowaste composting was conducted under mesophilic conditions (45 °C) with passive aeration for the initial 14-day phase, using three initial moisture levels: 31.6% (variant M100), 21.6% (M90), and 12.6% (M80), and periodic H 2 O addition in M100 and M90. Monitoring of CO, CO 2 , and O 2 concentrations, complemented by scanning electron microscopy of composts, revealed a non-monotonic moisture effect on CO formation. The intermediate-moisture treatment (M90; ~41–50%) was associated with the highest CO production, reaching a maximum of 681 ppm and 18.2 mg CO∙kg wet mass −1 , whereas high moisture (M100; ~51–64%) with lower CO levels (max. 276 ppm, 4.4 mg CO∙kg wet mass −1 ), matrix compaction, elevated CO 2 and lower O 2 concentrations. The driest treatment produced trace CO (<20 ppm, max. 0.4 mg CO∙kg wet mass −1 ) and retained a rigid, porous microstructure consistent with limited biodegradation. The results showed rapid but transient CO pulses after H 2 O addition, implicating moisture-driven shifts in biological activity and/or abiotic formation. These findings identify an optimal moisture window for reproducible CO generation.