Chemical Conditioner-Free Sludge Dewatering via CO2 Hydrate Crystallization: Coupling Organic Matter Retention, Water Extraction, and Carbon Utilization

Abstract Effective sludge dewatering is essential for reducing sludge volume and associated transportation and disposal burdens. However, conventional approaches often require substantial chemical addition or energy input, increasing operating costs and potential secondary environmental impacts. Here, we present a CO2 hydrate crystallization-based dewatering approach that avoids conventional chemical sludge conditioners. Residual water in mechanically dewatered sludge is converted into separable solid hydrate structures, with water molecules incorporated into hydrogen-bonded hydrate cages and sludge-derived solutes and particulates excluded from the hydrate lattice. Low-field nuclear magnetic resonance measurements showed that hydrate crystallization promoted the mobilization of mechanically bound water, whose relative signal fraction decreased from 68.2% to 40.7%. With methionine as a kinetic promoter, repeated hydrate formation and separation reduced the sludge water content to 59.3%. The screening-level process electricity demand was estimated at 0.247–0.298 kWh/kg of removed water, depending on the cold energy recovery scenario. The process also retained most sludge-derived volatile solids and produced hydrate-dissociated water with reduced aqueous constituent levels following hydrate layer separation. These results highlight CO2 hydrate crystallization as a phase-transition-driven strategy for deep sludge dewatering, with energy integration and solid–liquid separation remaining key considerations for further process development.