Displacement dewatering of sludge with supercritical CO2

•Supercritical CO2 displaces water from alum, wastewater, and paper sludge.•Multistage dewatering increases the degree of dewatering.•The expanding CO2 blows out the water during decompression. Supercritical CO2 (sCO2) displaces water from wastewater, alum, and papermill sludge. The sCO2 appears to enter the sludge matrix through viscous fingering through the entrained water. Because the water removed far exceeds the solubility of water in sCO2, it must be displaced by the sCO2 rather than dissolved out. Adding a small amount of soap to the sludge converts some of the bound water into free water, which can then be displaced by sCO2. Application of the sCO2 in multiple stages greatly enhances dewatering as compared to a single stage process. Approximately 70, 70 and 85% of the initial water can be removed from alum, wastewater and paper sludges, respectively, through a five-stage process. Staged application of sCO2 doubles the efficiency of water removal over a single-stage process of the same duration. It is proposed that when the sCO2 entrained in the sludge is decompressed between stages some of the water is explosively displaced by the expanding CO2. [Display omitted]