Displacement dewatering of sludge with supercritical CO2.

• Supercritical CO 2 displaces water from alum, wastewater, and paper sludge. • Multistage dewatering increases the degree of dewatering. • The expanding CO 2 blows out the water during decompression. Supercritical CO 2 (sCO 2) displaces water from wastewater, alum, and papermill sludge. The sCO 2 appears to enter the sludge matrix through viscous fingering through the entrained water. Because the water removed far exceeds the solubility of water in sCO 2 , it must be displaced by the sCO 2 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 sCO 2. Application of the sCO 2 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 sCO 2 doubles the efficiency of water removal over a single-stage process of the same duration. It is proposed that when the sCO 2 entrained in the sludge is decompressed between stages some of the water is explosively displaced by the expanding CO 2. Image, graphical abstract [ABSTRACT FROM AUTHOR]