Displacement dewatering of sludge with supercritical CO₂

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.