Supercritical water (SCW) systems and methods using the properties of supercritical water to allow a raw water stream including water and a contaminant to be purified. A supercritical water oxidation (SCWO), water purifier may generally include water and oxidizer (such as oxygen in air) pumping and mixing apparatus, a supercritical water reactor, and a system for separating the resultant purified water from other relatively harmless outputs of the supercritical water reactor.