Provided herein are methods, systems, and apparatuses for energy-efficient supercritical water oxidation of waste. The supercritical water oxidation processes and systems described herein may incorporate one or more of the following features: compression of large amounts of oxidant for plant-scale operations in an energy-efficient manner; the use of air as an oxidant; using reactor effluent to drive a turbine or other gas expander for energy recovery; and recovery of pressure and heat of reactor effluent. In some embodiments, the systems and methods are energy-neutral or energy-positive.