A reactor for supercritical water oxidation includes an essentially vertical reactor section and an essentially non-vertical reactor section. The vertical reactor section has a cross-sectional area which is substantially larger than the cross-sectional area of the non-vertical reactor section. The vertical reactor section has an inlet in an upper portion thereof for receiving a flow containing organic material and water, and an outlet in a lower portion thereof for outputting the flow. Both the vertical and the non-vertical reactor sections are configured to oxidize organic material in the flow through supercritical water oxidation.