Combing hydrothermal carbonization (HTC) of sewage sludge with wet oxidation of the resulting process waters (PWs) is a favorable economic process leading to a wastewater that is better amenable to biological treatments. Five PWs from HTC of sewage sludge and straw were oxidized with molecular oxygen. A good removal of chemical oxygen demand and dissolved organic carbon was shown. The oxidized waters proved good substrates for a subsequent anaerobic digestion. However, the oxidized PWs were identified as more toxic in germination tests with cress (Lepidium sativum L.) compared to the untreated PWs.