Combination of hydrothermal carbonization and wet oxidation of various biomasses

• Process waters from HTC of various biomasses were treated by wet oxidation.
• Iron-salts increased the efficiency of the wet oxidation.
• Chemical oxygen demand of the process waters was reduced by up to 55%.
• Oxidized waters showed improved biodegradability.
• Wet oxidation at mild conditions of 120 °C generates sufficient heat for HTC process.

Wet oxidation (WO) treatment was performed on process waters obtained from hydrothermal carbonization (HTC) of sucrose, cellulose, wood, and brewer’s spent grains with oxygen at 3 MPa and temperatures of 120–200 °C. The aim was to reduce the dissolved organic carbon (DOC) content and chemical oxygen demand (COD) of the process water in order to reduce toxicity, enhance biodegradability and supply heat to improve the preceding HTC process. Success of the treatment was evaluated on reduction of COD and DOC. Iron-salts were identified as suitable catalyst to increase the efficiency of the WO. At 200 °C DOC and COD elimination was for all biomasses around 40% and 55%, respectively. At 120 °C, DOC and COD reduction of sucrose, cellulose and wood process waters was around 20–30% and 40%, respectively. Products from brewer’s spent grains HTC proved more resistant towards oxidation. The biodegradability of the process waters was improved as shown by increased BOD/COD ratios. The results provided evidence that WO at mild conditions of 120 °C can generate sufficient heat to sustain the HTC process in an autothermal regime. Moreover, WO showed the potential to degrade the recalcitrant chlorinated aromatics 2,4-dichlorophenol and 2-chloronaphthalene. In addition to HTC process waters, unseparated HTC slurries, i.e. hydrochars and process waters, were subjected to WO. At mild oxidation conditions of 120 °C, the dissolved organic matter was preferentially oxidized while the hydrochar retained its quality. However, oxidation temperatures of 200 °C led to a significant degradation of the hydrochars, while DOC and COD content were not reduced.