Influence of Process Water Reuse on the Hydrothermal Carbonization of Paper

Process water reuse was exercised in the hydrothermal carbonization (HTC) of paper at 200 degrees C for 16 h, four times. Results showed that hydrochar mass yields increased significantly in the first recirculation step but only slightly in further steps. At the same time, solid carbon content did not increase, and a constant organic carbon distribution between the three product streams, hydrochar, process water (PW), and gas phase, was observed over all recycling steps. Dissolved organic carbon content (DOC) and chemical oxygen demand (COD) of the PWs increased, as well as total acids content, among those specifically lactic, acetic, formic, and propionic acid. Surprisingly, the aerobic biodegradability of the PWs decreased as indicated by lower biochemical to chemical oxygen demand (BOD/COD) ratios in reused process waters. Subsequent methanogenesis of the PW revealed that a high amount of biogas was produced; however, no increase was seen with each recirculation step.