Direct-Contact Membrane Distillation of Process Water from Hydrothermal Carbonization of Rice Husk to Concentrate Sugars

At present, many secondary agricultural residues are landfilled or burned in the open field, leading to the loss of the valuable nutrients they contain. The valorization of agricultural biomass provides a promising route for resource recovery and circular bioeconomy development, allowing for symmetry in their reuse. Hydrothermal carbonization (HTC) offers a sustainable method for converting agricultural waste to both solid and liquid carbon-containing products. Since the liquid product is highly dilute, it is a challenge to apply it at large scales. In this work, a low-energy separation process, direct contact membrane distillation (DCMD), has been used to concentrate the liquid product from HTC of rice husks while recovering water from it. The effect of feed temperature on DCMD performance in terms of water recovery percentage and concentration factors for the fermentable sugars xylose and glucose was investigated at feed temperatures of 50 °C, 65 °C, and 80 °C. DCMD conducted at a feed temperature of 80 °C yielded the highest value for average water flux at 3.4 L/m2·h. DCMD at 80 °C also showed the highest water recovery (56.9%) compared to the lower temperatures, and concentrated xylose and glucose by 2.06 and 3.20 times, respectively3. Incremental flux measurements revealed evidence of limiting mechanisms, such as temperature and concentration polarization, with prolonged operation time. The results from this study thus establish an integrated HTC–DCMD system for efficient water recovery and the concentration of HTC liquor, thereby enhancing its potential for commercial use.