Fouling mitigation and carbon recovery in nanofiltration processing of hydrothermal liquefaction aqueous waste stream

A significant fraction of the embedded energy, carbon, and nutrients in biomass is lost to the aqueous phase during the hydrothermal liquefaction of biomass to bio-oil. The feasibility of concentrating the aqueous phase to recover and recycle the organics back to the hydrothermal liquefaction reactor using nanofiltration was investigated. The dissolved organic carbon content (~ 1 wt%) of the hydrothermal liquefaction aqueous phase was mostly comprised of carboxylic acids, phenolics, and ketones. Several pretreatments, including pH adjustment, thermal treatment, ultrafiltration, and adsorption, were conducted prior to the nanofiltration process to mitigate membrane fouling. Adjustment of pH to 8 and higher was observed to improve the permeate flux. Minor amounts of humic-like organic materials were determined to be the major contributor to membrane fouling. Adsorption using coal was especially effective at selectively removing organic foulants and enabled the volume reduction of the hydrothermal liquefaction aqueous phase by a factor of ~8. The dissolved organic carbon concentration in the retentate increased fourfold and an organic carbon mass recovery of ~50% was achieved. The results show promise as a cost-effective way to the recovery of dissolved organic carbon in the hydrothermal liquefaction aqueous phase. Further integrated process development is needed to maximize valorization.