Treatment of biofuel production wastewater by a combined freezing method for resources recovery and waste reduction

Biofuel production wastewater (BPWW), which contains ultra-high concentration of complex substances including bio-oil and acid contents, is a hazardous by-product produced from the process of biomass-to-biofuel conversion (fast pyrolysis and hydrodeoxygenation). In the present study, a combined freezing method of freeze concentration and freeze-thaw demulsification was employed to treat BPWW. The effects of freezing parameters and BPWW properties on concentration efficiency in suspension crystallization and demulsification performance in freeze-thaw treatment were thoroughly investigated. COD and oil contents decreased sharply with removal efficiencies over 96% using multi-stage freeze concentration, and near 60–80% of the original volume wastewater could be purified and recovered. Moreover, the oil droplets in concentrates coalesced evidently (mean diameter grew from 10.02 μm to 286.98 μm) after freeze-thaw treatment, resulting in near 85% of the bio-oil in the concentrated BPWW could be separated and recovered. The results proved the feasibility and capability of the freezing methods for water purification, contaminates concentration and oil-in-water emulsion breaking in BPWW treatment. Based on these results, a conceptual mode of biofuel production process which includes the combined freezing method for resources recovery, including purged water and bio-oil, and wastewater volume reduction was proposed, which is of great significance to the sustainability of the biomass-to-energy technology. [Display omitted] •A combined freezing method was first applied to the treatment of BPWW.•BPWW concentration and water recovery were achieved by suspension crystallization.•Demulsification and oil/water separation was realized by freeze-thaw treatment.•Performance of the combined freezing method was thoroughly investigated.•A novel sustainable mode for biofuel production was proposed.