Nanofiltration of synthetic HTL-AP: rejection, fouling analysis, and membrane autopsy

Valorization or recycling of the hydrothermal liquefaction aqueous phase (HTL-AP) pose a substantial obstacle to the commercial use of the HTL process for biomass (especially algal). This is due to its high concentration of organic constituents and nutrients. This article focus on nanofiltration (NF) of acidic and basic synthetic HTL-AP using a DuPont FilmTec™ NF90 membrane in a batch crossflow module. The emphasis is on rejection performance and fouling behavior, supported by membrane autopsy. Overall, almost 90 % rejection of COD, NH4+, and PO43−, and TDS/conductivity was achieved. HTL-AP(pH = 4) led to higher NH4+ rejection (94 ± 1 %) compared with pH = 8.5 (83 ± 3 %). This can be attributed to the formation of a gel layer and the equilibrium between NH4+ and NH3, favoring the more permeable NH3 at higher pH. Flux monitoring revealed that NF90 performs better (J30 wt% recovery/J0 ca. 0.77 ± 0.02) with HTL-AP(pH = 8.5) than at pH = 4 (0.56 ± 0.06) since at pH = 8.5 most of the small organic compounds are deprotonated and therefore exhibit weaker interactions with the negatively charged NF90 surface. At both HTL-AP pH levels irreversible fouling is negligible while gel layer and concentration polarization are major contributors which are reversible through water flushing. Resistance due to the gel layer formation was more prominent for HTL-AP(pH = 4). The presence of a thin gel layer was confirmed through attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), scanning electron microscopy (SEM), water contact angle measurements, and thermal gravimetric analysis (TGA). In addition, fouling resistance due to pore blockage and persistent adsorbed foulants inside the membrane matrix is also a significant contributor. This was removed successfully by basic cleaning (NaOH).