Ash-pretreatment and hydrothermal liquefaction of filamentous algae grown on dairy wastewater

Hydrothermal liquefaction (HTL) of wastewater microalgae is constrained by the high ash content in the biomass. High ash content causes slagging, fouling, and catalyst deactivation. In this study, batch experiments for hydrothermal liquefaction (HTL) of filamentous algae were carried out in a 1.8-L autoclave reactor to measure the effects of reaction conditions and demineralization on the yields and quality of the products. Nitric acid was the most efficient demineralization acid, halving ash content (27.1 to 13.5 wt%). HTL of untreated algae showed that, as temperature increased from 310 to 350 °C, total bio-crude oil (light + heavy) yield slightly increased (25.4 to 28.0 wt%, dry ash-free). HTL of demineralized algae gave higher total bio-crude oil yields (27.2 to 43.4 wt%, dry ash-free), greater energy recovery (29.5 to 50.9%), higher char yields (22.1 to 32.5 wt%, dry ash-free), and decreased energy consumption ratios (from 1.0 to 0.6) compared to HTL of untreated algae. Fourier transform infrared spectroscopy (FT-IR) of the feedstocks revealed slightly higher carbohydrate content in the acid-treated biomass compared to the untreated biomass. Both FT-IR and FT ion cyclotron resonance mass spectroscopy of the bio-crude oils showed no significant difference in the functional groups and heteroatoms in the hexane-soluble bio-crude oil derived from untreated and acid-treated biomass. These results demonstrate that demineralization of wastewater algae is relevant for improving the feasibility of algal wastewater-treatment-to-biofuel systems.