Promising Pathway for Algal Biofuels through Wastewater Cultivation and Hydrothermal Conversion

The purpose of this study is to demonstrate feasibility of an integrated wastewater algae-to-biocrude process that can sustainably cultivate algal biomass for biofuel production. This process used pilot-scale algal cultivation ponds fed with municipal wastewater as the nutrient source. The open ponds were self-inoculated from the wastewater source, resulting in a mixed-culture microalgal community with distinct differences compared to laboratory-maintained and fertilized monocultures: 29.0% dry weight (dw) ash, 48.9% ash-free dry weight (afdw) carbon, 37.5% afdw oxygen, and 14.0% afdw lipid. The harvested algae was processed using hydrothermal liquefaction at 350 °C (autogenous pressures up to 2000 psig) for 1 h using 3 g of freeze-dried algae and 50 mL of water. The yield of biocrude was 44.5 ± 4.7% afdw, with an elemental weight percent composition of 78.7% carbon, 10.1% hydrogen, 4.4% nitrogen, and 5.5% oxygen and an energy content of 39 MJ/kg. Hydrothermal processing also resulted in the formation of 18.4 ± 4.6% afdw aqueous co-products (ACPs) and 45.0 ± 5.9% dw solid biochar. The ACPs contained 4550 ± 460 mg L –1 organic carbon, 1640 ± 250 mg L –1 total nitrogen, and 3.5 mg L –1 total phosphorus. The solid biochar product contained >20% dw carbon with an energy density between 8 and 10 MJ kg –1 . This study is the first hydrothermal liquefaction paper of wastewater-derived microalgae. The municipal wastewater matrix and resultant mixed-culture biomass significantly influenced liquefaction product distribution, yielding a higher proportion of biochar, which may be a valuable co-product. This paper explores the potential for wastewater-fed algal systems integrated with hydrothermal liquefaction, which together overcome challenges identified by the 2012 National Research Council’s report on algal biofuel sustainability.