Hydrothermal liquefaction could be a sustainable approach for valorization of wastewater grown algal biomass into cleaner fuel

Abstract

Studies highlighting the actual roadblocks in sustainability of algal fuels require systemic analysis using real-time experimental inventories. In view of this, the authors performed Life Cycle Assessments (LCAs) for 3 previously developed algal biofilms-based conversion systems; 1) wastewater grown algae (WWA) processed via anaerobic digestion (AD), 2) WWA processed via hydrothermal liquefaction (HTL) and 3) synthetic media grown algae (SMA) processed via HTL. Their environmental impacts were compared for resources depletion, ecosystem damage, human health deterioration and climate change. Results showed that HTL of WWA had 41.1 % lesser impact compared to AD of the same biomass, because of the huge energy input, production of CO2 and huge volume of algal digestate. Due to the short reaction time in HTL (20 min), energy input and resources usage was minimal. Further, the HTL of WWA showed 98% lesser impact than HTL of SMA. HTL of SMA had larger impact on the human health (3.95Pt), compared to HTL of WWA (0.25Pt). The latter also showed positive impact on the eutrophication reduction due to the treatment of wastewater during algal cultivation. The global warming potential for biocrude production from WWA was 10 times lower than biomethane production from the same biomass and 21 times lower than the biocrude production from SMA. The results of the comparative LCAs proved that HTL of WWA is a sustainable route for valorizing the biomass. Based on the LCA outcomes, shortcomings of the respective algal technologies were identified, and improvement strategies were suggested.