Electrochemical oxidation of nitrogen-rich post-hydrothermal liquefaction wastewater

Abstract The aqueous phase co-product from the hydrothermal liquefaction of algae contains a significant fraction of the decomposed feedstock components that are suspected of having detrimental effects on the environment, requiring treatment before reutilisation or disposal. We used for the first time an electrochemical oxidation process to decrease the soluble organics content in the aqueous phase co-product from the hydrothermal liquefaction of microalgae Chlorella sp. using boron-doped diamond as the anode. The original aqueous phase co-product contained relatively high levels of chemical oxygen demand (36,000 mg·L−1), ammonia (1500 mg N·L−1), and organic nitrogen (3000 mg N·L−1). The process removed up to 99% of the carbon and attained a 96% discolouration of the wastewater. Nitrogen was converted from organic to inorganic forms, predominantly remaining in solution as ammonia and nitrate. The global current efficiency was 85% at a constant current density of 30 mA·cm−2, and the specific energy consumption to remove dissolved carbon was 28 kWh per kg of chemical oxygen demand.