Electrolysis of HTL-AP for nutrient recovery by converting cyclic nitrogen to nitrate-N fertilizer

Valorization of hydrothermal liquefaction aqueous phase (HTL-AP) can be achieved through its use as a nutrient source for lettuce production in hydroponic systems after being treated to reduce the nutrient imbalance. Removing nitrogen cyclic compounds in HTL-AP may impact the availability of some nutrients, such as nitrate-N, that are necessary for plant growth. Previous studies indicate that electrolysis enables nitrate-N accumulation in algal-HTL-AP. In this study, HTL-AP derived from food waste was electrolyzed to convert available nitrogenous compounds into nitrogen forms that are preferred by plants such as nitrate-N. Biochemical properties were assessed for the HTL-AP samples before and after two years of storage. Results from this study show that it is viable to convert heterocyclic amines in HTL-AP into inorganic nitrogen forms such as nitrite-N, nitrate-N, ammonia-N, and fatty acids. Specifically, this study showed that accumulation of 609 mg/L of nitrate-N in the HTL-AP with an initial concentration of 25 mg/L was achieved at the lowest current density. Additionally, electrolysis treatment removed 48%–61% of COD from the HTL-AP at different current densities. Furthermore, water quality characterization before and after storage for two years showed decreased organic matter in the HTL-AP, leading to reduced inorganic nitrogen recovery. Overall, this study indicates that electrolysis can increase the concentration of inorganic nitrogen in the HTL-AP both before and after long-term storage.