Nutrient recycle from algae hydrothermal liquefaction aqueous phase through a novel selective remediation approach

Abstract

Algae have received increasing interest in the past several decades as a biofuel feedstock source. However, sustainable nutrient supply has presented algal biofuels with a major obstacle in the value chain. At a scale where algal biofuels would meet a significant portion of transportation fuel needs, the demand for nutrients, specifically nitrogen and phosphorus, would exceed current global agricultural production. One downstream conversion pathway, hydrothermal liquefaction (HTL), produces bio-crude oils from wet algal biomass with a waste aqueous phase (HTL-AP), containing a significant amount of carbon and nitrogen. While this stream is rich in organic content and nutrients, it also contains toxic components, which include heterocyclic nitrogen compounds and phenolic compounds. Thus, the recyclability and potential toxicity of HTL-AP need to be studied in detail. The feasibility of utilizing nutrients available in HTL-AP was experimentally determined for Chlorella vulgaris and Desmodesmus armatus monocultures. Our work focused on determining the tolerance of these algae species toward HTL-AP toxicity through varying dilutions. Nitrogen replacement in the growth media was varied from a low of 18% to a high of 141% across both species. The most notable of these results show that addition of a 100× dilution (35% nitrogen replacement) of untreated HTL-AP decreased growth in C. vulgaris by 47 ± 7% with respect to a control medium. Adsorption treatments, including activated carbon and various resins, were introduced to remediate the HTL-AP toxic effects. Treatment of the HTL-AP portion with an ion-exchange resin, Dowex 50WX8, supported C. vulgaris growth at a 100× dilution (35% nitrogen replacement) with no statistical change compared to the control. An in-depth molecular profiling demonstrated for the first time the selective removal of high‑nitrogen containing components by resin treatment. This work provides a foundation for studying the toxic components of HTL-AP and possible mechanisms by which treatments can remove these components.