Treatment of Post-Hydrothermal Liquefaction Wastewater (PHWW) for Heavy Metals, Nutrients, and Indicator Pathogens

Abstract

Recycling post-hydrothermal liquefaction wastewater (PHWW) may allow the use of nutrients in the aqueous phase that may otherwise go unused. PHWW is an attractive option for use as fertilizer in systems like crop production. However, there are potential contaminants in the PHWW that may inhibit crop growth or pose a food safety risk. This study investigated the concentrations of heavy metals and nutrients in the PHWW, as well as the presence of indicator pathogens. In addition, four different water treatment methods were used: (1) dilution of raw PHWW, (2) sand filtration after dilution, (3) sand and carbon filtration after dilution, and (4) reverse osmosis after dilution. Our results indicate that the concentrations of cadmium, lead, and arsenic in raw PHWW were well below the maximum recommended concentrations set by the US Environmental Protection Agency (US EPA) for Water Reuse. In addition, the treatment methods in this study achieved percent removals ranging from 82–100% for cadmium, 99–100% for mercury, 75–99.5% for lead, and 71–99% for arsenic. Nitrogen in raw PHWW was predominantly in the total N form, preventing it from being accessible to plants. After nitrification was induced, the concentration of NO3 + NO2 increased by 1.75 mg/L in the untreated 5% PHWW mixture, but remained unchanged or decreased for all other treatments and mixtures. There were no E. coli or coliform colonies detected in the raw PHWW, or in any PHWW mixtures. All PHWW mixtures with and without treatment are within US EPA guidelines for metals for irrigation water reuse. However, fertilizer supplementation may be required for PHWW to be suitable for crop production, as the low concentrations of NO3 + NO2 may prove challenging for growing crops.