Evaluation of the Sustainability of the Hydrothermal Processing of Select Organic Waste Feedstocks: Accounting for Water Quality and Nutrient Recovery Impacts

In this dissertation, a combination of laboratory experiments, water chemistry modeling, and energy accounting techniques are used to evaluate the sustainability of the hydrothermal liquefaction (HTL) processing of select non-food, organic waste feedstocks, with emphasis on water quality impacts. Laboratory experiments include: (1) experimental characterization of the hydrothermal processing of select organic waste feedstocks, (2) evaluation of corresponding HTL products, specifically so-called aqueous co-product (ACP), and (3) assessment of ACP quality and suitability for discharge into receiving waters or a municipal wastewater treatment plant (WWTP). Water chemistry modeling includes an assessment of the treatability of ACP via the recovery of valuable, scarce nutrients (i.e., nitrogen [N] and phosphorus [P]) from the post-HTL ACP of select organic waste feedstocks as a means of both managing the ACP and producing valuable materials. Energy accounting includes adjusting the “energy return on investment” (EROI) of HTL systems to account for the production and management of ACP from the HTL processing of select organic waste feedstocks.

Experimental results of this research indicate that while the hydrothermal processing of select organic waste feedstocks generates liquid biofuel, HTL processing also produces substantial quantities of potent wastewater (i.e., ACP). The ACP arising from HTL processing contains very high concentrations of traditional wastewater pollutants (i.e., total nitrogen [TN], total phosphorus [TP], and dissolved organic carbon, measured as chemical oxygen demand [COD]), which has been largely overlooked by the current literature. The potency of the ACP renders it more noxious than relevant benchmark wastewaters, requiring management of the ACP prior to discharge into the receiving waters of a municipal WWTP. Adjustment of published energy ratio metrics to account for ACP management reveals that the energy consumption required to remove TN, TP, and COD from the ACP to achieve typical permitted levels of WWTPs is on the same order of magnitude as that of liquefaction. The results of water quality modeling to assess the management of ACP via nutrient-based precipitation of valuable nutrients (i.e., N and P) from the ACP indicate that pH adjustment and the addition of magnesium facilitate the theoretical precipitation of N and P as solid compounds (i.e., struvite and hydroxyapatite [HAP]) from the ACP. This is promising from an environmental perspective as precipitation-based nutrient recovery could enhance the appeal of waste HTL systems as a means of both valorizing waste materials into renewable energy and producing valuable nutrient-based materials. Additional work will comprise: (1) evaluating the impacts of various HTL processing conditions on the quantities and composition of HTL products, specifically ACP, (2) characterizing possible toxicity impacts of ACP, and (3) validating theoretical nutrient recovery yields from water chemistry modeling via laboratory experiments. The results from this work will offer insight into the water quality impacts of waste HTL systems, as well as mitigating the effects of ACP on water quality via novel ACP management and recovery of valuable, scarce nutrients.