Hydrothermal Liquefaction of Acid Whey: Effect of Feedstock Properties and Process Conditions on Energy and Nutrient Recovery

Improper management of dairy wastewater (acid whey) leads to water basin eutrophication and greenhouse gas emissions. We evaluated hydrothermal liquefaction as a means to sustainably manage and valorize acid whey wastes by converting them into biocrude oil and recovering nutrients in aqueous-phase byproducts. In a set of well-defined experiments, we studied the effects of reaction temperatures (280-360 °C), reaction times (7.5-37.5 min), feedstock pH values (2.5-8.5), and feedstock salt contents (0-2 wt ) on the energy recovered and the yields of products and elements. Response surface analysis showed that an alkaline feedstock pH (8.5) and a short reaction time (7.5 min) combined with a specific optimal reaction temperature for certain salt content in the feedstock were required to maximize biocrude energy recovery and nutrient recovery for a range of feedstock compositions. In general, 280-290 °C was optimal for salt contents ≤1.675 wt and 360 °C for salt contents 1.675-2 wt . Carbon and nitrogen were mostly distributed between biocrude and aqueous-phase products, added with distribution to the gas phase at higher temperatures. Partitioning of inorganics, for example, calcium and phosphorus, into aqueous-phase and solid hydro-char products depended on reaction conditions. This study provides new information for controlling target product composition as well as specifying desirable operating conditions for practical systems. © 2021 American Chemical Society.