Sustainable Resource Recovery from Dairy Waste: A Case Study of Hydrothermal Co-liquefaction of Acid Whey and Anaerobic Digestate Mixture

Abstract

Uncontrolled emission of carbon and nutrients due to mismanagement of dairy waste leads to global warming, eutrophication of water bodies, aerosol pollution, and acidification of ecosystems. We studied hydrothermal liquefaction (HTL) to sustainably recover resources by converting the dairy wastes into carbon-dense biocrude oil and a nutrient-rich aqueous-phase coproduct. We evaluated acid whey as an alternative source of acid catalyst, replacing acetic acid, for the HTL of anaerobically digested cattle manure (manure digestate). Using well-designed HTL experiments, we investigated the effects of several factors on the product formation and the fate of elements as well as the underlying chemistry. The investigated factors included reaction temperature (280–360 °C), reaction time (10–50 min), mixing ratio of acid whey to manure digestate (AW/MD of 0–2), and the amount of additional acetic acid required, that is, specified based on the final feedstock pH between 3.5 and 5.5. The experimental results suggested the following optimal conditions for enhanced biocrude oil formation and maximal nutrient yield in the aqueous-phase coproduct: AW/MD of 1.21, feedstock mixture pH of 4.5, and reaction temperature and time of 354 °C and 21 min, respectively. Under such conditions, the biocrude oil yield was enhanced to 40–50% (equivalent to 60–80% energy recovery) via cyclic acetalization, hydroxymethylation, isomerization, Piancatelli rearrangement, oxidation, and condensation with heteroatom removal via several deoxygenation and denitrogenation mechanisms. Moreover, these conditions provided adequate acidification to solubilize Mg-carbonate and Ca-phosphate minerals and catalyzed the deamination of N-heterocyclics and other nitrogenous compounds, producing a higher recoverable yield of Mg, NH3-N, P, and K nutrients in the aqueous-phase coproduct, that is, 60–70, 30–40, ∼40, and >95%, respectively. This study demonstrated the potential of using HTL for coprocessing manure digestate and acid whey to measurably valorize biomass waste as a key component of achieving a circular bioeconomy for the dairy industry.