Grease trap waste valorization through hydrothermal liquefaction and anaerobic digestion: a circular approach to dairy wastewater treatment†

Grease traps are commonly used in the dairy industry to separate fats from their generated wastewater. Due to its properties, grease trap waste (GTW) is predominantly incinerated or landfilled despite its high energy content. In this study, hydrothermal liquefaction (HTL) was used to convert dairy industry GTW into biocrude while the generated HTL-wastewater (AP) was subjected to anaerobic digestion (AD) to recover biomethane. To maximize organic carbon to biocrude conversion, and to minimize the use of freshwater, a fraction of the AP was recirculated in subsequent HTL reactions. AP recirculation increased biocrude yields (73 vs. 78 wt%) but decreased both the higher heating value (HHV) (38 vs. 37 MJ kg⁻¹) and the fraction (72 vs. 64%) of lighter hydrocarbons. Continuous AD using an EGSB reactor proved to be an effective method to further reduce the COD of the AP from 6.5 g L⁻¹ to 0.7 g L⁻¹ and enhance the overall energy recovery of the GTW from 81% (HTL only) to 83.1% (HTL-AD). Integrating HTL with AD and recycling a fraction of the AP in the HTL process allows for efficient wastewater treatment and a recovery of up to 84.8% of the energy contained in the GTW.