Primary Sludges Drive the Scalability and Power Production of Hydrothermal Liquefaction for Energy Resource Recovery at Wastewater Treatment Facilities

Abstract

This research assesses the impact of wastewater sludges from different treatment stages and facility scales on the biocrude power potential from hydrothermal liquefaction (HTL). HTL offers a promising method for energy resource recovery through biocrude production, yet its viability for smaller facilities [<1 million gallons per day (MGD)] remains uncertain. Sludges from facilities of varying scales (0.8, 13, and 76 MGD) were analyzed. We found that the treatment stage influences the energy content and chemical composition more than the facility scale. HTL experiments showed that primary sludges (PS) yield more biocrude than waste-activated sludges (WAS); however, the carbon fractionation remained similar across facility scales. The power generated by PS converted to biocrude was 1.05–1.55 times higher than that if it were converted to methane. Meanwhile, WAS resulted in lower power generation from biocrude than methane. At small plants where primary treatment is not incorporated, HTL is not an ideal energy recovery technology, and instead other waste-management solutions might be better explored. This study emphasizes applying HTL only at plants where it can be truly viable for matching or exceeding the power consumption by evaluating HTL in the context of practical wastewater treatment parameters, e.g., treatment flow, sludge density, energy density, and the realistic conversion potential for energy resource recovery technologies.

This study highlights the importance of the sludge quantity and quality in determining the scalability of sustainable energy resource recovery for wastewater treatment.