Hydrothermal liquefaction for sludge-to-energy conversion: An evaluation of biocrude production and management of waste streams

Hydrothermal liquefaction (HTL) is a rapidly developing technology that converts waste biomass, such as municipal sludge, into a petroleum-like biofuel known as biocrude. At the HTL temperature/pressure of 350 degrees C/ 170 bar that simulates an HTL unit designed for a wastewater treatment plant (WWTP), this study found that >64% of energy could be recovered as biocrude from mixed primary and secondary sludge. HTL process also generates two waste streams, hydrochar and HTL aqueous. Heavy metals tend to accumulate in hydrochar, which raises concerns about its valorization. Hydrochar had concerning amounts of Cd, Mo, and Zn for land application, while it was non-hazardous for landfilling. Hydrochar had the most P distribution, resulting in a high concentration (4.6% by weight). Almost all P could be recovered by acidic extraction from hydrochar. The impacts of returning HTL aqueous to wastewater treatment processes were evaluated for the first time. HTL aqueous could be aerobically treated. However, its return increased the final effluent COD by 16.3% and 20.5% and decreased UV disinfection performance by 4% and 8% for average flow and low flow (dry season) conditions, respectively. In conclusion, this study yielded significant information in guiding the development of wastewater biorefinery by incorporating HTL into WWTPs.