Effects of municipal sludge composition on hydrothermal liquefaction products: optimizing energy recovery via combination with anaerobic digestion

Abstract

This study evaluated the performance of an optimized hydrothermal liquefaction (HTL) process for municipal sludge with downstream anaerobic digestion (AD) for aqueous by-product valorization. Mixed sludge with various primary and secondary sludge ratios and digested sludge cake from mesophilic and thermophilic AD from wastewater treatment plants under seasonal and operational fluctuations were tested. Results showed shifts in product yield, biocrude and hydrochar composition, and energy recovery (ER), with higher secondary sludge ratio leading to increased hydrochar heavy metals and phosphorus. However, biocrude maintained consistent C (72–75 %), H (9–10 %), and N (4–5 %) contents, higher heating value (35–37 MJ/kg), dry-ash-free yield (48 ± 3 %), and ER (69 ± 1 %), demonstrating adaptability of the optimized HTL condition for varying feedstock. Biocrude ER was driven by sludge composition, ranking lipids > proteins > carbohydrates. Mesophilic AD effectively treated HTL aqueous by-product, achieving the highest overall ER (78–82 %), energy return on investment (10.8–11.1), and net energy yield (15.7–16.2 MJ/kg, dry basis) in HTL-AD system, outperforming AD and AD-HTL-AD configurations for sludge treatment. These findings demonstrate the robustness of optimized HTL condition across diverse sludge feedstocks and highlight the potential of HTL-AD integration to enhance ER and resource sustainability in wastewater treatment practice.

This study was funded by the Natural Sciences and Engineering Research Council of Canada (NSERC) and Metro Vancouver Industrial Research Chair Program in Advanced Resource Recovery from Wastewater (IRCPJ 548816-18) and supported by the NSERC Canada Graduate Scholarship – Doctoral program.

Peer Reviewed

Postprint (published version)