Bench‐Scale Evaluation of Hydrothermal Processing Technology for Conversion of Wastewater Solids to Fuels

TLDR

The study tested bench‑scale hydrothermal liquefaction (HTL) and catalytic hydrothermal gasification (CHG) to evaluate hydrothermal treatment of municipal wastewater sludge. HTL was conducted at 300–350 °C and 20 MPa on primary, secondary, and digested sludge, followed by CHG at 350 °C and 20 MPa on the HTL aqueous phase using a ruthenium catalyst. The HTL‑CHG process produced 25–37 % biocrude with algae‑like quality that, after hydrotreating, matched crude oil properties, yielded 47–64 % methane on a carbon basis with siloxane below engine limits, and achieved >99.9 % COD removal and 94–99 % reduction in residual solids.

Abstract

Hydrothermal Liquefaction (HTL) and Catalytic Hydrothermal Gasification (CHG) proof-of-concept bench-scale tests were performed to assess the potential of hydrothermal treatment for handling municipal wastewater sludge. HTL tests were conducted at 300 to 350 °C and 20 MPa on three different feeds: primary sludge, secondary sludge, and digested solids. Corresponding CHG tests were conducted at 350 °C and 20 MPa on the HTL aqueous phase output using a ruthenium-based catalyst. Biocrude yields ranged from 25 to 37%. Biocrude composition and quality were comparable to biocrudes generated from algae feeds. Subsequent hydrotreating of biocrude resulted in a product with comparable physical and chemical properties to crude oil. CHG product gas methane yields on a carbon basis ranged from 47 to 64%. Siloxane concentrations in the CHG product gas were below engine limits. The HTL-CHG process resulted in a chemical oxygen demand (COD) reduction of > 99.9% and a reduction in residual solids for disposal of 94 to 99%.