Effect of operating conditions on the hydrothermal valorization of sewage sludge

Abstract

This study followed a quantitative approach, with the purpose of converting municipal sewage to valuable bio-oils. The sewage sludge was subjected to hydrothermal liquefaction in 60 mL stainless steel batch reactors, where the effects of temperature, solvent composition and solvent content were investigated, and all the other process parameters were maintained at constant values. Temperatures were explored between 220 and 370°C. The two solvents investigated were de-ionized water (H2O) and ethanol (E) which were applied in the following compositions: 1:0, 1:1 and 0:1 (H2O:E). The five solvent contents (moisture contents) investigated were 75, 80, 85, 90 and 95 wt%. The temperature of 310°C emerged categorically as the optimal temperature across the various solvent contents. The best yield was achieved when H2O/E (1:1) performed as a co-solvent at 85% solvent content and 310°C, which amounted to 40.69 wt%. Bio-oils derived by the use of pure H2O solvent were overall the least effective in yielding high bio-oil quantities. The lowest bio-oil yield obtained was 6.42 wt% where pure H2O was the solvent at extreme conditions of 370°C and 95% solvent content. The order of efficiency of solvent content was as follows: 85% > 80% > 75% > 90% > 95%. Among the six temperatures investigated, the intermediate temperature, 310°C, gave the highest conversions. The highest conversion across all of the runs was 85.65% at 310°C with 90% solvent content using the co-solvent H2O/E. The order of superiority of solvent was H2O/E > pure ethanol > pure water. © 2022 The Authors. Biofuels, Bioproducts and Biorefining published by Society of Industrial Chemistry and John Wiley & Sons Ltd.