Utilizing Hydrothermal Liquefaction to Create Higher-Value Products from Municipal Wastewater Sludge

Currently, sewage sludge is pressed and land-applied directly as fertilizer. However, it has the potential to contaminate soil due to its toxic contents, such as pharmaceuticals, toxic metals, and E. Coli. Hydrothermal liquefaction (HTL) can offer a solution by transforming nutrient-rich sewage sludge into higher-value products. This reaction uses water at moderate temperatures (250-350°C) and high pressures (5-22 MPa) to convert lipids, proteins, and carbohydrates in sewage sludge into four main products: biocrude, biochar, gas, and an aqueous product.

Current studies on HTL have focused on converting biomass to biocrude, and the usefulness of the biochar phase is often overlooked. This study compares how an Enhanced Biological Phosphorous Removal (EBPR) process and a non-EBPR wastewater treatment process affect HTL product distribution and yields, with an emphasis on the biochar product. Furthermore, this study compares the use of wastewater and deionized (DI) water for HTL on both EBPR and non-EBPR processes, and the residence time before HTL for the EBPR-processed sludge.

Hydrothermal liquefaction was performed on EBPR and non-EBPR processed sludges for 1 hour at 350°C and 8500 psi in a fluidized sand bath. Deionized water and wastewater were used as reaction mediums to determine if they influenced product yields. Characterization techniques included Thermogravimetric Analysis (TGA), a Nitrogen and Carbon soil analyzer, and an Inductively Coupled Plasma Optical Spectrometer (ICP-OES).

Results showed that the non-EBPR process produced the highest biochar yields due to the addition of lime in its treatment process. However, the EBPR process increased the phosphorous concentration within its biochar compared to the non-EBPR process. The use of wastewater as the reaction medium minimally increased biochar yields compared to deionized water. Lastly, the residence time before performing HTL on the EBPR sludge had a negligible effect on product yields, suggesting that wastewater treatment plants can implement hydrothermal liquefaction long after processing EBPR sludge. Subsequent studies could involve the use of catalysts to increase the biochar yield from the EBPR-processed sludge.