Available at: https://digitalcommons.calpoly.edu/theses/3432
Date of Award
8-2026
Degree Name
MS in Civil and Environmental Engineering
Department/Program
Civil and Environmental Engineering
College
College of Engineering
Advisor
Tryg Lundquist
Advisor Department
Civil and Environmental Engineering
Advisor College
College of Engineering
Abstract
Hydrothermal Liquefaction (HTL) is a thermochemical process for converting wet biomass into renewable biocrude oil which can be further upgraded into a variety of liquid fuels. One of the main candidate feedstocks for this process is municipal sewage sludge (MSS). Using MSS for HTL opens a wide range of synergistic benefits for wastewater treatment plants (WWTPs) to improve the economics and environmental impact of their sludge management. The main challenge is that HTL generates an aqueous phase coproduct (AP) which is a high strength wastewater that cannot be directly discharged into the WWTP because it inhibits nitrification. To address this challenge, wet air oxidation (WAO) was evaluated as a treatment step to reduce the nitrification inhibition of AP before it is returned to the WWTP. WAO is another thermochemical process which is used to treat high strength wastewaters that are unsuitable for biological treatment. WAO is controlled by several operational parameters including temperature, residence time, and pressure, which in turn affect the qualities of the treated wastewater. This study quantifies the nitrification inhibition effects of thirteen samples of WAO treated AP to model the relationships between these parameters and inhibition so they can be optimized for efficient and effective treatment. Previously determined concentrations of many AP and WAO treated AP constituents were also used to gain further insight into which compounds are the most relevant to nitrification inhibition and which WAO parameters are most effective at removing them. AP was found to inhibit nitrification in activated sludge by 61% on average at a dilution of 0.2% v/v while WAO treatments varied in effectiveness from 46% inhibition to under 2% inhibition which made it statistically indistinguishable from the controls. Temperature and residence time correlated strongly with inhibition and these relationships were found to be largely explainable in terms of COD reduction. Some other compounds were found to have mild correlations with inhibition, but they were either not effectively removed during WAO treatment or were removed under the same conditions as COD. These results suggest that WAO optimized for COD reduction could serve as a standalone pretreatment step for AP, allowing HTL of MSS to become integrated into a WWTP.