Considerations in Applying Thermal Hydrolysis for Advanced Residuals Stabilization


  • C. Michael Bullard PE, Wendell J. Khunjar PhD, PE, Ronald L. Latimer PE

Thermal hydrolysis (THP) is a residuals pre-treatment process typically applied upstream of conventional mesophilic anaerobic digestion (MAD) in order to realize several process benefits, including:

• Pasteurization to achieve 40 CFR 503 time-and-temperature requirements;
• Increase in the digestibility of organics bound in the sludge matrix;
• Ability to operate at higher volumetric loading rates reducing digestion volume requirements; and
• Increased dewaterability of post-digestion sludges.

The aggregate result of these benefits is typically a final residuals product that will meet Class A pathogen reduction requirements and have a lower dry mass and wet cake mass than conventional anaerobic digestion and require less digester volume to achieve the increased process performance.

However, multiple investigations in the scientific literature indicate that THP pre-treatment may result in the formation of refractory organic (COD) and nutrient (rDON and rDOP) compounds. These compounds, when returned to the liquid treatment train following post-dewatering, could be problematic in facilities with stringent effluent nutrient limits. Therefore, mitigation of sidestream loadings from THP+MAD processes may be necessary in facilities with stringent effluent nutrient limits.

Specifically, this presentation will:
• Provide participants with a overview of available THP technologies currently available in the marketplace;
• Present several process approaches and configurations for THP implementation;
• Present results from a batch THP treatment assessment on a blended primary and waste activated sludge stream quantifying refractory compound formation and potential impacts on effluent nutrient concentrations;
• Discuss possible issues associated with inhibitory compounds formed during THP pretreatment on the application of ammonia removal using annamox bacteria; and
• Present a preliminary treatment concept for advanced oxidation treatment to remove refractory compounds in effluent prior to discharge to achieve stringent COD and nutrient limitations.

For more information, please contact the author at

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Horizons Fall 2017 (pdf)

Horizons showcases significant water, wastewater, reuse, and stormwater projects and innovations that help our clients to achieve their goals, and can help you achieve yours. Articles are written by top engineers and process group leaders, demonstrating and explaining the beneficial application of a variety of technologies and tools.

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