Volatile organic compounds (VOCs), present in the exhaust gas from the biosolids drying system, are required to be destroyed prior to the exhaust gas being released to the ambient air. Biogas, a byproduct of the waste sludge treatment process, can fire the dryer instead of natural gas, a common source of energy. Biogas' components, typically a combination of methane, carbon dioxide, and other traces gases, and the destruction of VOCs released from biosolids drying will depend on the type of waste sludge being treated at the plant. Therefore, biogas utilization can raise concerns as to whether using biogas adds to the VOC emissions that could subject the plant to hazardous air pollutant (HAP) regulations. Research was conducted at an industrial scale to study potential emission differences from the final regenerative thermal oxidizer (RTO) of the dryer exhaust gases when the biosolids drying system was fueled by natural gas and biogas at the Morris Forman Water Quality Treatment Center (MFWQTC) in Louisville, Kentucky. Analyzed samples taken from the RTO's exhaust demonstrated that the RTO's flue gas emission difference between the two modes averaged between 2 and 10 μg/m3 for the selected compounds, an insignificant difference. This indicates that biogas, a free byproduct at the plant, is most likely as clean as the costly natural gas, and should be considered an economical renewable source of energy without causing an unsound elevation in harmful emissions.
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