Information is presented to support the numeric limits proposed in the regulations for sewage sludge incineration for the metals: arsenic, beryllium, cadmium, chromium, lead, mercury, and nickel, and total hydrocarbons (THCs). For THCs, the numeric limit is a stack combustion gas concentration, based on allowable ambient air quality at ground-level, site characteristics, and meteorological conditions. The limits for metals are allowable sludge concentrations, based on allowable ambient air quality at ground-level, allowable emission rates, air dispersion factors, and data on facility operating conditions. A major process in determining the limits was the risk methodology. In the methodology, cancer potency values were applied and risk levels were developed based on worst-case emission and exposure scenarios. Air dispersion models were used in the methodology to predict the maximum emission levels. They were also used to develop a simplified approach for determining compliance with the metal and THC limits.

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