The primary purpose of a multiple hearth furnace when used with carbon adsorption is to ______ ___ _____ ______ adhering to the spent carbon.

• A. Cool and store spent carbon
• B. Filter residual organics
• C. Replenish activated carbon
• D. Heat and expel molecules adhering to the spent carbon

Answer: (D)

Thermal regeneration of activated carbon is used to remove the contaminants that remain adsorbed on the carbon after it has treated the water. In a multiple hearth furnace, the spent carbon is heated in stages so those adsorbed organic molecules are driven off and expelled as off‑gas, often with oxidation of the organics. This desorption and destruction/oxidation of the adsorbed compounds regenerates the carbon so it can be reused in the adsorption system. Cooling, storing, filtering residual organics, or replenishing carbon are not the primary regeneration goal; the key purpose here is to heat and drive off the molecules adhering to the spent carbon.