Abstract: A system for controlling the operation of a multiple hearth furnace over a diverse range of operating conditions at a minimum level of energy consumption. Relatively low temperature gases are controllably recirculated from the drying zone to the combustion zone of the furnace, and/or gases are controllably discharged from the combustion zone and/or drying zone to regulate drying zone temperature, combustion zone temperature and the quantity of excess air required to provide optimum furnace operating conditions. The system is particularly useful for controlling the incineration in a multiple hearth furnace of sludge from waste water treatment plants.