In most multiple hearth furnaces, partially dewatered sludge is fed onto the perimeter of the top hearth. The rabble arms move the sludge through the incinerator by raking the sludge toward the center shaft where it drops through holes located at the center of the hearth. In the next hearth, the sludge is raked in the opposite direction. The effect of the rabble motion is to break up solid material to allow better surface contact with heat and oxygen.
Scum may also be fed to one or more hearths of the incinerator. The scum quantities are generally small compared to those of other wastewater solids.
Ambient air is first ducted through the central shaft and its associated rabble arms. A portion, or all, of this air, is recirculated from the top to the bottom as preheated combustion air. The combustion air flows upward through the drop holes in the hearths, countercurrent to the flow of the sludge, before being exhausted from the top hearth. Air also enters the bottom to cool the ash. Provisions are usually made to inject ambient air directly into the middle hearth.
Multiple hearth furnaces can be divided into three zones:
- The drying zone (upper hearths) where most of the moisture in the sludge is evaporated
- The combustion zone (middle hearths) where the temperature is higher
- The cooling zone (lowermost hearth), where the ash is cooled as its heat is transferred to the incoming combustion
Multiple hearth furnaces are sometimes operated with afterburners. Afterburners further reduce odors and concentrations of unburned hydrocarbons. In afterburning, furnace exhaust gases are ducted to a chamber where they are mixed with supplemental fuel and air and completely combusted.
Under the normal operating condition, 50 - 100% excess air must be added to a multiple hearth furnace to ensure complete combustion of the sludge. Besides enhancing contact between fuel and oxygen in the furnace, these high rates of excess air are necessary to compensate for normal variations in both the organic characteristics of the sludge feed and the rate at which it enters the incinerator.