Wet sludges cannot be incinerated because of the large amounts of fuel that would be needed. However, a filter cake of sludge or a well-thickened sludge may be burned. If the solids concentration can be raised to roughly 30 to 40 per cent by weight, the heat of combustion of the organic material will about match the heat of vaporization of the water, thus minimizing or eliminating the need for fuel except for start up. If solid wastes are being incinerated, their fuel value may be enough to compensate for adding sludge that is not very dry. At 35 per cent solids, a sludge resembles common dirt.

Although there are many incinerator designs, the rotary hearth is quite common. Here multiple hearths are arranged vertically, with rakes moving the solids across one to fall to the next. An animated computer sketch of such a unit is needed.

The gas entering is usually air except at startup when fuel is needed to initiate burning. Gases rising from the combustion zone drive off moisture from the feed. Below the combustion zone, the solids are still red hot and react with the air to oxidize the last traces of anything that will burn leaving only mineral ash.

  • Much more detailed information about incineration