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.