9.3 What are the preconditions and requirements for burning of sludge?

To determine if burning sludge is a suitable option, sludge/biosolids need to be characterised and analysed. It is essential to know the composition, moisture content, calorific value, and potential contaminants of the sludge.

The water content must be reduced by a large amount through dewatering, followed by drying to guarantee process efficiency and take advantage of the sludge’s fuel potential. The recommended dry matter content depends on the specific thermochemical process. There needs to be at least 35% dry matter content, although the process efficiency increases with higher concentrations. Lower dry matter or higher moisture contents will usually result in losses of thermal efficiency and increased production of gases such as carbon monoxide.

While dewatering has essential importance, stabilisation of the sludge is less important. Considering heat recovery, the received sludge should be undigested to have a significant fuel value because of the high organic content. Also, sludge that may still contain pathogens does not pose a problem. The high process temperatures will ensure the elimination of these pathogens.

Aside from the pre-treatment, the burning of sludge in a controlled environment requires a reliable design of the containment and process. This is of high importance as the burning process produces exhaust gases. Components such as a chimney or stack to exhaust gases and particulates into the atmosphere, as well as a filtration system that captures and removes particulates from the exhaust gases before they are released through the chimney, may need to be considered.