sludge characterisation on the energy level

The energy utilisation potential is defined by the raw sludge’s net calorific value (NCV). This potential is calculated from the NCV of the organic matter (between 20,900 and 23,000 kJ · kg^–1 of organic matter) and this NCV is used to deduce the condensation heat of water contained in the sludge concerned.

Compared with other waste, sludge has a low NCV. For guidance, untreated household waste has an NCV of approximately 8,500 to 9,000 kJ · kg^–1 of household waste whereas mechanically dewatered sludge has a NCV of between 1,200 and 1,500 kJ · kg^–1 of dewatered sludge (25 % dry matters).

All heat applications are also heavily affected by the specific pollutants covered by atmospheric emissions regulations. In this context, it is essential for the following to be quantified :

• halogen pollutants (Cℓ, F);
• sulphur pollutants (S, mercaptans, H₂S…);
• volatile heavy metal pollutants (Hg, Cd, Tℓ);
• heavy metal pollutants regarded as slightly, if at all, volatile (Sb, As, Pb, Cr, Cu, Co, Mn, Ni, V);
• dioxin and furan type pollutants.