Thermal drying has become necessary given the technical restrictions on mechanical dewatering (see sections belt filters, centrifugation and filter press). Furthermore, drying offers many advantages and allows diversifying the sludge recovery channels to be diversified (see section treatment line design: limitations).
A temperature gradient has to be created between the inside and the outer surface of the granules or floc that make up the sludge so that this bound water can be eliminated by evaporation. When this process occurs, the water vapor formed in the core diffuses towards the surface layer, designated border bed. The energy involved is the sum of the latent heat and of this water’s binding energy (low compared with the former).
As the energy from evaporation is very high, it is advised that dewatering is optimised before drying in a dryer in order to reduce its size. The equipment upstream to the dryer must be suited to the type of dryer used.
Moreover, it is essential to correctly define the dry content to be achieved in the material leaving the dryer :
- The dry content must be high enough for the dried sludge to be stabilised and sanitized, but
- low enough to avoid creating dust particles and over consuming energy.