Perfectly dried, the sludge can thus be turned into fuel combustibles with as much calorific power as household waste, and can thus be used in electricity production and cogeneration systems. Cement manufacturing here has a double interest, the dry material being used both as a combustible and as a mineral supplement for the cement.

New technologies of sludge value enhancement are today at the research stage: pyrolysis (taken to a very high temperature in the absence of oxygen, the sludge distils a gas which can be retrieved, and the carbonised material can be made use of as a secondary combustible) or gasification (the sludge is oxidised by reaction with oxygen, releasing a gas composed of C0, C0² and H² which can also be retrieved).

Furthermore, hydrothermal oxidation is the technology used by the new water purification plant at Brussels North (1,100,000 Inhabitant Equivalents, one of the biggest in Europe). Heat oxidised, the sludge, still in a liquid state, produces a mineral residue which is recycled at a rate of 97% in concrete, and also produces a small amount of gas.

Monitoring drying by X-ray microtomography

At the University of Liège, Angélique Léonard, an FNRS qualified researcher and assistant lecturer at the Chemical Engineering Laboratory, devotes a part of her research to the convective drying of waste water sludge (by forced circulation of hot air). This method has just formed the subject of a new publication (2).

In the convention drying machine systems with which the laboratory is equipped, micro-dryers and pilot dryers, she studies the physical mechanisms which govern the drying process by monitoring the three operational variables (temperature, superficial speed and absolute air humidity).