Process for the treatment of sludge

A process for the treatment of sludge issuing from waste water treatment plants, containing less than 85% of water, comprising the steps of:

• a) mixing the sludge with a solution of formaldehyde alkalized with an effective amount of caustic soda, inside a mixer in order to eliminate all the microorgamsms contained in the sludge;
• b) adding to the resultant sludge of step a) sufficient urea to transform residual formaldehyde to mono methylene urea;
• c) mixing the resultant sludge of step b) with a product containing ammonium sulfate and organic matter;
• d) feeding the mix of step c) into a pre-dryer including an exit connected to a fines collector, into which is fed a hot gaseous stream containing sulfur trioxide and sulfur dioxide in order to polymerize the mono methylene urea contained in the mix of step c) to yield a resin rich in nitrogen and resulting in an exothermic reaction between the gaseous sulfur trioxide, the water, and the ammonium sulfate contained in the mix of step c) by means of which the ammonium sulfate is transformed into acid ammonium sulfate and part of the water contained in the mix of step c) reacts with the sulfur trioxide and another part of the water is evaporated by the heat of said exothermic reaction thus obtaining a semi-dried sludge;
• and
• e) feeding the semi-dried sludge of step d) to an ammoniator operating with an excess of ammonia into which the remaining water Contained in the semi-dried sludge of step d) is evaporated by an exothermic reaction between the ammonia and the acid ammonium sulfate contained in the semi-dried sludge of step d) which is transformed to ammonium sulfate thus obtaining a dry sludge free of pathogenic and non pathogenic microorganisms enriched with nitrogen.