Indirect heat drying and simultaneous pelletization of sludge

A process for treating liquid sludge obtained from a waste water treatment plant, comprising:

• producing an organic fertilizer product by following the steps of:
• dewatering the liquid sludge to obtain a dewatered sludge product, mixing dry fines as nuclei for pellets into the dewatered sludge to produce a mixture, inputting the mixture into a dryer performing indirect heat drying and simultaneous pelletizing of the mixture by coating the nuclei with the dewatered sludge during said mixing and drying to build up the pellets layer by layer to a predetermined size, supplying sweep-air to the dryer for removing an air and water vapor mixture produced by the drying, extracting a dried product and the air and water vapor mixture from the dryer separately from one another, classifying the dried product extracted from the dryer into oversized, on-spec and fines fractions, wherein the oversized and fines fractions are greater than and smaller than the on-spec fractions respectively, crumbling the oversized fraction to a smaller predetermined size, recycling and using the fines and crumbled fractions as said dry fines in said mixing step, and maintaining a negative pressure in the dryer for preventing the escape of malodorous gases from the dryer.