The process comprises the biological transformation of organic waste sludges both of domestic and industrial origin by means of the spontaneous fermentation, in heaps (2), of a mass (1) formed from said sludges and a mechanical substrate of wooden splinters having a high resistance to the biological attack. The heaps are periodically turned over in order to ventilate the mass and are of such sizes that, by means of the turning over, it is mantained a micro-aerobic fermentation with an oxygen content at the center of the heaps in the range from 0,1% to 0,5%. After a fermentation of about 60 days, the mass is screened in order to separate two components, one comprising the fertilizer and the other the mechanical substrate of wooden splinters, which are recycled at the head of the process as a biological inoculum for the fermentation (Figure 2).