This study aimed to identify possibilities to improve the quality of the stabilized biowaste coming from the mechanical–biological treatment of municipal solid waste produced in Rome for possible recovery rather than landfilling. The waste sampled before and after the first and the fourth week of aerobic biodegradation as well as different particle size classes composing the biostabilized materials were characterized in order to investigate the content and distribution of contaminants. Results showed, firstly, that the stabilized biowaste was not biologically stable after 4 weeks of biostabilization process, presenting dynamic respiration index >1,000 mg O2 kg VS−1 h−1. Heavy metal (Cd, Cr, Cu, Ni, Pb, Zn) content fulfilled the requirements for utilization, but by contrast, their release in water phase was quite high and not complying with Italian regulatory limits for waste recovery. In order to raise the quality, the biological process management should be improved by increasing the water amount to add to the rotting waste since the output material had moisture (19.7 % WM) lower than the recommended content for an optimized aerobic biodegradation (>40 %). Furthermore, fractions having particle size higher than 10 mm, composed of high content of impurities and heavy metals, should be separated by obtaining a reduction in chemical–physical contamination. Finer waste flow (<10 mm) can be recovered in environmental remediation activities, whereas coarser fractions (>10 mm) can be used in waste-to-energy plants, given the great content of high calorific value materials and the compliance with characteristics defined for solid recovered fuels.