This study evaluated the use of two-stage composting (the first stage an aerated semi-closed reactor and the second a pile turning) for industrial-scale composting of sewage sludge (SS) and its effect on the quality of the final compost. The results showed that the two-stage system was more effective in obtaining a compost with stabilized and mature organic matter. The material at the reactor outlet presented an adequate level of sanitization in accordance with the limits established by Spanish regulations for the content of pathogenic microorganisms in organic fertilizer products. However, the resulting material was unstable and immature, with a high content of easily degradable organic matter and phytotoxic compounds, such as NH4+. It was also observed that the Zn and Cu content was high in the final compost, and control of these elements in the SS feeding is necessary for safe agricultural use of the compost.