Composting was evaluated as option to produce a soil organic amendment, using as raw material the biosolids (stabilized sludges by methanogenic process) from wastewater treatment plant "El Salitre" located in Bogota, Colombia (EAAB). Different concentrations of biosolids as C material were evaluated in four treatments (T1=60%, T-3=50%, T-2 and T-4=40% and Control 100% of biosolids), in randomized experiment, under greenhouse conditions for 80 days. To establish compost piles, biosolids were mixed with Pennisetum clandestinum (kikuyo) grass, horse manure and rice husk. pH, temperature and humidity were measured weekly. The final compost was tested for chemical and microbiological properties, including toxicological effects on plant assays (Lactuca sativa L.). The best treatment was used to improve compost quality adding a thermophilic bacterial assortment. All statistical analyses were performed using Statistix 9 for Windows. Results showed a composting process in 74 days, with 50 degrees C as higher temperature for T-3 and T-4, enabling reduction and/or pathogen elimination, while control had 30 degrees C as higher temperature during process. T-3 showed better chemical characteristics (C/N 19.08, % CO 28.6, pH 6.3), than the other treatments according to Colombian National Standard for compost quality (NTC 5167/11) but inhibited the germination of Lactuca sativa L. at 75% (m/v) and 100% (m/v). Finally with thermophilic bacterial inoculum, T-3 reduces the composting process down to 52 days (22 days less), while presenting higher temperature (62.4 degrees C), better physicochemical properties (C/N 14.67, % CO 30.07, pH 5.83) (NTC 5167/11), and reduction of human pathogens (fecal coliforms) from >2.3x10(10) down to 16x10(4) MPN/4 g.