A piling test with urban refuse (R-pile) and urban refuse + sewage sludge (R + S-pile) was carried out to study the evolution of organic matter during composting, and compost maturity. The R + S-pile maintains during around 20 days the limit of thermal inactivation of the micro-organisms. Therefore, the exclusive determination of temperature in piling composting plants cannot be considered in all cases as the conclusive criterion to estimate the grade of biological stability of compost. The bio-oxidative phase was 75 days and the optimization of C/N ratio in the water-soluble phase (C/N(w)) and cation-exchange capacity (CEC) occurs in the maturation phase. Likewise, the C/N ratio in the solid phase (C/N(s)) cannot be considered as a valid parameter to evaluate the maturity degree of compost: the R-pile reached the optimal value of 15 in full thermophilic phase, with an optimal value of the initial material of 28. The period in which practical temperature stabilization is achieved with a C/N(w). ratio lower than 6 and a CEC value higher than 60 meq 100 g-1, on an ash-free material basis, may constitute the most valid criterion for establishing the optimum degree of maturity.
