Agricultural recovery is the primary option for the final destination of sewage sludge, which must be properly treated, stabilized and sanitized. However, these processes are occasionally eliminated or not carried out properly for economic reasons or due to the design and size of the treatment plant. In this study, 62 samples of sewage sludge (21 and 41 samples from Seville and from Huelva, respectively) were characterized. Variable annual sludge production and equivalent inhabitants are virtually proportional (related by a potential power function of 0.9134). The results were evaluated through multivariate statistical analysis in order to assess the operability status of the WWTPs and the final quality of the sludge generated in the processes. Principal components analysis provided information on two separate groups for the bulk of waste water treatment plant (WWTPs) analyzed: (1) a nickel group, where the samples showed outliers above 50 mg/kg with respect to other WWTPs; and (2) the group where total nitrogen, organic matter and total potassium (K2O) showed abnormally high values. Linear discriminant analysis models allowed the detection of WWTPs that used inadequate treatments but where geographical area and number of equivalent inhabitants were not significant. (C) 2017 Published by Elsevier B.V. on behalf of Institution of Chemical Engineers.