other countries, such as Italy, are more recent and less common. In this study, results about sludge stabilization occurring within beds after the entire period of operation in two different urban wastewater treatment plants situated in Denmark (Nakskov, 30,000 p.e.) and in Italy (La Fontina, 30,000 p.e.) were presented. In order to evaluate the process of sludge stabilization parameters that highlighted the biochemical and chemico-structural properties of sludge organic matter have been determined. In addition, the residual sludge biotoxicity have been investigated through a microbiological parameter, phyto-test, availability of inorganic and organic pollutants. The results showed that stabilization of the sludge similarly occurred in both treatment plants, even though in different landscape ecosystems, as shown by the low content of water soluble carbon and dehydrogenase activity. Moreover, the organic matter humification was demonstrated by the pyrolytic indices of mineralization and humification and by the significant level of humic carbon content. Furthermore, parameters related to sludge biotoxicity have been evaluated. Results about Escherichia coli have shown that the stabilised sludges were sanitized in both treatment plants; moreover, data about germination tests, bioavailable heavy metals and hazardous organic compounds demonstrated that the stabilised sludges were not toxic for environmental purposes
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