Background During the 1990s a considerable number of wastewater treatment plants were constructed in Greece in order to handle the major environmental problem of sewage treatment and disposal. The majority of plants have achieved the prime objectives of sewage treatment, but none were concerned with final sludge management and disposal. Almost all the sludge is taken to landfills for final disposal after dewatering. This practice has severely affected the capacity of sanitary landfills. It has also greatly increased the production of heavily polluted leachate and the generation of greenhouse gases, especially methane, by introducing biodegradable organic products to the sanitary landfills. Objectives The aim of the “Humification of sludge STP” project was to use the waste generated by municipal sewage treatment to produce soil, thereby creating an environmentally friendly sludge management process. For the adoption of this sludge-to-soil treatment method, the following measures were proposed: the preparation of a mixture of sludge tailored to specific soil requirements; the evaluation of the properties of the generated product; the use of the produced mixture as a soil; and the possible large-scale application of the method. Results The project succeeded in transforming dewatered sewage sludge into soil. The method, known as humification, was applied in Thessaloniki, near the Tsagarades landfill. The unit treated approximately 600 tons of dewatered sludge, together with 1,000 tons of inert material and 400 tons of green waste, to produce 2,000 tons of earth material. Though the soil product was intended for landfill cover, it was found to be of sufficient quality for parks and recreation areas as well. The area required 6,000 square meters of space (e.g. for storing and windrow rotting), and the demonstration site was situated in the vicinity of the Tsagarades landfill. Chemical analysis of nearly 100 parameters was used to determine the quality of the raw materials, the sludge and the final product. The mixtures – consisting of sewage sludge, inert materials and green wastes, such as wood and green cuttings, and defined according to the simulation of the final product composition - were layered in windrows and were constantly aerated and mixed with a special turning machine. Mixing guaranteed the homogeneity of the mixtures and the enrichment with oxygen to accelerate thermal rotting. Thermal rotting is an aerobic, exothermic, microbiological and biochemical transformation through which the decontamination of the mixture is ensured. Temperatures ranged from 55 to 75 degrees in the piles, resulting in the destruction of all pathogenic bacteria and spores. At the same time, the growth of thermophilic fungi enriched the final product. Continuous monitoring of the process included measurements of temperature, moisture, other physical parameters, and emitted gases, while samples were collected for chemical analysis of the intermediate products. At the end of the humification process, earth material equivalent to natural soil, free of sludge contaminants, was produced. Final laboratory analysis confirmed that the produce complied with all existing EU and Greek regulations regarding chemical composition, including heavy metals. It is suitable for supporting vegetation and will be used to cover the existing landfill in Thessaloniki. This project has been selected as one of the 22 "Best" LIFE Environment projects in 2006-2007