1 SEWAGE SLUDGE: CHARACTERISTICS AND RECOVERY OPTIONS

In this paper, an overview on sewage sludge characteristics and recovery routes is presented. Starting from an analysis of typical sewage sludge characteristics as found in literature, the most promising disposal options are presented. A focus on incinerated sewage sludge ash (ISSA) follows. Most of the authors agree on considering pyrolysis and gasification as the most viable treatments for sludge in the near future. For this reason, the last two chapters offer a brief description of these two processes and their products. Sewage sludge Characteristics Sewage sludge is a kind of waste coming from municipal wastewater treatment plants. In these facilities, high quantities of water are treated every day to eliminate bacteria, viruses and pollutants. As main output, these plants produce treated water and sewage sludge. A qualitative example in Fig.1 shows the main steps of the process followed by municipal wastewater treatment plants in Bologna district (Italy). The calculations refer to 100 cubic meters of wastewater entering the municipal treatment plant. Sludge, which constitutes 1 % of wastewater entering the plant, is digested anaerobically and dehydrated. Sewage sludge, at the outlet after mechanical drying, is made of approximately 80% moisture and 20 % dry matter. The management of this product has become a serious problem in Europe over the last years because of legislation and environmental issues. Typical destinations for sewage sludge were agriculture, forestry, incineration, land building or landfills, but most of them have been banned or limited by recent EU and local laws. Sludge constituents are organic and inorganic compounds, including traces of heavy metals such as chromium, zinc, mercury, lead, nickel, cadmium and copper. These elements restrict the use of sludge in agriculture, because their accumulation is harmful to the environment and particularly to the food chain