Energy-efficient thermal treatment of sewage sludge for its application in blended cements

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Although landfilling and disposal into the ocean present still the most common methods used in the sewage sludge management, the share of recycling and reuse based on drying and incineration procedures continuously increases during the last decades. In this paper, an energy-efficient thermal treatment of sewage sludge is proposed, aimed at its use in blended cements. A practical application of the designed treatment is performed for the sewage sludge obtained from the municipal wastewater treatment plant in Patras, Greece. The material characterization shows that the sludge thermally treated at 700 °C has significant amounts of silica, alumina, calcium oxide and iron oxide, while the content of amorphous phase is 41%, which results in a good pozzolanic activity. The mechanical properties of hardened cement pastes prepared using blended Portland cement-thermally treated sewage sludge binders are after the 28-days curing period not satisfactory yet, indicating that a substantial part of the sludge acts still as filler only. This discrepancy between the pozzolanic activity and mechanical tests can be explained by too coarse sludge particles which slow down the pozzolanic reaction. The results of chemical tests show that the safe limit for the practical use of sewage sludge as partial Portland cement replacement is 10%, which is mainly due to the relatively high content of chlorides and alkalis. This limit can though be increased by the removal of soluble salts from the raw sludge using water treatment. The life cycle assessment analysis of the blended cements shows that the decrease in energy consumption is nearly 10% for each 10% of sewage sludge in the blend. The emission of greenhouse gases during the thermal treatment of the sludge is found almost twenty times lower than in the Portland cement production. The cost analysis reveals substantial economical benefits related to the application of sewage sludge as partial cement replacement.