Effect of the stabilization process of sewage sludge on the mercury content of municipal sewage sludge

• Investigations of the physicochemical parameters of 17 different sewage sludges. • Analysis of the effect of the stabilisation process of sludge on mercury content. • The stabilization process does not guarantee the same properties for sludges. • The source of the sludge has a decisive influence on the mercury content. • Sludge stabilization processes mainly reduce the organic matter content of sludge. • Sludge stabilization processes increase the mercury content of sewage sludge. Sewage sludge, the amount of which is increasing every year, must be disposed of on the one hand, and on the other hand it makes a pretty good alternative fuel. However, due to the high mercury content of sewage sludge, its use for energy purposes requires further research. The effect of the technological process of sewage sludge treatment on the mercury content of this end product of wastewater treatment is not fully understood. Therefore, the present study examined 17 sewage sludges from municipal wastewater treatment plants in Lower Silesia for energy properties and mercury content before and after the following technological processes: autothermal aerobic stabilization at temperatures above 50 °C (ATSO), aerobic separated stabilization at effluent temperature (KTSO), separated digestion at ambient temperature (OKF) and separate digestion at 37 °C (WKF). The study showed that the mercury content ranged widely from 0.47 to 6.05 mg/kg Hg, indicating several times higher atmospheric emissions than when using conventional fuels. Among the sewage sludge samples tested, there were samples with very high mercury content, significantly different from the average mercury content of the sludge from the same process line. On this basis, it is concluded that the sewage sludge disposal process leads to the formation of significantly heterogeneous material. Sludge stabilization processes (ATSO, KTSO, OKF, and WKF) increase the mercury content of sewage sludge.