Effect of thermal conversion of municipal sewage sludge on the content of Cu, Cd, Pb and Zn and phytotoxicity of biochars

Degradation of various compounds with the simultaneous formation of new or activation of initially unavailable chemically bound substances or elements can occur very often during the thermal conversion of organic materials. Thus, the aim of this research was to determine the effect of thermal conversion of municipal sewage sludge on the content of total and water-soluble forms of Cu, Cd, Pb, Zn, and on the phytotoxicity of the solid products generated in the process. The research material consisted of stabilized sewage sludge from municipal sewage treatment plants in Krakow (SS1), Krzeszowice (SS2) and Słomniki (SS3), Malopolska Province. The treatment plants are equipped with systems of mechanical-biological sewage treatment. The content of bioavailable forms was determined after a 24-hour extraction of a sample with redistilled water (material : water = 1: 5). Phytotoxicity of the extracts (material : water = 1 : 5) prepared from unprocessed sewage sludge and from biochars was examined against Lepidium sativum. Thermal conversion of municipal sewage sludge, with reduced air access, caused an increase in the content of total forms of copper, cadmium, lead and zinc in stable products of the process. The process of thermal conversion caused a reduction in the content of water-soluble forms of the elements, which points to their lower bioavailability. Phytotoxicity of sewage sludge was influenced more by its origin, and in consequence by the type of sewage treated, than by thermal conversion.