Relevance of Pyrolysis Products Derived from Sewage Sludge for Soil Applications

The recovery of sludge produced in the wastewater treatment process in WWTPs is often limited by the high content of toxic forms of contaminants of both an inorganic and organic nature. One of the options for the effective treatment of the world’s ever-increasing quantities of sewage sludge is the pyrolysis process. Thermochemical conversion of sewage sludge is emerging as a promising method for treating these heterogeneous and highly complex wastes with increasing research work. Pyrolysis-treated sewage sludge (PM) prepared at 603–615 °C was characterized by pH, EC, and CHN-S analysis; total and bioavailable concentrations of P and heavy metals (Cd, Cu, Fe and Zn); fractionation of bound forms of P and heavy metals in the material and determination of the presence of polycyclic aromatic hydrocarbons (PAHs). The studied material was subjected to ecotoxicological tests (Daphnia pulex L.) and cultivation tests (Lactuca sativa L.). Elemental analysis revealed the concentrations of heavy metals in PM: Fe (137,600 mg/kg), Zn (2602 mg/kg), Cu (582 mg/kg), Cr (107 mg/kg), Pb (87 mg/kg), Ni (67 mg/kg), As (<1 mg/kg), Hg (<2 mg/kg) and Cd (<1 mg/kg). The highest values of extractability of the investigated heavy metals from PM were found in the cases of Zn (HCl) and Fe (Mehlich 3), both values not exceeding 500 mg/kg. BCR sequential extraction showed the major concentrations of Cu and Fe were predominantly bound in the residual fraction (F4) and Zn in the reducible fraction (F2) of PM. The results of heavy metal bioavailability suggest that the addition of PM does not negatively affect the growth of lettuce biomass and the metal contents of plant tissues. Based on the results obtained, the pyrolysis material prepared from municipal sewage sludge seems to be a promising and innovative soil additive and a potential alternative to conventional inorganic fertilizers.