Elemental Migration and Transformation from Sewage Sludge to Residual Products during the Pyrolysis Process

The elemental distribution and physicochemical properties of residual products-namely, char, tar, and produced gas obtained during the pyrolysis of anaerobically treated dried sewage sludge was investigated and compared with those of raw sludge. Experimental investigations were performed in a laboratory fixed-bed reactor and revealed the migration fate of heavy metals (Cd, Co, Cr, Cu, Ni, Pb, Ti, and Zn), alkaline earth metals and alkali metals (Ba, Be, Ca, K, Mg, and Na), intermediate metals (Fe, Mn, Al, and Si), and nonmetals (P, S, Cl, C, H, and N) in pyrolysis products. Primary attention was focused on heavy metal migration. The highest heavy metal recoveries were found for Cd > Co > Cr > Cu from sewage sludge into the gas-phase, with 68.17% of the Cd eliminated. This study also showed that high-temperature pyrolysis technology is an effective and promising thermochemical treatment for sewage sludge to reduce its volume.