Co-incineration effect of sewage sludge and municipal solid waste on the behavior of heavy metals by phosphorus

The effects of sewage sludge phosphorus (P) content on heavy metal behavior during co-incineration of sewage sludge and municipal solid waste (MSW) were evaluated. Thermogravimetric differential thermal analysis revealed that MSW incineration was mainly caused by organic matter and fixed carbon, while sewage sludge incineration was caused by volatile matter. During co-incineration, the peak weight loss at 460 °C shifted to slightly higher temperatures and the sludge ratio increased, indicating that interaction effects during co-incineration delayed pyrolysis and polymer/fixed carbon incineration. The residual heavy metal ratios after mono-incineration of sewage sludge were higher than those after MSW mono-incineration. The Cl content of MSW (0.757%) was much higher than that of sewage sludge (0.068%), which resulted in the conversion of heavy metals into metal chlorides and then volatilized during MSW mono-incineration. A synergistic effect of co-incineration was evident for Cu, but not for lead (Pb) or cadmium (Cd). X-ray absorption fine structure (XAFS) measurement revealed that Cu in MSW ash was in the form of CuO(s), but was Cu3(PO4)2 in sewage sludge and co-incineration ashes. CuO(s) is relatively unstable and may be transformed to CuO(g) or CuCl(s) before volatilizing at high temperature or in the presence of Cl. Phosphorus has the effect of stabilizing Cu in sewage sludge during co-incineration.