Comprehensive Evaluation of the Control Efficiency of Heavy-Metal Emissions during Two-Step Thermal Treatment of Sewage Sludge

Recycling the phosphorus in sludge by incineration has received great interest at home and abroad. However, heavy metals (HMs) is a restrictive factor for SS thermal treatment. In this study, a comprehensive evaluation method was adopted to evaluate the comprehensive control efficiency of HM emissions during two-step thermal treatment (incineration-calcination). The effects of temperature, calcination time, and additives (CaO and NaCl) on leaching rates, stabilized rates, and comprehensive control efficiency of HM emissions were investigated. Results showed that comprehensive control efficiency increased significantly with an increase of temperature because of the transformation of chemical speciation from a leachable to a more stable combined form. Additives Cao and NaCl promoted the volatilization of HMs and reduced the comprehensive control efficiency. The highest comprehensive control efficiency of HM emissions was 78% when the incineration temperature reached 950 °C. Furthermore, a comparison was made between leaching rates, stabilized rates, and a comprehensive evaluation method. The results were inconsistent when leaching rates and stabilized rates were adopted. In contrast, when the comprehensive evaluation method was used, the results were coordinated and unique. This work can provide a promising approach for the evaluation of control efficiency of HM emissions during the process of thermal treatment of sludge.