Stabilization of heavy metals during co-pyrolysis of sewage sludge and excavated waste

Abstract

In this paper, excavated waste was added to sewage sludge for co-pyrolysis, aiming to stablize the heavy metals in sewage sludge. The effect of co-pyrolysis with various pretreatment (e.g. cooling, drying and hydrothermal pretreatment) on heavy metals stabilization was studied using orthogonal test. The results showed that the optimal conditions are 600 °C, nitrogen flow rate of 200 mL/min, mixing excavated waste with sewage sludge (25:75, wt%) and hydrothermal pretreatment. 90% of the heavy metals in the sewage sludge and excavated waste mixtures were transformed to biochars after co-pyrolysis. Moreover, the state of heavy metals changed from bio-available fractions to stable state, thereby reducing the potential ecological risk index (RI) from 116.8 to below 50, which represented a reduction in contamination levels and ecological risks from considerate to low. Finally, the study found that the synergy between hydrothermal and pyrolysis made full use of the moisture in sewage sludge and was more conducive to the solidification of heavy metals. This paper provides a good option to dispose multiple wastes and reduce their environmental risks.