Research progress on the characteristics of heavy metal transfer and transformation in municipal sludge treatment

A large amount of sewage sludge is produced during the municipal wastewater treatment process with complex composition, and improper disposal would cause secondary pollution to the environment. Heavy metals in the sludge cannot be degraded in the sludge treatment and disposal process with only solid-liquid phase migration and chemical speciation transformation, and they will accumulate and migrate in the environment and finally enter the food chain and endanger human health. Not only the environmental risk of heavy metals is determined by their types and contents, but the chemical speciation distributions of the heavy metals are also the key factors affecting their bioavailability, migratability and ecological toxicity. Focusing on common treatment methods used in the harmlessness, stabilization, reduction and recycling of municipal sludge, including physical methods (thermal hydrolysis, ultrasonic, microwave) and chemical methods (pyrolysis, acid treatment, oxidation, electrochemistry), the impacts of these methods on interphase migration and chemical speciation transformation of heavy metals were described briefly. The reasons for the migration and transformation of the heavy metals were analyzed, and the relevant mechanisms were summarized and discussed. On this basis, recommendations were put forward for risk control of heavy metals in the sewage sludge, including total amount reduction and stabilization of heavy metals in the sewage sludge, as well as the control of heavy metal content in the wastewater.