Fate and distribution of heavy metals during thermal processing of sewage sludge

Thermal processing of sewage sludge (SS) has received increasing attention in recent years. Thermal processes valorise the carbon rich organic fraction of SS, while effectively reducing SS volume. However, the fate and distribution of heavy metals (HMs) during thermal processing of SS is an important issue to address because it has impact on the generation of secondary pollutants and the environmental acceptability of the residues for reuse and reclamation. The refractory metals (thermally stable, i.e. Cr, Mn and Ni) are less volatile at typical temperature ranges (200-1100 °C) of thermal processes, and they are enriched in the residues. On the contrary, HMs with lower thermal stability (i.e. Hg, Cd, As and Pb) are prone to volatilisation. However, volatilisations and enrichments of HMs in the residues strongly depend on the characteristics of SS and nature of the thermal process. This review article discusses the volatilisation, enrichment and speciation (or stabilisation) of HMs in the residues formed during thermal processing of SS (incineration, pyrolysis, gasification and hydrothermal treatment). First, it summarises the fundamental aspect of SS in each thermal process. The influencing factors on the fate and distribution of HMs are discussed in terms of process principles, reactor types, operating conditions, pre-treatment of SS, use of additives and co-processing with secondary feedstocks. The use of advanced analytical techniques and modelling tools to analyse the complexity of HMs redistribution during thermal processing is described. Practical and economic challenges associated with HMs in SS during operation of full-scale thermal processing facilities are also addressed. Finally, a brief comparison of HMs redistribution and stabilisation during SS incineration, gasification, pyrolysis and hydrothermal treatment is provided.