Migration characteristics and ecological risk assessment of heavy metals in ash from sewage sludge co-combustion in coal-fired power plants

Co-combustion in coal-fired power plants is one of the main methods to dispose of municipal sewage sludge (MSS). The high heavy metals content of MSS increases the ecological risk of ash from the coal and MSS co -combustion. In response to this problem, the three types of fly ash or bottom ash from MSS co-combustion in the three coal-fired power plants were sampled. The properties of fly ash and bottom ash were analyzed in terms of ash composition, particle size distribution, heavy metals content and leaching characteristics. The potential ecological risks were evaluated using the ecological risk index assessment method. The results showed that the co-combustion of MSS had no significant effect on the ash composition order and heavy metals content order. However, the total content of Cu, Mn, Pb and Zn, Hg in the co-combustion ash was higher than that in the coal-fired ash without sludge blending but did not exceed the risk screening value for soil heavy metal contamination of residential land. Heavy metals showed significant enrichment in fly ash and dilution in bottom ash. The leaching concentrations of Cr and Ni were several times the limit value. The potential ecological risk levels of fly ash and bottom ash were moderate risk and high risk respectively. Cr and Hg were the main contributors to potential ecological risks. The contents of Cr, Hg and Ni need to be paid attention to in the subsequent utilization of coal ash mixed with co-combustion of sewage sludge.

Automated summary

Co-combustion of municipal sewage sludge (MSS) with coal in power plants is a common method for disposal, but it increases the ecological risk due to the high heavy metals content. This study analyzed the properties of fly ash and bottom ash from MSS co-combustion, including ash composition, particle size distribution, heavy metals content, and leaching characteristics. The results showed that the co-combustion had no significant effect on ash composition and heavy metals content order. However, the total content of Cu, Mn, Pb, Zn, and Hg in co-combustion ash was higher than that in coal-fired ash without sludge blending. The leaching concentrations of Cr and Ni exceeded the limit value, indicating a potential ecological risk. The potential ecological risk levels of fly ash and bottom ash were moderate and high, with Cr and Hg as the main contributors.