(352d) Toxicity Assessment of Bottom Ash from Biomass and Sewage Sludge Co-Gasification

The harmful substances in sewage sludge, such as heavy metals, can pollute the environment and pose severe threat to human health. Gasification, a process that converts organic or fossil based carbonaceous materials into carbon monoxide, hydrogen and carbon dioxide, can be used to treat sewage sludge and turn it into solid residues which consist mainly of inert mineral ash. The bottom ash can be 8-15% of the original volume and is usually for landfill. However, toxins such as most heavy metals still persist in the ash and can be possibly leached out into environment by rain or surface water. Therefore, it is important to examine the possible toxic effects of the bottom ash on human health.

In this study, toxicity analysis was conducted for sewage sludge and bottom ash – feedstock and products respectively, from the co-gasification of sewage sludge and woody biomass process in a fixed-bed downdraft gasifier. Extractions of both sewage sludge and bottom ash exhibited toxic effects on human cells of HepG2 and MRC-5 when they were present in high concentrations in the cell culture medium, represented by the decreased cell viability and abnormal cell morphology. The toxic effect of these materials was found to be more significant with longer duration of exposure, and the toxicity could be greatly alleviated upon further dilution. The effects of PH value, heavy metals and PAHs were investigated to evaluate their roles in the toxicity observed.