Report: Factors affecting the content of potentially toxic elements in the biodegradable fraction of autoclaved household waste

Abstract

A total of 3.4 tonnes of household waste (HW) that had a controlled composition were autoclaved in the presence of saturated steam for 1 h at the nominal temperature levels of 130, 160 and 200°C. The levels of Cd, Cr, Cu, Pb, Hg, Ni, and Zn were examined in samples of the fibrous, predominantly-biodegradable fraction (floc) derived from the autoclaved HW, aqueous extracts from floc, and the condensate from steam that was released from the reactor. This data was used to study the impact of the overall levels of potential toxic elements (PTEs) in HW feed and autoclaving process parameters on the levels of PTEs in floc. No consistent correlation was found to exist between the levels of PTEs in HW feed and those in the produced floc. This was primarily attributed in the inherent variability of PTEs in waste materials. Autoclaving temperature, pressure and mixing were all found to affect the mobilization and removal of PTEs from HW and their presence in floc. Temperature/pressure appeared to increase PTE removal through the condensate path, although that removal had only a negligible impact on the contents of PTEs in floc. Autoclaving temperature correlated positively, although not significantly at a 95% confidence level, with the levels of Pb and Zn in floc. On the other hand, it correlated negatively with the Hg content in floc. Increasing temperature/pressure also increased the water extraction of PTEs from floc, with the exception of Cr. However, in the context of producing high quality compost, the extraction rates were too low to bring about a substantial decrease of PTEs in floc.