Sources and levels of potentially toxic elements in the biodegradable fraction of autoclaved non-segregated household waste and its compost/digestate

Abstract

Waste that reflected the average UK composition of household waste was treated by autoclaving at the three set pressure/temperature levels of 2.7 bar/130 °C, 6.2 bar/160 °C and 15.5 bar/200 °C. The biodegradable fraction of the autoclaved household waste (`floc') was manually separated by screening and underwent characterization for its Cd, Cr, Cu, Pb, Hg, Ni, and Zn content. Autoclaving did not guarantee the production of compost/digestate that met the UK specification for compost, BSi PAS100, without restrictions being made on the composition of the waste feedstock. Results indicate that the levels of Zn and Cd associated with floc materials alone could lead to compost limit values being exceeded. For all other potentially toxic elements (PTEs), the estimated excessive (i.e. above levels of compliance) PTEs levels for compost/digestate were mainly due to external (i.e. non-floc) materials, primarily electronic/electrical waste. Batteries may have also contributed to the high levels of Zn and Hg. In this study, for all PTEs examined, with the exception of Cd and Zn, autoclaving had a performance comparable to that of the most effective mechanical biological treatment systems.