Improved heavy metal fractionation of dewatered activated sludge via sequential physical extraction (SPE)

Detoxification of dewatered activated sludge is a pressing issue for countries that apply biological wastewater treatment. Heavy metals account for a major fraction of the toxic substances. To select the appropriate removal technique or to evaluate the removal efficiency, an analysis of the load and the fractionation of heavy metals is among the first to be done. Previous studies often inspect the speciation of heavy metals using sequential chemical extraction (SCE), which has been developed for non-activated-sludge systems. Beyond the known deficiencies of SCE, SCE was found to be insensitive to the variation of dewatered sludge upon acidification and aerobic/anoxic digestion. Thus, a sequential physical extraction (SPE) based on heat-assisted extracellular polymeric substance extraction was proposed to supplement or replace the SCE. Based on the SPE results, the commonly studied heavy metals were categorized into three groups, which are supported by previous results from X-ray absorption spectra and other non-SCE methods. By pinpointing the deficiencies of SCE, evaluating, and testing the performance of the SPE, we demonstrated that fractionation based on the spatial structure of activated sludge can be applied to better characterize the heavy metals in this organic-rich matrix. Further perfection of the SPE will facilitate the detoxification of dewatered activated sludge and eventually will promote resource recovery from activated sludge.