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Fate of trace elements during and after anaerobic digestion: a sequential extraction method and DGT technique to assess bio-accessible trace elements in digestate

Research output: Book/ReportDoctoral thesisCollection of Articles


Original languageEnglish
PublisherTampere University
Publication statusPublished - 22 May 2019
Publication typeG5 Doctoral dissertation (article)

Publication series

NameTampere University Dissertations


Different chemical interactions between trace elements and organic/inorganic compounds originating from the substrate and generated during the anaerobic digestion process will determine the speciation of trace elements in anaerobic digesters. After anaerobic digestion, digestates are exposed to oxidizing conditions which may favor a change of trace elements’ speciation and consequently bio-accessibility for soil microorganisms and plants when digestates are spread on lands as organic amendment. Several techniques were used to assess the mobility, accessibility, and potential bio-availability of
trace elements in digestates for environmental risk assessments of digestate utilization as a soil fertilizer. The aim of this thesis is to evaluate a sequential extraction procedure and the diffusive gradients in thin films technique (DGT) to assess bio-accessible trace elements in digestate samples. Samples were taken from full-scale anaerobic digestion plants treating a mixture of industrial and municipal solid wastes or sewage sludge. The elements investigated include Al, As, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Pb, Sb, Se, Zn and W.

A sequential extraction procedure, originally conceived for organic matter fractionation, was implemented to simultaneously extract organic matter and trace elements in a substrate and digestate sample. It was observed that more than 60% of total As, Cd, Co, Fe, Mn, Ni and Zn were extracted along with the operationally defined organic matter fractions in both samples. In contrast, a lower recovery was observed for Al, Cr, Cu, Mo and Pb. These elements were mainly found in the dissolved organic matter fraction where soluble trace elements (e.g. free ions and complexed with organic/inorganic ligands) are
likely bio-accessible for microbial up-take. Moreover, a high portion of elements was found in the mineral fraction (e.g. sulfide), which was considered poorly bio-accessible. However, the feasibility of using the aforementioned method was questioned following the low efficiency of extraction of certain trace elements during the extraction procedure. Moreover, it was acknowledged that chemical reagents employed during the extraction procedure could have promoted a dissolution/precipitation of trace elements and therefore a change in their fractionation.

Therefore, DGT technique was tested to fractionate trace elements and it was observed that this technique increased the sensitivity of trace elements monitoring compared to conventional dissolved elements measurements in digested sewage sludge. However, it was observed that the DGT samplers’ deployment time in digested sewage sludge should be carefully evaluated. Additionally, the digestate matrix lowered the accumulation of some trace elements in the DGT samplers. Therefore, DGT labile trace elements (i.e. most bio-accessible species) can be correctly estimated provided a careful adapta-
tion of the deployment time as well as an evaluation of the matrix effect is performed in digestate samples. Unless this, general trend of labile trace elements over time could be estimated such as the distribution of labile trace elements over time in digestate exposed to air. Therefore, the effect of atmospheric air on the mobility and bio-accessibility of trace elements, including labile and soluble fractions, in digested sewage sludge was investigated. The exposure of digestate to air promoted dissolution of Al, As, Co, Cr, Cu, Fe, Mn, Mo and Pb, suggesting that a possible increase in their mobility may likely occur
during digestate storage in open tanks or handling before land spreading. Labile elements’ fraction increased only during an increase of aeration (except for Fe and Mn), suggesting that their short-term bio-accessibility can increase only after significant aeration as the one assumed to occur when digestate land spreading takes place.

These results open new fields of investigation for improving estimation of bio-accessible trace elements in digestate samples. For example, DGT technique should be further explored to accurately estimate labile trace elements concentrations in digestates.

Field of science, Statistics Finland