Comparison of fractionation methods to assess Ni impact on anaerobic digestion: Filtration versus DGT

Excess of trace elements (TE) significantly alters the performances of anaerobic digestors (AD). Due to interactions with organic matter in particular, only a small fraction of TE can effectively interact with the biomass. However, assessing the bioavailable fraction of TE remains an issue. This study focuses on two chemical methods to monitor the potentially bioavailable Ni, selected as a model TE. The determination of the dissolved fraction by microfiltration was selected because it is commonly used, while the determination of the labile fraction by DGT (Diffusive Gradients in Thin films) was tested as it might better assess the bioavailable fraction. Different levels of Ni and ligands (EDTA and extracellular polymeric substances) were added in lab-scale AD reactors to induce AD performance alteration by changes of Ni amount or speciation. The AD performances were evaluated through biogas production and methane content. The results show that monitoring dissolved Ni highlights the alteration of AD performance due to variations of Ni amount but failed to detect alteration by change of speciation. The monitoring of DGT-labile Ni highlights reactor performance alteration by both variation of Ni amount and speciation. DGT therefore appears as an interesting complementary tool to detect potential alterations induced by TE.