Abstract
The metal(loid) and in particular the Arsenic (As) burden of thirteen agricultural biogas plants and two sewage sludge digesters were investigated together with the corresponding microbial consortia. The latter were characterized by ARISA (automated ribosomal intergenetic spacer analysis) and next generation sequencing. The consortia were found to cluster according to digester type rather than substrate or metal(loid) composition. For selected plants, individual As species in the liquid and gaseous phases were quantified, showing that the microorganisms actively metabolize and thereby remove the As from their environment via the formation of (methylated) volatile species. The As metabolites showed some dependency on the microbial consortia, while there was no statistical correlation with the substrate mix. Finally, slurry from one agricultural biogas plant and one sewage sludge digester was transferred into laboratory scale reactors (“satellite reactors”) and the response to a defined addition of As (30 and 60 µM sodium arsenite) was studied. The results corroborate the hypothesis of a rapid conversion of dissolved As species into volatile ones. Methanogenesis was reduced during that time, while there was no discernable toxic effect on the microbial population. However, the utilization of the produced biogas as replacement for natural gas, e.g. as fuel, may be problematic, as catalysts and machinery are known to suffer from prolonged exposure even to low As concentrations.
Highlights
Biogas is considered a possible substitute for fossil fuels such as natural gas in a sustainable energy mix
In this study we investigated metal(loid)s found in agricultural biogas plants and sewage sludge digesters and in particular the production of As species under anoxic conditions in these plants
The most abundant 30 taxonomic groups are shown averaged over all samples; the full dataset is available in Additional file 2: Fig. S1
Summary
Biogas is considered a possible substitute for fossil fuels such as natural gas in a sustainable energy mix. The number of elements entering a biogas plant via the substrate and/or various additives is much larger and includes a wide selection of metal(loid)s. Volatile compounds exist for European Biogas Association, that they do not occur in significant amounts in the produced gas (Svensson 2014). This includes As species, even though natural biogas formed in swamps and marshes, and in rice paddies, has been shown to contain, inter alia volatile As species (Jia et al 2012; Zhang et al 2016)
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