Elemental composition of biogas residues: Variability and alteration during anaerobic digestion

Biogas production and the amount of thereby incurred digestates increased remarkably in the last decade. Digestates should be used as soil fertilizers to close nutrient cycles. However, knowledge about the elemental composition of digestates from biogas production and element losses or accumulations during fermentation process is insufficient so far. Intending to enlarge the database for the elemental composition of digestates and to investigate element in- and outputs of biogas fermenters, we measured the concentrations of C, N, P, K, S, Ca, Mg, Fe, Mn, Zn, Cu, Pb, Cd, Ni, Mo and Se of digestates and feedstock (ingestates) of four full-scale biogas plants in Germany monthly over a one year period. Ingestates were sewage sludge, fat and mash (SEW1), sewage sludge and fat (SEW2), pig slurry, treacle and food residues (SL) and maize silage (M). We developed a statistical method to calculate the number of required sampling dates which have to be integrated for the calculation of reliable element budgets between ingestates and digestates for the case when information about the amount and composition of the produced biogas is not available. Our results suggest that two (SEW2), five (SEW1, M) and 10 (SL) sampling dates had to be integrated for reliable balances. All fermenters revealed losses of N, most likely due to volatilization of NH3. Losses of S (probably H2S), Mg (precipitation of struvite), Cd and Zn (precipitation of sulfides) could be detected in some cases. Iron and Mn accumulations can be attributed to attrition of the stirrer.