The effect of copper sulfate addition on the performance of an anaerobic digester treating dairy manure

Abstract. Inhibitory substances are often found to be the leading cause of anaerobic digester instability and failure. Copper, a known inhibitor, may be significantly spiked in dairy manure slurry when large volumes of CuSO₄, frequently used as a footbath to prevent lameness, are drained. This study evaluated the effect of CuSO₄ addition on the operational parameters and bacterial community composition of a stably operating mesophilic bench-scale dairy manure digester (30 day HRT, 1.61 kg VS/m³/day OLR) and used batch respirometer tests to determine the toxic dosage. Over 1.8 HRTs the addition of feedstock containing CuSO₄ footbath waste resulted in decreased biogas production from an average of 272 L/kg VS added to a low of 19 L/kg VS added while the total VFA concentration increased to 9321 mg/L from 141 mg/L. Within 2 HRTs of using fresh feedstock the digester recovered to values close to its previous steady-state levels. Batch test results where dairy manure was spiked with CuSO₄ confirmed expectations that while a small amount of copper may be beneficial to the anaerobic digestion process (1 – 10 mg/L), increasing concentrations can be inhibitory (20 – 40 mg/L) and even toxic (100 mg/L). High-throughput Illumina sequencing of the V4 region of bacterial 16S rRNA gene in digestate samples showed a significant change in the bacterial community composition followed by a return to the original composition. Genera that were suppressed during the inhibition period include Candidatus Cloacamonas, Syntrophomonas, Butyrivibrio, and Caldicoprobacter, while the relative abundance of Acholeplasma, Desulfobulbus, Aminobacterium, Treponema, and YRC22 peaked.