Stimulation and Inhibition of Anaerobic Digestion by Nickel and Cobalt: A Rapid Assessment Using the Resazurin Reduction Assay

Stimulation of anaerobic digestion by essential trace metals is beneficial from a practical point of view to enhance the biodegradability and degradation rate of wastes. Hence, a quick method to determine which metal species, and at what concentration, can optimize anaerobic digestion is of great interest to both researchers and operators. In this present study, we investigated the effect of nickel(II), cobalt(II), and their mixture, on the anaerobic digestion of synthetic municipal wastewater. Using a volumetric method, that is, measuring methane production over time, revealed that anaerobic digestion was stimulated by the addition of 5 mg L^-1 nickel(II), and cobalt(II), and their mixture in day(s). However, using a novel resazurin reduction assay, and based on its change in rate over time, we evaluated both inhibition at 250 mg L^-1 nickel(II) and cobalt(II), and also the stimulatory effect of 5 mg L^-1 nickel(II), and cobalt(II), and their mixture, in just 6 h. By investigating the dynamic distribution of these metals in the liquid phase of the anaerobic system and kinetics of resazurin reduction by nickel spiked anaerobic sludge, the concentration of nickel(II) on anaerobic digestion performance was profiled. Three critical concentrations were determined; stimulation starting (around 1 mg L^-1), stimulation ending (around 100 mg L^-1) and stimulation maximizing (around 10 mg L^-1). Hence, we propose that the resazurin reduction assay is a novel and quick protocol for studying the stimulation of anaerobic bioprocesses by bioavailable essential trace metals.