Dynamics and the interaction effects of trace elements and volatile fatty acids concentrations on methanization processes during thermophilic anaerobic digestion

Abstract

Volatile fatty acids (VFA) and trace elements (TEs) influence methane production and VFA metabolism in positive and negative ways during methanization. There is insufficient knowledge on why the interactions could go either way and this has generated unpleasant consequences for biogas operators. To fill the knowledge gap, statistically designed thermophilic batch experiments were conducted with VFA mixture as substrate to investigate the dynamics and interaction effects of TEs including Nickel (Ni), Cobalt (Co), Selenium (Se) and Molybdenum (Mo) on the methanization processes of CH₄ production and VFA degradation rate. Response surface model and desirability functions were used to determine TEs and VFA interaction effects and the dynamics of the optimum TEs configurations for CH₄ production and VFA degradation rate at different VFA concentrations. The results showed that TEs supplementation influenced Y_CH4 production by -14% to 11%: whereas the positive interaction effects of VFA and TEs were by Co*Mo, VFA*Se and VFA*Mo; the interaction effects of Se*Mo and VFA*Co were negative. -15% to 45% increase in Y_VFA-DR was obtained with TEs supplementation: Ni*Co interaction produced the positive influences; and Co*Se produced negative effects. The optimum TEs configuration for CH4 production and VFA degradation rate varied with changes in VFA concentrations.

Keywords: Thermophilic methanization, trace elements, VFA metabolism, methane production, trace elements synergism, trace elements interaction