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

N.C Ezebuiro

doi:10.4314/njb.v35i1.7

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 YCH4 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 YVFA-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

Highlights

Methanization involves anaerobic digestion (AD) of biomass to produce energyrich methane (Ch4)(Appels et al, 2008).Common substrates used for methanization include grease trap residue and wastewater; biowaste from restaurants and households; and agricultural residues
The results showed that trace elements (TEs) supplementation influenced YCH4 production by -14% to 11%: whereas the positive interaction effects of volatile fatty acids (VFA) and TEs were by Co with Mo (Co*Mo), VFA*Se and VFA*Mo; the interaction effects of Se with Mo (Se*Mo) and VFA*Co were negative. -15% to 45% increase in YVFA-DR was obtained with TEs supplementation: Ni*Co interaction produced the positive influences; and Co*Se produced negative effects
VFA metabolism and CH4 production could be improved by supplementing trace elements (TEs) to a methanization digester or performing methanization at thermophilic temperature(Appels et al, 2008; Mata-Alvarez, 2003)

Summary

Introduction

Methanization involves anaerobic digestion (AD) of biomass to produce energyrich methane (Ch4)(Appels et al, 2008).Common substrates used for methanization include grease trap residue and wastewater; biowaste from restaurants and households; and agricultural residues. Methanization involves anaerobic digestion (AD) of biomass to produce energyrich methane (Ch4)(Appels et al, 2008). VFA are metabolized to produce Ch4 (Appels et al, 2008; Mata-Alvarez, 2003). VFA metabolism and CH4 production could be improved by supplementing trace elements (TEs) to a methanization digester or performing methanization at thermophilic temperature(Appels et al, 2008; Mata-Alvarez, 2003). TEs are micro-nutrients that influence the expression and dominance of VFA degradation pathways, rate of VFA metabolism and methane production (Halarnkar and Blomquist, 1989; Osuna et al, 2003; Pobeheim et al, 2011). TEs are widely reported in literature as being associated with the enzymology of the acetyl CoA pathway, a dominant pathway during VFA metabolism and CH4 production. Reported TEs used in co-supplementation during methanization include Nickel (Ni), Cobalt (Co), Selenium (Se), Tungsten (W), Molybdenum (Mo), and Iron (Fe)

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