Loss of thermodynamic spontaneity in methanogenic consortium with ammonia contents

Sungtaek Oh, Alastair D. Martin

Research output: Contribution to journalArticle

3 Citations (Scopus)

Abstract

Over 60% of the primary and secondary sludge produced in the UK is now processed by mesophilic anaerobic digestion in order to “stabilise” and extract bio-energy from the organic material. However inhibition and destruction of microorganisms is frequently reported and related to elevated ammonia concentrations. Ammonia is released from proteinaceous material in the wastewater and concentrated in the sludge. Empirically observed inhibition or loss of bio-kinetic rate is seldom attributed to specific underlying causes such as loss of enzyme activity or loss of thermodynamic driving force. This work takes a theoretical electrochemical modelling approach to investigate thermodynamic limitations by free energy, enthalpy and entropy analysis. From the Gibb’s free energy results, there is no evidence for thermodynamic limitation arising from even very high concentrations of ammonium ion. However entropy analysis shows a sudden loss of spontaneity when the initial ratio of ammonia to the other solutes, R, exceeds a, concentration dependent, threshold value. The loss of spontaneity is not directly related to the rising pH value which is indicative of the addition of ammonium ions. The loss is however attributable to the depletion of carbonates occurring in anaerobic ammonium oxidation. These results suggest that empirically observed ammoniacal inhibition in anaerobic digestion arises from reduced activity of hydrogenophilic methanogens which are dependent on the simultaneous availability of hydrogen and bi-carbonate ion.
Original languageEnglish
Pages (from-to)244-253
Number of pages10
JournalChemical Engineering Journal
Volume243
Early online date11 Jan 2014
DOIs
Publication statusPublished - 1 May 2014
Externally publishedYes

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Ammonia
ammonia
thermodynamics
Thermodynamics
Ammonium Compounds
ammonium
Anaerobic digestion
Carbonates
Ions
entropy
ion
Entropy
sludge
Methanogens
carbonate
Gibbs free energy
Enzyme activity
bioenergy
enthalpy
Microorganisms

Cite this

Oh, Sungtaek ; Martin, Alastair D. / Loss of thermodynamic spontaneity in methanogenic consortium with ammonia contents. In: Chemical Engineering Journal. 2014 ; Vol. 243. pp. 244-253.
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Loss of thermodynamic spontaneity in methanogenic consortium with ammonia contents. / Oh, Sungtaek; Martin, Alastair D.

In: Chemical Engineering Journal, Vol. 243, 01.05.2014, p. 244-253.

Research output: Contribution to journalArticle

TY - JOUR

T1 - Loss of thermodynamic spontaneity in methanogenic consortium with ammonia contents

AU - Oh, Sungtaek

AU - Martin, Alastair D.

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AB - Over 60% of the primary and secondary sludge produced in the UK is now processed by mesophilic anaerobic digestion in order to “stabilise” and extract bio-energy from the organic material. However inhibition and destruction of microorganisms is frequently reported and related to elevated ammonia concentrations. Ammonia is released from proteinaceous material in the wastewater and concentrated in the sludge. Empirically observed inhibition or loss of bio-kinetic rate is seldom attributed to specific underlying causes such as loss of enzyme activity or loss of thermodynamic driving force. This work takes a theoretical electrochemical modelling approach to investigate thermodynamic limitations by free energy, enthalpy and entropy analysis. From the Gibb’s free energy results, there is no evidence for thermodynamic limitation arising from even very high concentrations of ammonium ion. However entropy analysis shows a sudden loss of spontaneity when the initial ratio of ammonia to the other solutes, R, exceeds a, concentration dependent, threshold value. The loss of spontaneity is not directly related to the rising pH value which is indicative of the addition of ammonium ions. The loss is however attributable to the depletion of carbonates occurring in anaerobic ammonium oxidation. These results suggest that empirically observed ammoniacal inhibition in anaerobic digestion arises from reduced activity of hydrogenophilic methanogens which are dependent on the simultaneous availability of hydrogen and bi-carbonate ion.

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