Table 3 Free energy values of key acetogenic and methanogenic reactions of anaerobic digestion

Acetogenesis

 Propionate → Acetate

\({\text{CH}}_{ 3} {\text{CH}}_{ 2} {\text{COO}}^{ - } + 3 {\text{H}}_{ 2} {\text{O}} \to {\text{CH}}_{ 3} {\text{COO}}^{ - } + {\text{H}}^{ + } + {\text{HCO}}_{3}^{ - } + 3 {\text{H}}_{ 2}\)

+76.1

 Butyrate → Acetate

CH₃CH₂CH₂COO⁻ + 2H₂O → 2CH₃COO⁻ + H⁺ + 2H₂

+48.1

 Ethanol → Acetate

CH₃CH₂OH + H₂O → CH₃COO⁻ + H⁺ + 2H₂

+9.6

 Lactate → Acetate

\({\text{CH3CHOHCOO}}^{ - } + 2 {\text{H}}_{ 2} {\text{O}} \to {\text{CH}}_{ 3} {\text{COO}}^{ - } + {\text{H}}^{ + } + {\text{HCO}}_{3}^{ - } + 2 {\text{H}}_{ 2}\)

−4.2

 Formate → Acetate

\(2 {\text{HCO}}_{3}^{ - } + 4 {\text{H}}_{ 2} {\text{O}} + {\text{H}}^{ + } \to {\text{CH}}_{ 3} {\text{COO}}^{ - } + 4 {\text{H}}_{ 2} {\text{O}}\)

−104.6

Methanogenesis

 Acetate → Methane

\({\text{CH}}_{ 3} {\text{COO}}^{ - } + {\text{H}}_{ 2} {\text{O}} \to {\text{HCO}}_{3}^{ - } + {\text{CH}}_{ 4}\)

−31.0

 H₂/CO₂ → Methane

4H₂ + CO₂ → CH₄ + 2H₂O

−131.0

 Formate → Methane

\({\text{HCO}}_{3}^{ - } + 4 {\text{H}}_{ 2} + {\text{H}}^{ + } \to {\text{CH}}_{ 4} + 3 {\text{H}}_{ 2} {\text{O}}\)

−135.6