Stoichiometry of the Anaerobic Digestion Process

Typically contains Biogas from anaerobic digestion of:

-          Sewage

-          Food Processing

-          Animal

-          And other wastes

         CH4 about  55% to 70%

         CO2 about  30% to 45%

         Biogas production from organic substrates involves an internal redox reaction that convert organic molecules to CH₄ and CO_2, the proportion of the gases being dictated by:

-          Composition substrates

-          Biodegradability substrates

         Simple case Conversion Carbohydrates such as:

-          Sugar (e.g, glucose, C6H12O6)

-          Starch atau Cellulose (CnHn-2On-1)

         Equal amount of CH₄ and CO₂ is produced (50:50 ratio)

         C_nH_n-2O_n-1 + nH₂O à ½ n CH₄ + ½ n CO₂

        In the case wastes containing proteins and fats, a larger amount of methane is produced.

        For Protein

        C₁₀H₂₀O₆N₂ + 3H₂O à 5,5 CH₄ + 4,5 CO₂ + 2NH₃

-          This yields a CH4 : CO2 ratio of 55 : 45

-          The exact composition will depend on the individual substrate protein

       For fats and vegetable oil (triglycerides), a typical CH4 : CO2 ratio is 70 : 30

      C₅₄H₁₀₆O₆₆ + 28H₂O à 40 CH₄ + 17 CO₂  

These simplified examples can change according to effect from several factor:

- Reaction are often incomplete.

- By – product are produced and voided in the digester effluent.

- Bacteria use these reaction to make more bacteria .

Increasing methane gas production:

-          Two Phase Anaerobic Digestion

-          CO₂    Dissolution in process water

Conclusions

Biogas produced by dairy wastes in typical AD processes is somewhat enriched in CH₄ ,  compared to what would be expected from the metabolic processes organics degradation. However, the observed and expected enrichment is rather modest, from about 50% or 60%. There is also a near-doubling of CH₄ to CO₂ ratios, from 1:1 closer 2:1 (e.g., 66% methane), which is about the maximum that would likely be achievable.