Michael S. answered • 08/05/26
B.S. in Chemistry, Indiana University; Organic Chem Teaching Intern
Treat it as two stages: hydrolysis of the polyester back to monomer, then anaerobic digestion of that monomer to biogas.
Stage 1 - hydrolysis. PLA is polylactic acid, repeat unit C3H4O2. Water cleaves the ester linkages and regenerates lactic acid:
(C3H4O2)n + n H2O -> n C3H6O3
Stage 2 - methanogenesis. For the biogas step use the Buswell equation, which predicts CO2/CH4 split from formula alone:
CnHaOb + (n - a/4 - b/2) H2O -> (n/2 - a/8 + b/4) CO2 + (n/2 + a/8 - b/4) CH4
For lactic acid, n=3, a=6, b=3. The water coefficient works out to 3 - 1.5 - 1.5 = 0, so no additional water is consumed, and both gas coefficients come out to 1.5:
C3H6O3 -> 1.5 CO2 + 1.5 CH4 (or 2 C3H6O3 -> 3 CO2 + 3 CH4)
Overall, clearing fractions:
2 (C3H4O2)n + 2n H2O -> 3n CO2 + 3n CH4
Check the balance: the left side is C6n H12n O6n and so is the right. It works.
What the math is telling you: lactic acid sits at exactly the oxidation state where biogas comes out 50% methane and 50% CO2 by mole, and where the digestion step needs no net water. Only the hydrolysis step consumes water.
Rough yield if you need a number: one repeat unit is 72.06 g/mol and gives 1.5 mol CH4, so at STP that is about 0.47 L of methane per gram of PLA. Real digesters fall short because some carbon goes to biomass.
Assumptions worth stating in your answer: complete conversion, no carbon diverted to microbial growth, and no nitrogen or sulfur in the feed, so no ammonia or H2S in the biogas.
One practical caveat that often gets missed. Stage 1 is rate limiting and is slow at ambient temperature. PLA has a glass transition near 55-60 C, and below it the polymer is glassy and water barely penetrates. That is exactly why PLA is labeled industrially compostable rather than home compostable - it needs a thermophilic digester around 55 C. Written on paper the reaction looks easy; in a backyard bin at 20 C it essentially does not proceed.
