Anaerobic degradation of bioplastics is a controversial challenge. Size reduction is a must for degradation while it requires a significant amount of energy, which lowers the overall energy efficiency of the system. On the other hand, inoculum to substrate ratio has interaction effects in the process. The present work aimed to optimize these two parameters for the improvement of energy efficiency through response surface methodology. The central composite design procedure was implied. The levels of the experimental variables were 0.72, 4.3, 7.87 mm for particle size and 2, 3, and 4 for inoculum to substrate ratio. The input variable effects on biomethane yield were estimated, discussed, and then also optimized using the genetic algorithm. Moreover, energy balance analysis was done for the samples. The highest biomethane yield was found at the particle size of 4.3 mm and inoculum to substrate ratio of 4, which corresponds to 23% energy efficiency. Despite the high energy consumption for size reduction to less than 1 mm, more biomethane yield was not observed. Inoculum to substrate ratio showed more effect on biomethane yield than particle size.

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