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Effects of Co-substrate Type and Mixing Ratio on Methane Production and Synergistic effect of Mesophilic Co-digestion of Polylactic Acid (PLA) Waste
Summary
Co-digesting polylactic acid (PLA) waste with food waste improved methane yields by 16–22% through synergistic effects, demonstrating that bioplastic waste can be more efficiently managed through anaerobic co-digestion rather than mono-digestion. This is relevant to microplastic pollution research because PLA's limited standalone degradability means it can persist and fragment into microplastics if not properly processed.
Polylactic acid (PLA) is a biodegradable plastic widely used as an alternative to conventional plastics due to its high biodegradability; however, when treated by anaerobic digestion, PLA waste exhibits limited degradation and energy recovery efficiency. Therefore, this study evaluated the anaerobic co-digestion of PLA waste with food waste (FW) or waste activated sludge (WAS) to enhance energy recovery from PLA waste. Batch experiments were conducted under mesophilic conditions, and methane production characteristics, methane yields, and synergistic effects were compared between mono-digestion and co-digestion systems. The significance of synergistic effects was evaluated using analysis of variance (ANOVA). The mono-digestion results showed that methane yield and methane production rate followed the order of FW, WAS, and PLA waste. In PLA–FW co-digestion, methane yield decreased with increasing PLA fraction; however, synergistic effects ranging from 16.2% to 21.9% were observed. In contrast, PLA–WAS co-digestion exhibited a similar decreasing trend in methane yield with increasing PLA fraction, but no consistent pattern in synergistic effects was observed with respect to the mixing ratio. One-way ANOVA indicated that differences in synergistic effects among mixing ratios were observed under the FW added conditions, whereas no significant differences were found under the WAS added conditions. Two-way ANOVA with replication further revealed that synergistic effects differed significantly depending on substrate type, while the overall main effect of mixing ratio was not significant (p>0.05). These results quantitatively demonstrate that food waste acts as a more effective co-substrate than waste activated sludge in the anaerobic co-digestion of PLA waste and suggest that co-digestion performance strongly depends on the degradation kinetics of the combined substrates.