Thermophilic and mesophilic biogas production from PLA-based materials: Possibilities and limitations.

Recently, the use of bio-based products, including biodegradable poly(lactic acid) (PLA), has increased, causing their rapid growth in municipal waste streams. The presence of PLA in biowaste may increase biogas production (BP). However, the rate of PLA biodegradation, which affects the time frame of anaerobic digestion, is a key parameter for an efficient process. In this study, detailed kinetics of BP from PLA were determined at 58 °C and 37 °C. At both temperatures, lag phases were observed: 40 days at 37 °C, and 10 days at 58 °C. After the lag phase BP proceeded in two phases, differed in process rate. At 58 °C, during the 1st phase (up to day 30), the rate of BP (r) equaled about 35 L/(kg OM·d). At the end of this phase, the amount of biogas was 710 L/kg OM, which constituted 84% of the maximal BP (831-849 L/kg OM). In the 2nd phase (10 days), only 13% of maximal BP was produced (r of 16.1 L/(kg OM·d)). At 37 °C, maximal BP (obtained after 280 days) was 1.5-fold lower (558-570 L/kg OM) than at 58 °C. In the 1st phase (100 days), r was 1.4 L/(kg OM·d); at the end of this phase, BP constituted merely 14% of the maximal BP. A majority of biogas was produced in the 2nd phase (the next 180 days), and r doubled to 2.6 L/(kg OM·d)). At 58 °C, intensive biogas production took place when PLA pieces were still visible. At 37 °C, in contrast, biogas was mainly produced when the PLA pieces had been disintegrated. Although PLA anaerobically biodegrades and produces a high yield of biogas, the time frame of PLA digestion is much longer than that of biowaste and, in thermophilic conditions requires separate digesters. In mesophilic conditions, however, is unacceptable at technical scale.