Improvement of methanogenic activity of anaerobic digestion using poly(l-lactic acid) with enhanced chemical hydrolyzability based on physicochemical parameters

Abstract

Because packing bags and disposable items of poly (l-lactic acid) (PLLA) waste are discharged together with other organic waste including garbage, anaerobic co-digestion of PLLA and other organic waste is required. However, because of low hydrolyzability of PLLA products, the chemical hydrolyzability must be improved for PLLA treatment during anaerobic digestion. This study aimed to assess weight-average molecular weight (M_w) and crystallinity (X_c), to determine the chemical hydrolyzability of PLLA, for PLLA treatment during anaerobic digestion. Moreover, the possibility of anaerobic co-digestion of the PLLA after improvement of chemical hydrolyzability and other organic waste was also discussed. Detectable methanogenic activity of the mesophilic and thermophilic anaerobic sludges of PLLA occurred in the M_w range of 6,800 to 16,500, and 6,800 and 38,000, respectively. The methanogenic activity of mesophilic and thermophilic anaerobic sludge was higher with PLLA with a high crystallinity (X_c = 39.9-46.1%) than with nearly amorphous PLLA (X_c = 0.3-3.5%). The maximum methanogenic activity of anaerobic sludge using PLLA with an X_c of approximately 40-45% and with a M_w of 10,300 and 16,500 for mesophilic and thermophilic anaerobic sludge were 0.013 gCOD·gVS^-1·d^-1 and 0.13 gCOD·gVS^-1·d^-1, respectively. A survey on the possibility of anaerobic co-digestion of PLLA after improvement in chemical hydrolyzability based on M_w and X_c and organic wastes revealed that thermophilic conditions at 55 °C are more advantageous than mesophilic conditions at 37 °C.

Keywords: Anaerobic digestion; Chemical hydrolysis; Crystallinity; Methanogenic activity; Poly(l-lactic acid); Weight-average molecular weight.