Process performance of in-situ bio-methanation for co-digestion of sewage sludge and lactic acid, aiming to utilize waste poly-lactic acid as methane.

This study examined hydrogen conversion efficiency and operational stability in pilot-scale in-situ bio-methanation during the co-digestion of sewage sludge and lactic acid (partially derived from waste poly-lactic acid). Parallel laboratory-scale experiments were also conducted. In the pilot, hydrogen conversion efficiency decreased from 98.9 % to 84.4 % as the hydrogen feed rate increased from 240 to 1,200 mL/L _R /d. Conversely, laboratory experiments maintained efficiencies above 95 % at a feed rate of 3,600 mL/L _R /d, suggesting that hydrogen gas-liquid transfer limited hydrogen conversion efficiency in the pilot. Lactic acid degradation was observed both with and without hydrogen injection in the pilot. Methane yields from the acid were 310 ± 30 and 300 ± 30 mL/g (chemical oxygen demand (COD))-added, close to the theoretical methane yield (350 mL/gCOD). These results demonstrate the importance of hydrogen gas-liquid transfer when scaling up bio-methanation processes. Moreover, they showed the potential of waste poly-lactic acid as a methane source.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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