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

• Published in: Bioresource technology 2025-02, Vol.418, p.131945, Article 131945
• Main Authors: Akimoto, Shinya, Tsubota, Jun, Tagawa, Shinya, Hirase, Tatsuaki, Angelidaki, Irini, Hidaka, Taira, Fujiwara, Taku
• Format: Article
• Language: English
• Subjects: Anaerobic digestion; Biodegradable plastics; Biogas upgrading; Hydrolysis; Power to gas
• ISSN: 0960-8524; 1873-2976; 1873-2976
• DOI: 10.1016/j.biortech.2024.131945

[Display omitted] •In-situ bio-methanation for sewage sludge and lactic acid co-digestion was studied.•Sufficient lactic acid degradation was observed both with and without H2 injection.•H2 feed at pH 7.5–8.0 may activate Dictyoglomus, known for cellulolytic capacity. 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/LR/d. Conversely, laboratory experiments maintained efficiencies above 95 % at a feed rate of 3,600 mL/LR/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.