Evaluating compostable bags in food waste streams: a semicontinuous anaerobic digestion trial

Abstract

This study investigated the semicontinuous mesophilic anaerobic digestion of food waste, assessing process stability, performance, and microbial communities across emerging treatment scenarios. Following the EU mandate for separate bio-waste collection, food waste streams are expanding from medium/large-sized producers (FW-ML) to household food waste (FW-HH) likely to introduce biodegradable bags. Consequently, four scenarios were evaluated: one representing current FW-ML treatment and three simulating its treatment alongside FW-HH containing cellulose-based bags, PBAT/starch-based bags, or thermoalkaline pretreated PBAT/starch-based bags. The PBAT/starch-based bags led to a progressive acidification, with volatile organic acids reaching 4.1 ± 0.6 gHAc/L and total alkalinity decreasing to 8.3 ± 0.8 gCaCO₃/L. Correspondingly, methane yield declined to 469 ± 10 NLCH₄/kgVS, compared to 518 ± 9 NLCH₄/kgVS for FW-ML alone. The qualitative detection of terephthalic acid, a monomer of PBAT, suggests that incomplete biodegradation may have contributed to acidic stress. Conversely, the thermoalkaline pretreatment (3 M KOH, 70 °C, 4 h) provided the dual benefit of plastic solubilization and alkalinity enhancement (11 ± 1 g CaCO₃/L), minimizing performance losses (504 ± 46 NL CH₄/kg VS). Microbial analyses revealed stable overall diversity and a resilient methanogenic community dominated by Methanoculleus sp. Redundancy and discriminant analyses indicated active community structure shifts associated with the presence of FW-HH and the PBAT/starch-based bags, despite their low concentration (2.2 ± 0.3% of the organic loading rate). This study outlines operational challenges and mitigation strategies for treating FW-HH alongside biodegradable plastics, establishing a baseline that invites future techno-economic and life cycle assessment studies.