Plastic Abundance and Degradation in the Anaerobic Digestion of Food Waste

Food waste (FW) is a pervasive issue across the supply chain, with an estimated 30-40% of the food supply left unsold or uneaten annually (ReFED, 2022). In response, in 2015 the U.S. set a goal to halve FW by 2030 (U.S. EPA, 2020f). Valorization, the process of recovering value from FW, has become a key strategy, with anaerobic digestion (AD) playing a pivotal role. AD transforms organic waste into biogas and digestate, a potential soil amendment. However, plastic contamination in the organic waste stream poses a significant challenge. During digestion and storage, plastic debris can break down into microplastics (MP), which may enter agricultural soils through digestate application. Despite its significance, the fate of plastics in AD and the abundance of MP in food waste-derived digestate remain poorly understood. This research aimed to assess plastic degradation across the AD life cycle and to quantify and characterize MP variation in digestate. A novel method was developed for extracting MP from digestate using wet peroxide oxidation and EDTA/Triton X-100. MP were detected in all samples of manure, fresh digestate, and storage lagoon digestate, with PET fibers the most prevalent polymer type and morphology. Digester conditions, rather than storage lagoon environments, were identified as the likely drivers of plastic degradation, significantly altering the Carbonyl Index and Young’s Modulus of incubated plastics. Additionally, when incubated in digestate, plastic produce stickers showed minimal signs of degradation while paper stickers rapidly disintegrated into countless fragments. These findings offer valuable insights into contamination levels in an anaerobic co-digestion system and reveal how larger plastics may shed MP. However, further research is necessary to fully understand contamination sources and the plastic degradation cycle to mitigate the risks of plastic pollution in FW management and its impact on agricultural and aquatic ecosystems.