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Kinetic models of anaerobic digestion (AD) are widely applied to soluble and particulate substrates, but have not been systematically evaluated for bioplastics. Here, five models are evaluated to determine their suitability for modeling of anaerobic biodegradation of the bioplastic poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV): (1) first-order kinetics with and without a lag phase, (2) two-step first-order, (3) Monod (4) Contois, and (5) Gompertz. Three models that couple biomass growth with substrate hydrolysis (Monod, Contois, and Gompertz) gave the best overall fits for the data (R>0.98), with reasonable estimates of ultimate CH production. The particle size limits of these models were then evaluated. Below a particle size of 0.8mm, rates of hydrolysis and acetogenesis exceeded rates of methanogenesis with accumulation of intermediates leading to a temporary inhibition of CH production. Based on model fit and simplicity, the Gompertz model is recommended for applications in which particle size is greater than 0.8mm.

Composites made with bio-based resins are promising candidates for replacement of conventional plastic composites made with petroleum-based resins in many applications (e.g., decking, paneling, furniture, molded automotive parts). For any such applications, end-of-life management needs consideration. Here, we describe a methodology to assess end-of-life anaerobic degradation to methane (CH 4) within landfills or anaerobic digestion (AD) facilities in batch anaerobic microcosms. The core methodology combines stoichiometric considerations, chemical oxygen demand (COD) analysis, a CH 4 production assay, and modeling. Additional analyses, such as thermogravimetric analysis (TGA), can complement this core set of analyses. We apply the methodology to injection molded poly(hydroxybutyrateco-hydroxyvalerate) (PHBV) composites with wood fiber (WF) (0%, 20%, 40%) and two fiber-matrix compatibilization treatments that enhance in-service performance: (1) hydrophobic silane treatment of the WF and (2) grafting of hydrophilic maleic anhydride groups to the PHBV matrix. The methodology successfully quantifies process kinetics, ultimate CH 4 production capacity, and biodegradability, and allows comparison to reference materials (positive controls).

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