Enhancing the interpretation of biochemical methane potential assays through methane production kinetics: A case study with grass clippings and fruit-vegetable waste

Abstract

Anaerobic co-digestion (AcoD) of grass waste (GS) with fruit and vegetable waste (FVW) was evaluated to improve methane generation from lignocellulosic biomass. Batch BMP tests were performed using raw grass (GSr), size-reduced grass (GSp), and GS:FVW mixtures. FVW exhibited high biodegradability (453 Nml CH4/gVS; BDI 97.7%), whereas GS showed markedly lower conversion (169 Nml CH4/gVS; BDI 41.2%). Particle-size reduction of grass from 20 mm to <5 mm increased the methane production rate but did not significantly affect ultimate methane yield. Co-digestion behavior was strictly additive, with CPI values approximately equal to 1 for all mixtures. Among the kinetic formulations, the Two-fractions First-order and Multi-stages models provided the best description of methane production, particularly when BMP values were fixed to their theoretical predictions derived from CHNSO. A mathematical demonstration showed that the analytic expression of the Multi-stage model can be transformed into the Two-fractions First-order formulation, confirming their mathematical equivalence despite differing conceptual interpretations. These results highlight the suitability of FVW as a co-substrate for GS digestion and demonstrate the value of kinetic modeling for interpreting and predicting biomethane production from slowly degradable feedstocks.