Decoupling Dissolution and Biological Kinetics in Polycaprolactone-Based Denitrification: Direct Determination of Yield and Maximum Specific Growth Rate.

Solid-phase denitrification using biodegradable polymers such as polycaprolactone (PCL) is increasingly proposed as a sustainable alternative to conventional soluble carbon dosing. However, reported kinetic and stoichiometric parameters for PCL-based systems are often confounded by mass-transfer and dissolution limitations, hindering accurate determination of biological kinetics. In this study, the dissolution step was explicitly decoupled from microbial growth by presolubilizing PCL prior to use, enabling the direct experimental determination of biomass yield (Y) and maximum specific growth rate (μmax) under strictly anoxic batch conditions. Methanol was investigated in parallel as a benchmark soluble carbon source. Yield assays conducted over 12 independent replicates produced statistically similar yields of 0.35 ± 0.08 and 0.38 ± 0.08 g COD per g COD for methanol and PCL, respectively, indicating comparable stoichiometric efficiency for biomass synthesis. Dynamic nitrate depletion profiles were resolved through high-frequency batch testing and fitted using SUMO process modeling. The resulting μmax values were 1.3 day-1 for methanol and 2.1 day-1 for presolubilized PCL, indicating that when carbon availability is not limited by dissolution, under the tested decoupled batch conditions, PCL-derived substrates can support higher growth rates than conventional methanol. Long-term acclimation tests showed lower net biomass accumulation with PCL, attributable to its slower carbon release rather than to microbial capacity limits. These results provide the first direct determination of true biological kinetic parameters for PCL-based denitrification, independent of polymer hydrolysis effects. The findings demonstrate that the apparent kinetic constraints previously reported for solid PCL systems primarily reflect physicochemical mass-transfer limitations and not biological capacity. This work establishes design-ready kinetic parameters for integrating biodegradable polymers into predictive denitrification models, supporting the rational implementation of biodegradable polymeric carbon sources in sustainable nitrogen removal processes.