Modeling of Polyhydroxyalkanoate Synthesis from Biogas byMethylocystis hirsuta

Abstract

Methylocystis hirsuta, a type II methanotroph, has been experimentally demonstrated to be able to efficiently synthesize polyhydroxyalkanoates (PHA) from biogas under nutrient-limited conditions. A mechanistic model capable of describing the relevant processes of M. hirsuta, which is currently not available, would therefore lay a solid foundation for future practical demonstration and optimization of the PHA synthesis technology using biogas. To this end, dedicated batch tests were designed and conducted to obtain experimental data for different mechanistic processes of M. hirsuta. Through utilizing the experimental data of well-designed batch tests and following a step-wise model calibration/validation protocol, the stoichiometrics and kinetics of M. hirsuta are reported for the first time, including the yields of growth and PHA synthesis on CH4 (0.14 ± 0.01 g COD g–1 COD and 0.25 ± 0.02 g COD g–1 COD), the CH4 and O2 affinity constants (5.1 ± 2.1 g COD m–3 and 4.1 ± 1.7 g O2 m–3), the maximum PHA consumption rate (0.019 ± 0.001 g COD g–1 COD d–1), and the maximum PHA synthesis rate on CH4 (0.39 ± 0.05 g COD g–1 COD d–1). Through applying the developed model, an optimal O2:CH4 molar ratio of 1.6 mol O2 mol–1 CH4 was found to maximize the PHA synthesis by M. hirsuta. Practically, the model and parameters obtained would not only benefit the design and operation of bioreactors performing PHA synthesis from biogas, but also enable specific research on selection for type II methanotrophs in diverse environments.