Modelling of Polyhydroxyalkanoates Synthesis from Biogas by Methylocystis hirsuta

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 Methylocystis 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 Methylocystis 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 Methylocystis 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). 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 Methylocystis 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.