Impact of nitrogen feeding regulation on polyhydroxyalkanoates production by mixed microbial cultures

A sequencing batch reactor (SBR) is typically used for selecting mixed microbial cultures (MMC) for polyhydroxyalkanoate (PHA) production. Since many waste streams suitable as process feedstock for PHA production are nitrogen-deficient, a nutrient supply in the SBR is typically required to allow for efficient microbial growth. The scope of this study was to devise a nitrogen feeding strategy which allows controlling the nitrogen levels during the feast and famine regime of a lab-scale SBR, thereby selecting for PHA-storing microorganisms. At the beginning of the cycle the reactor was fed with a synthetic mixture of acetic and propionic acids at an overall organic load rate of 8.5gCODL^-1d^-1 (i.e. 260CmmolL^-1d^-1), whereas nitrogen (in the form of ammonium sulphate) was added either simultaneously to the carbon feed (coupled feeding strategy) or after the end of the feast phase (uncoupled feeding strategy). As a main result, PHA production was more than doubled (up to about 1300±64mgCODL^-1) when carbon and nitrogen were separately fed and the higher PHA production also corresponded to an 82% increase in the polymer HV content (up to 20±1%, wtwt^-1). Three SBR runs were performed with the uncoupled carbon and nitrogen feeding at different carbon to nitrogen (C/N) ratios (of 14.3, 17.9, and 22.3CmolNmol^-1, respectively) which were varied by progressively reducing the concentration of the nitrogen feeding. In spite of a comparable PHA storage yield at 14.3 and 17.9CmolNmol^-1 (0.41±0.05 gCOD_PHA gCOD_VFA^-1 and 0.38±0.05 gCOD_PHA gCOD_VFA^-1, respectively), the storage response of the selected MMC significantly decreased when the C/N ratio was set at the highest investigated value. Notably, an increase in this parameter also resulted in a change in the HV content in the polymer regardless the composition of the organic acids solution.