Integrated glycerol biorefinery: volatile fatty acid platform for polyhydroxyalkanoates production under feast/famine operation

: This study evaluated aerobic (Stage 1) and anaerobic (Stage 2) feeding strategies in a sequencing batch reactor (SBR) for polyhydroxyalkanoate (PHA) accumulation, using fermented effluent from a glycerol-fed acidogenic reactor operated at hydraulic retention times (HRT) of 4 and 8 days. Coordinating upstream acidogenic fermentation with downstream reactor feeding strategies is necessary to enhance carbon conversion within biorefinery concepts. Extending the acidogenic HRT from 4 to 8 days increased volatile fatty acid (VFA) production to 3167 mg COD/L (a 21% increase), reduced residual glycerol, and shifted the metabolic profile from a predominance of acetic and butyric acids to butyric and caproic acids. In feast/famine SBR operations, representative cycles showed that PHA accumulated faster under anaerobic feeding (Stage 2), reaching 10.0 mg PHA/L after 2 h compared to 7.5 mg PHA/L under aerobic feeding (Stage 1). Stage 2 also achieved a volumetric productivity of 59.77 mg PHA/L/h and specific yields of 0.37 g PHA/g VFA and 0.64 g PHA/g glycerol, outperforming Stage 1 yields of 0.28 and 0.20 g/g, respectively. Although the higher concentration of medium-chain carboxylic acids did not result in a corresponding increase in downstream polymer accumulation, the operational feeding strategy was the primary determinant of performance. These experimental findings provide robustness to the concept of coupling upstream acidogenesis with downstream PHA production, demonstrating how integrating anaerobic feeding with a prolonged HRT improves carbon conversion yields from a glycerol substrate.