Stable aerobic granulation for the selection of a mixed culture producing polyhydroxyalkanoates at high phenol loads

Background

Polyhydroxyalkanoate (PHA) production by pure culture or activated sludge (AS) using phenolic compounds is constrained by their toxicity against dispersed bacteria and flocs. Aerobic granular sludge (AGS) is a promising option for mitigating inhibition; however, achieving stable PHA-producing AGS from phenol constitutes a challenge.

Results

In the present study, aerobic granulation and accumulation of intracellular carbon reserve were evaluated in laboratory-scale sequential batch reactors (24 h cycle), inoculated with AS at different phenol loads and aeration rates. At upflow air velocity of 1.9 cm s⁻¹, acclimation with a mixed carbon source (acetate, phenol) was achieved. At 500 mg (L d)⁻¹ with phenol as the only carbon source and food/microorganism (F/M) ratio of 0.7 C-mmol (C-mmol)⁻¹, the reactor showed stable microgranules (mean diameter of 150 μm) but low PHA production. At 750 mg (L d)⁻¹, F/M ratio increased to 1.5 improving the PHA production (200 mg L⁻¹); however, granule disintegration led to sequential batch reactor failure. At low aeration rate (0.9 cm s⁻¹), larger granules (300 μm) with greater reactor stability were achieved at 750 mg (L d)⁻¹ (F/M = 0.5); however, glycogen was mainly accumulated (y_Gly/S = 0.24, yield per substrate unit, COD units). At 1000 mg (L d)⁻¹ (F/M = 1.5), the highest PHA production (270 mg L⁻¹, 16.3 wt%) and yield (y_PHA/S = 0.23) were achieved, although the effluent quality fluctuated. PHA was identified as polyhydroxybutyrate using DSC, FTIR and NMR techniques.

Conclusion

High F/M ratio allowed the selection of PHA-producing AGS with phenol uptake turned towards PHA synthesis rather than microbial growth or glycogen storage. Phenol toxicity was attenuated by the development of larger granules at low shear forces. © 2025 Society of Chemical Industry (SCI).