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

Abstract

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 upflowairvelocityof1.9 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 ∼m) but low PHA production. At 750 mg (L d)−1, F/M ratio increased to 1.5 improving the PHAproduction (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 ∼m) with greater reactor stability were achieved at 750 mg (L d)−1 (F/M = 0.5); however, glycogen was mainly accumulated (yGly/S = 0.24, yield per substrate unit, COD units). At 1000 mg (L d)−1 (F/M = 1.5), the highest PHAproduction (270 mg L−1, 16.3 wt%)and yield (yPHA/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.