Dependence of poly-β-hydroxybutyrate accumulation in sludge on biomass concentration in SBRs

The combination of wastewater treatment with polyhydroxyalkanoate production has attracted increasing interest in the context of the circular economy. Recent studies have thus attempted to optimize the conditions for polyhydroxyalkanoate accumulation in sludge when treating wastewater. The effects of biomass concentration and sludge morphologies in reactors on PHB storage, however, were neglected in the literature. Therefore, in this study settling time and organic loading rate were manipulated to adjust sludge morphology and biomass concentration in sequential batch reactors (SBRs) to investigate their influence on PHB storage in the feast phase. Our study shows that reducing settling times in SBRs from 10 to 0 min under organic loading rate of 3 g L^-1 d^-1 resulted in the decrease in biomass concentration at steady states from 4.2 to 1.0 g L^-1 and the change of sludge morphology from well-settled granules to poorly settled pinpoint flocs, but PHB content in sludge at the end of feast phase increased from 7.7 to 26.7%. The well-fitted regression lines between PHB content, SRT, feast/famine and food/microorganisms ratios and biomass concentration under different settling times suggest that PHB was highly dependent on biomass concentration but independent on sludge morphology. Under settling time of 0 min, the increase in OLR from 3 to 7.5 g L^-1 d^-1 resulted in an increased biomass concentration from 1.0 to 2.1 g L^-1 and an increase in PHB content from 26.7 to 33.8%. The batch and fed-batch experiments with different biomass concentrations also showed the influence of biomass concentration on PHB accumulation in sludge. The conclusion of the dependence of PHB content on biomass concentration under a fixed OLR and varied OLRs drawn from this study enables sludge PHB content as high as possible by adjusting biomass concentration in SBRs apart from the selective enriching strategies for PHB accumulating organisms when treating VFA-rich wastewater.