Effect of famine-phase aeration on polyhydroxyalkanoate accumulation in aerobic granules
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Date
2018-07-01
Authors
Vjayan Thrumakunasagran
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Abstract
Polyhydroxyalkanoate (PHA) is a biodegradable polymer which has attracted a lot of interests among researchers lately. PHA can be feasibly produced using aerobic granules treating wastewater. However, high production costs, particularly due to the high aeration used, have limited the applications of this biopolymer. In this study, the effect of variable aeration rates in the famine period on the PHA accumulating capability and morphology of aerobic granules was investigated. Four sequencing batch reactors were inoculated with aerobic granules and operated at different aeration rates using palm oil mill effluent as the sole substrate. A feast-famine constant aeration rate of 2.0 L/min was used for Reactor 1 (control). Meantime, the feast period aeration rate of Reactor 2, 3 and 4 was similar to that of Reactor 1 while the famine period aeration rate was decreased to 1.0 L/min, 0.5 L/min and 0 L/min in Reactor 2, 3 and 4, respectively. The findings of Reactor 1 show that more than 90% of the influent chemical oxygen demand (COD) was removed on average and a maximum PHA content of 56% cell dry weight was accumulated in the aerobic granules. The PHA accumulated was a copolymer of poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (P3(HB-co-HV)) with 21 % hydroxyvalerate (HV) content. The aerobic granules had an average granular size, sludge volume index and extracellular polymeric substance (EPS) content of 1030 μm, 41 mL/g and 829.50 μg/g biomass, respectively. Meanwhile, the variable aeration study reveals that regardless of the aeration rates used in the famine period, all aerobic granules achieved a similar COD removal and PHA content. Notably, under a famine period with low aeration rate, the time taken to reach the maximum PHA content had decreased significantly. Moreover, PHA compositional changes occurred, where the HV content had increased. Further, the decrease in the famine period aeration rate enhanced the total EPS content of the aerobic granules, indirectly improving the settling ability of the granules. The reduction in aeration rate from 2.0 L/min to 0.5 L/min has resulted in an increased size of granules with the highest volume percentage. On the other hand, the thickness of the layers of PHA accumulating microorganisms at the surface of the granules was found to have decreased when the famine period aeration rate was reduced. Overall, the results indicate that the requirement of aeration for PHA accumulation in aerobic granules is highly insignificant in the famine period. PHA production in aerobic granules under zero aeration in the famine period may result in an energy input reduction of up to 74%, which could essentially reduce the overall production cost of this biopolymer.