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Effects of microplastic accumulation on floc characteristics and fouling mechanism in sequencing batch membrane bioreactor
Summary
This study found that microplastic accumulation in membrane bioreactors disrupts the formation of biological flocs, reducing sludge aggregation and altering membrane fouling patterns. The findings indicate that microplastic contamination of wastewater treatment systems can impair treatment performance and membrane lifespan.
This research investigated the effects of microplastic accumulation on the floc characteristics and mechanism of membrane fouling in a sequencing batch membrane bioreactor (SB-MBR). At first, the jar test experiment was performed for 120 min. The results indicate that spiking microplastics of 100-1,000 MPs/L resulted in an increase in floc size by 27.9±9.2% to 70.9±5.1%, while the control set (without microplastic) showed a greater increase in the floc size at 113.3±5.6% of the initial floc size. Afterwards, the SB-MBR, which was equipped with a flat-sheet microfiltration membrane, was operated at 24 hours/cycle with 4 reactors at the influent COD of 1,500 mg/L. Microplastics were spiked to the reactors at 7, 15, and 75 MPs/L to accelerate microplastic accumulation, in comparison with a control (without microplastic). The SB-MBR achieved the COD removal of 98.5 – 98.7% and effluent turbidity of 0.26 – 0.31 NTU. When the microplastic accumulation occurred, the changes in the floc properties would be described as follows: decrease in floc size, decline of hydrophobicity, lower molecular size of extracellular polymeric substances (EPS, especially proteins), and higher calcium and magnesium ion uptake by microplastics. As for microbial community, Illumina Miseq technique and weight UniFrac analysis were conducted to study the microbial diversity among 4 reactors which had slightly difference of 3.9 – 7.9%. Moreover, the fouling mode, observed, started with pore adsorption at the filtered volume up to 1,000 L/m2, and then the cake formation followed, where difference between reactors can be seed from the second filtration cycle. Mechanism of fouling was predicted in terms of electrostatic double layer interaction and Lewis acid-base interaction. The large energy barrier occurred in the microplastic-spiking condition that hindered foulant adhesion onto the membrane surface. In addition, microplastics, which accumulated in a certainly high amount, behaved like scouring materials to the membrane surface. Overall, the SB-MBR is effective in removing COD and reducing effluent turbidity, and is likely to experience minimal negative impact by the accumulation of microplastics.