Effects of microplastic accumulation on floc characteristics and fouling behavior in a membrane bioreactor

Area of Science:

• Environmental Science
• Environmental Engineering
• Microbiology

Background:

• Microplastic accumulation in sewage sludge during wastewater treatment is a growing concern.
• The specific impacts of microplastics (MPs) on membrane bioreactor (MBR) systems, particularly sequencing-batch MBRs (SB-MBRs), remain understudied.
• Understanding these effects is crucial for optimizing wastewater treatment processes.

Purpose of the Study:

• To investigate the effects of microplastic accumulation on floc characteristics, microbial community composition, and membrane fouling in SB-MBRs.
• To quantify the influence of varying microplastic concentrations on key performance indicators.
• To elucidate the mechanisms behind microplastic interactions within MBR systems.

Main Methods:

• Operated SB-MBRs with controlled microplastic concentrations (0, 7, 15, and 75 MPs/L) for 124 days.
• Analyzed sludge floc properties, including size, hydrophobicity, extracellular polymeric substance (EPS) characteristics, and zeta potential.
• Assessed microbial community structure and diversity.
• Examined membrane fouling behavior and biofilm composition.

Main Results:

• Microplastic presence decreased sludge floc size, hydrophobicity, and EPS molecular size, while increasing EPS concentration and negative zeta potential.
• These changes were linked to microplastic-facilitated uptake of divalent cations (Ca2+, Mg2+), weakening ion-bridging interactions.
• Microplastics had a slight impact on microbial structure and diversity, with a notable decrease in Actinobacteria abundance.
• Microplastics acted as a scouring agent on membrane surfaces, altering biofilm structure and reducing protein content relative to nucleic acids.
• Despite negative effects on flocs, the scouring action counteracted severe cake fouling in SB-MBRs over four months.

Conclusions:

• Microplastics significantly alter sludge floc properties in MBRs by disrupting inter-floc bridging mechanisms.
• While microplastics impact microbial communities, their primary effect on fouling is through membrane scouring, which can mitigate severe cake formation.
• The net effect of microplastics in SB-MBRs involves a complex interplay between sludge alteration and membrane surface scouring, warranting further investigation for effective management.