Microplastic removal in batch and dynamic coagulation-flocculation-sedimentation systems is controlled by floc size

Area of Science:

• Environmental Science
• Water Treatment Engineering
• Materials Science

Background:

• Most microplastic (MP) removal studies use high coagulant doses, mimicking sweep flocculation, leaving other conditions understudied.
• Bench-scale jar tests are common but their batch hydrodynamics differ significantly from full-scale continuous-flow systems.
• The impact of various operating conditions on MP removal in water treatment remains largely unknown.

Purpose of the Study:

• To assess microplastic removal using conventional jar tests and a continuous-flow bench-scale system.
• To identify optimal pH and alum dosages for MP reduction under varied coagulation conditions.
• To compare MP removal efficiency between batch and continuous-flow water treatment models.

Main Methods:

• Conducted jar tests with diverse microplastic sizes and types to determine optimal pH and alum dosages for MP reduction.
• Employed a continuous-flow bench-scale system to compare MP removal against jar test results.
• Analyzed correlations between microplastic concentration, turbidity reduction, and floc characteristics (size and concentration).

Main Results:

• Aluminum hydroxide (Al(OH)₃) floc production was the primary driver for microplastic removal.
• Continuous-flow trials showed lower total MP removal compared to jar tests, indicating potential overprediction of performance.
• Strong correlations were found between MP concentration and turbidity reduction, suggesting turbidity as a viable surrogate.
• Significant correlations existed between floc size (90th percentile diameter) and concentration of larger flocs (>100 μm) and MP reduction.

Conclusions:

• Direct extrapolation of microplastic removal from jar tests to full-scale performance may be unreliable.
• Turbidity reduction can serve as a useful surrogate for monitoring microplastic removal efficiency.
• Floc characteristics, particularly larger floc sizes, are key indicators for effective microplastic removal in water treatment.