Efficient removal of microplastic particles from wastewater through formation of heteroagglomerates during the activated sludge process

Microplastic particles (MPs) are efficiently transferred from wastewater to sludge during wastewater treatment, but it remains unclear to what extent the physical-chemical properties (e.g., polymer type, shape, size) of the MPs affect their incorporation into heteroagglomerates. To assess the latter, different variants of MPs were individually spiked to activated sludge, which was aerated and mixed for durations between 0 and 120 min, followed by 30 min of sedimentation. In experiments with no aeration/mixing, already 70 - 80 % of all MP variants were found in the sedimented sludge. Including 120 min of aeration/mixing resulted in an additional accumulation of 15 - 20 % of the MPs in the sedimented sludge, with steady state conditions being reached after less than 15 min aeration/mixing. Overall, more than 90 % of all MP variants were accumulated in the sedimented sludge and up to 10 % remained in the supernatant. The similar relative affinities of spiked MP variants to sludge flocs derived from time resolved experiments (increasing aeration/mixing time followed by 30 min sedimentation time) agree with the similar behavior of all MPs during the activated sludge process. Images of flocs collected at the end of the experiments showed that MPs were dominantly associated with flocs. The formation of heteroagglomerates was likely favored by the close-to-neutral zeta potential of the MPs in filtered wastewater, suggesting that van der Waals forces control heteroagglomerate formation. Taken together, these results indicate that the removal of MPs during the activated sludge process is driven by the formation of heteroagglomerates, with differential sedimentation being more important than fluid shear.