Modelling and application of dissolved air flotation for efficient separation of microplastics from sludges and sediments

The pervasive presence of microplastics in our environment necessitates urgent measures to prevent further release and address existing contamination hotspots. This work focuses on the removal of microplastics from sludges and sediments through the application of dissolved air flotation (DAF). In addition to experimental investigations, the study presents the development of a predictive model for the formation of aggregates comprising air bubbles and microplastics in the DAF contact zone. First an empirical model was established to predict the attachment efficiency between air bubbles and particles (αdb). To this end, the experimental attachment efficiency was determined for various plastics, with varying particle sizes and shape factors, and system conditions. The obtained data set was utilized to establish an empirical model by PLS regression describing αdb based on particle size, bubble size, density, Hamaker constant, contact angle, solid load, aspect ratio, circularity, and sphericity, resulting in an MSPE of 0.011 and adjusted-R² of 0.64. Secondly, a semi-deterministic model based on the XDLVO theory was developed using MLR, where αdb was described by the natural logarithm of the Van der Waals, electrostatic and hydrophobic forces acting between the particle and air bubble, and the solid load, with an RSE of 0.067 and adjusted-R² of 0.63. Finally, the applicability of these models was demonstrated in a case study on industrial sludge consisting of sediment and PVC microplastics. Validation of the attachment efficiency and the dimensioning of a DAF unit for processing this sludge at a capacity of 20 m³.h-1 emphasize the practical implications of the developed models.