Fate of microplastics in sludge concentration: Experimental evaluation of gravity thickening and flotation processes

• MPs were tracked in real WWTP sludge using oxidative digestion and Raman • Up to 64% HDPE released to liquor after 36h gravity thickening • Floatation released 100% PTFE and 37% HDPE from Secondary Sludge to the liquor • Density and retention time strongly influenced MP’s distribution in both processes • Sludge concentration enables early MPs removal before final disposal Microplastics (MPs) are commonly retained in the sludge line of wastewater treatment plants (WWTPs), yet their behaviour during sludge concentration processes remains poorly understood. This study is the first to systematically evaluate MPs distribution in sludge concentration processes, revealing the critical role of polymer density and retention time. MPs behaviour was evaluated in gravity thickening and dissolved air flotation (DAF) using real sludge samples doped with three polymers of contrasting densities: HDPE, PET, and PTFE. MPs were quantified in both liquid and solid phases through oxidative digestion and Raman spectroscopy. Results showed that up to 64% of HDPE was released into the clarified stream during gravity thickening, while PET and PTFE remained mostly in the sludge. In DAF, 63% of HDPE was recovered in the floated sludge, whereas 100% of PTFE was found in the clarified water. These findings highlight the influence of polymer properties and operational conditions on MPs separation and suggest that sludge concentration units can play a key role in enhancing MPs removal prior to final sludge management.