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Microplastic Retention in Secondary Sewage Sludge: Characterization and Influence of Solid Concentration
Summary
Researchers examined microplastic retention in secondary sewage sludge from a municipal wastewater treatment plant and found that fibers made up 61% of the trapped particles. Testing different solid concentrations in settling experiments, they achieved a maximum microplastic capture rate of 63% without adding any chemicals. The study provides practical insights into how wastewater treatment plants can optimize sludge settling to better capture microplastics before they are released into the environment.
The majority of microplastics (MPs) removed from urban wastewater during conventional treatments end up in sewage sludge (around 95%). There are not many studies focused on the retention of MPs in secondary sewage sludge. This study explores the amount and kind of MPs retained in secondary sludge from a municipal wastewater treatment plant (WWTP) and the factors affecting the entrapment of MPs within the sludge flocs. The most abundant MPs in the sludges were fibres (61%), and the majority were within the size range of 250–500 μm. In addition, the effect of solid concentration on MP retention in settled sludge was analysed by carrying out a mixing–settling test. Without the addition of any chemicals, a maximum MP retention efficacy of 63% was obtained for a total suspended solids (TSS) concentration of 5.33 g/L. The effect of adding FeCl3 and non-ionic polyacrylamide (PAM) was also investigated, and the percentage of retained MPs increased to values of 82% and 70%, respectively (with only 0.63 g TSS/L). This improvement occurred predominantly in the case of fibres. The results suggest the possibility of adding chemicals before the secondary settling stage as a means of reducing MP contamination in treated water.