Filter Sand Characteristics in Long-Term Operating Water Treatment Plants: Impacts of Filtration Processes

As a pivotal unit in water treatment, filters undergo significant media aging during long-term operation, altering their physicochemical properties and heavy metal accumulation behavior. This study investigated long-term operated filter sands from a rapid sand filter (RSF) and a traveling bridge sand filter (TBSF) at a drinking water treatment plant in Guangzhou. Characterization revealed that prolonged operation induced the formation of poorly crystallized Fe–Mn oxide coatings on sand surfaces. The RSF exhibited markedly higher heavy metal enrichment than the TBSF—with substantially more fine particles (+9.5%) and coarse particles (+4.7%)—attributable to its higher proportion of fine particles and greater treated water volume per unit media mass. Distinct vertical distribution patterns were observed; metal concentrations decreased with depth in the RSF, while the TBSF showed minimal variation. Sequential extraction according to the European Community Bureau of Reference indicated that heavy metals (excluding Fe and Ti) were predominantly present in the reducible and acid-soluble fractions, with the RSF showing a higher proportion of the reducible fraction. Although particle size significantly influenced accumulation levels, it did not affect metal speciation within the coatings. Citric acid soaking partially removed Mn, Ni, and Cd; however, the associated Mn loss may compromise the coating’s adsorptive capacity. This approach proved insufficient to fully mitigate contamination risk—particularly given that acid-soluble fractions were not reduced, and even increased for Cd in the RSF—thereby substantiating the scientific basis for replacing long-term operated filter sands.