The synergy between particles and biofilms that drives drinking water discolouration processes in PVC pipes

Abstract

This study investigates how biofilms influence the accumulation and mobilization of iron oxide particles in drinking water distribution systems (DWDSs). Two experiments were conducted in a full-scale PVC pipe loop: one with biofilms grown over 28 days and one without biofilms. Iron oxide particles were injected into the pipes under steady flow conditions to promote particle attachment to the pipe walls, followed by four sequential flushing steps designed to mobilize the attached particles. Particle accumulation and mobilization were assessed using suspended sediment concentration (SSC), turbidity, and microscopy. Biofilms increased particle attachment from 66% to 72% and enhanced particle retention during flushing. In the first flush, 79% of the mobilized mass was released in the no-biofilm loop compared to 69% in the biofilm loop, indicating stronger adhesion in the presence of biofilms. Subsequent flushes mobilized more material from the biofilm experiment, particularly under higher shear stress. Microscopy revealed that biofilms captured both fine and large particles (up to 30 μm), and even with limited surface coverage (∼3%), substantially enhanced particle adhesion. While the biofilms developed under experimental conditions may differ from mature biofilms in actual DWDSs, the results demonstrate that biofilms have the potential to promote particle accumulation and resist their mobilization under high-shear events. Despite the ubiquity of biofilms in DWDS, these results may help water utilities improve pipe cleaning strategies and better manage material accumulation within the systems.