Particles in filter effluent: the roles of deposition and detachment.

Particles in the effluent of granular media filters can be classified as influent particles that were never removed or as particles that detached after prior deposition. To determine the effects of particle size, filter media depth and filter run duration on the relative fraction of each class, laboratory experiments were performed using suspensions of four sizes of polystyrene particles (0.2, 1.2, 2.5, and 4.0 microm diameters) that were destabilized with 0.04 M calcium chloride and continuously supplied to filters after flocculation. To investigate particle attachment alone, three sizes (1.4, 4.0, and 9 microm) of fluorescent microspheres (FM) were periodically pulse injected immediately ahead of the filter media. Detachmentwas assessed as the difference between net removal (particle counts) and deposition (FM counts). FM deposition followed theory, while results show that particle detachment was significant from an early phase of filtration (100 minutes). The detached fraction of effluent particles increased with particle size (1 to 12 microm range) and filter depth. These model system results suggest that detachment plays a significant role in the origin of filter effluent particles in full-scale water treatment systems.