Particles in filter effluent: Exploring the roles of deposition and detachment
Filtration is a final barrier in drinking water treatment plants for the removal of impurities and microorganisms in source water. Changes in regulations and the growing importance of the filtration process as a final barrier indicate that more sensitive analyses of particle removal in water treatment should be undertaken. The goal of this research was to investigate the relative roles of particle removal and detachment in controlling the origin of filter effluent particles. A conceptual mathematical model was developed and laboratory-scale experiments were conducted.
Laboratory experiments were performed to determine the fraction of filter effluent particles that are filter influent particles that were never removed as well as the fraction of filter effluent particles that were detached after deposition. In these experiments, four sizes of polystyrene particles were used as a primary source of particles in the raw suspension. Polystyrene particles destabilized using calcium chloride were supplied to the filter after flocculation. To investigate particle attachment alone in contrast to net removal from attachment and detachment, three sizes (1.4, 4.0 and 9 μm) of marker particles were injected ahead of the filter column as a pulse. Microscopic counting of filter effluent particles was assumed to reflect attachment. The net removal due to both attachment and detachment was assessed using particle counting (1∼300 μm). The difference between these methods is a measure of detached particles. Three different filtration depths of 0.5 mm silica sand media were utilized in a 25 mm diameter glass column to create the filter beds. Turbidity was continuously monitored throughout the run.
Experimental results indicated that particle detachment is significant beginning from the early phase of filtration. As expected, FM removal increased with filter run time, depth and particle size. For each size FM at one filter depth, FM removal increased with filter runtime to a maximum due to ripening and then decreased with filter runtime after ripening due to limited pore space remaining in the filter. The detached fraction of effluent particles increased with particle size and filter depth. The presence of detached particles and the increasing fraction of detached particles in deeper bed were confirmed.