Physicochemical aspects of particle breakthrough in granular media filtration
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Granular media filtration is used almost universally to remove particles from drinking water, and is usually the last particle removal process in water treatment plants. Therefore, superb particle removal efficiency is needed during this process to ensure a high quality of drinking water. However, particle breakthrough can occur by either the breakoff of previously captured particles (or flocs) or the direct passage of some influent particles through the filter. It is hypothesized that there are physicochemical differences among the particles, such that better destabilized particles are caught in a filter, while others that are not so well destabilized are allowed through. To investigate these differences, the zeta potential distribution (ZPD) and particle size distribution of effluent samples after filtration were analyzed. Filtration experiments were performed in a laboratory-scale filter using spherical glass beads with diameter of 0.55 mm as collectors. A single type of particle suspension (Min-U-Sil 5, nearly pure SiO2) and three different destabilization methods (pH control, alum and polymer destabilization) were utilized. The operating conditions were similar to those of standard media filtration practice: a filtration velocity of 5 m/h. More favorable particles, i.e., particles with smaller surface charge, were well attached to the collectors especially during the early stage of filtration when surface charge of particles and collectors were both negative. This selective attachment of the lower charged particles caused the ZPD of the effluent to move to a more negative range. On the other hand, the ZPD of effluent did not keep moving from less negative to more negative during the later stages of filtration, and this result was thought to be caused by two reasons: ripening effects and detachment of flocs. At the same time, to assess the possibility of particle detachment during the normal filtration, a hydraulic shock load (20% increase of flow rate) was applied after 4 hours of normal filtration. Less favorable particles, i.e., particles with larger surface charge, were easily detached during the hydraulic shock load. Therefore, proper particle destabilization before filtration is crucial for maximum particle removal as well as minimum particle breakthrough.