AWWA ACE54276

Laboratory dual media filtration experiments were conducted in direct filtration mode using a model raw water moderate in turbidity and low in dissolved organic carbon, and were also conducted in conventional filtration mode treating a watermoderate in turbidity and high in dissolved organic carbon. Effects of chemical pretreatment on the filter performance were investigated at conventional and high filtration rates. The results of this research indicated that direct filtration systems are much more sensitive to coagulant dose than are conventional systems. An increase in alum dosage in direct filtration mode, while improving filter performance, also showed some disadvantages including rapid development of head loss and, consequently, shorter filter service time. Suboptimal dosage in direct filtration significantly impaired the filter performance. In conventional mode, the effect of alum dosage on the filter performance, while obvious, was not as dramatic as in direct filtration. Laboratory jar tests provided useful estimates for the optimum dosage for the filtration in either direct filtration orconventional filtration mode. The conventional filtration results agreed with the jar test results using standard coagulation and sedimentation procedures. Jar tests with 48 hour extended flocculation were used in determining the optimum alum dosage for direct filtration experiments and proved to be a reasonable approximation. Ripening of particle removal occurred earlier than that of turbidity and, in most cases, breakthrough of particles started earlier than breakthrough of turbidity. These results suggest that turbidity can be used as a better or more conservative monitor of filter performance during the ripening period, while particle concentration measurements can be considered an earlier indicator of deteriorating filter performance during the breakthrough period. In dual media filtration, the lower sand layer serves as a multiple barrier for particles when the performance of the anthracite layer is not effective. When chemical pretreatment is inadequate, particles that pass through the anthracite layer reach the sand layer and if the coagulant dosage is not too low, can be removed due to the smaller size of the media particles. With adequate pretreatment, when the performance of the anthracite starts to break through, the particles dislodged from the anthracite media are effectively removed in the lower sand media. The optimal alum dosage for effective filter performance in direct filtration decreased as the pH of coagulation decreased. High filtration rates somewhat reduced the efficiency of the filter, while the general trends of the head loss development and turbidity and particle removals were consistent with those at low filtration rates. Includes 3 references, tables, figures.