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In order to determine whether adding alum to the raw water would improve filter performance, an alum dose and rapid mix were added to the experimental set up. In this trial, we used an alum dose of 25 mg/L, 3 inch depth of 90 ppi foam, with the flow rate ramped between 50 and 200 m/day.

The figure below Figure 1 shows both the raw water turbidity and the effluent turbidity as a function of the flow rate. The effluent turbidity increases with the flow rate. This shows that filter performance decreases with increasing flow rates, meaning that in order to achieve a desirable effluent turbidity, a lower flow rate and thus a larger filter tank size must be employed. As show in the graph, filtration velocities of less than about 2 mm/s produce an effluent turbidity of less than one, or a pC* of greater than 0.9 Image Modified
Figure 1: Raw Water and Effluent Turbidity vs Flow Rate Image Modified
Figure 2: pC* vs Flow RateWhile the results obtained using this alum dose were phenomenal, this is a very high, unrealistic alum dose. To actually achieve this alum dose level, it would be necessary to install an additional alum doser prior to the filtration unit.

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Figure 3 shows the three ramp states as a function of flow rate. The first ramp state has a significantly higher turbidity than the following two. This means that the layer of film on top of the foam sheets is affecting filter performance, and this needs to be considered when designing a filtration unit. Image Modified
Figure 3: Effluent Turbidity vs. Flow RateAfter it was determined that the film that forms on top of the foam sheets affects performance, it was necessary to test and see if foam filtration is actually a function of depth: Depth Filtration