Head Loss Calculations

Head Loss through Foam

In order to calculate the head loss through the foam with clean water running through it, we needed to slightly modify our experimental apparatus. Usually, a pressure sensor would be used to monitor head loss, however the head loss through foam with clean water it so small that noise in a pressure sensor reading is significant to the reading itself. Therefore, we decided to visually measure the headloss through the foam. To conduct this experiment, two free surfaces were used, one on top of the foam and another at the water exit. The initial height of water was marked with a piece of tape. Water was then pumped through the foam at a flow rate corresponding to a 4 mm/s velocity. The water height rose, and equilibrium point was recorded. The difference between the two water heights corresponds to the head loss through the foam itself and the filter column. We then ran the same test with the filter column but without the foam to determine the head loss through the filter column. These two values were subtracted to obtain the head loss through the foam itself. This experiment was run using a 10 inch foam depth with both 60 and 90 ppi foam.

Table 1. Head loss through 10 inches of foam at 4 mm/s

Head Loss through foam as over an Experimental Run and at Failure

In addition to knowing the head loss through the foam with clean water running through it, it is also important to know the head loss through the foam over time and when the foam eventually fails due to collapse. For this experiment, the head loss will be greater than the first experiment and it will be changing, so we used a pressure sensor to monitor head loss over time. The experiment was run using a 10 inch foam column with 60 ppi foam and a 1.5 mg/L alum dose. The influent turbidity was 5 NTU, and the experiment was run until the foam collapsed.

Figure 1: Head Loss vs. Time, 60 ppi foam, 10 inch depth, 1.5 mg/L alum

These results show a relatively constant head loss throughout the duration of the experiment and then a spike. This spike in head loss represents the time at which the foam collapsed and the filter failed. After collapse, the head loss remained constant for the rest of the experimental run. These results were similar to what we expected. Previous experiments had indicated that head loss in the filter was minimal. This experiment shows that for the first sixty hours of the experiment, the head loss was approximately 1 cm.
Additionally, this data is useful for the design of the foam point of use filtration unit. Disregarding effluent turbidity, we now know that under these conditions, the filter can run for approximately 60 hours before collapse. This represents the absolute maximum time the filter can be run, however the user will want to clean the filter well before 60 hours so that the filter continues to produce low effluent turbidities.

Lastly, while the time to collapse for this experiment was approximately 70 hours, we expect that the time to collapse will vary with different influent turbidities, alum doses and foam porosity layering. We will not further investigate the time it takes for a filter to reach collapse since point of use users would not want this to happen.