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In our research, we set up two filters in parallel that both received the same raw water but with one filter operating as a conventional rapid sand filter and the other filter having the raw water injected below the surface of the sand. The coagulant and clay water were both introduced into a clean water source before the influent turbidity of this water was tested. The water was then pumped to the two filter columns. After leaving the columns, the turbidity of the effluent water was measured and then left the system entirely. We measured both head loss and the effluent turbidity of each filter column as a function of time. We ran different tests at varying levels of influent turbidity, filter velocity, and coagulant dosage to see if any of these parameters significantly affected the head loss or effluent turbidity. In each experiment we also noted any differences between the appearances of the two columns which indicated differences in the location of particle capture.
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To backwash the columns, we used turned off the coagulant and clay pumps so that only tap water was used to clean the \[filters. This water was pumped up through the bottom of the filters to fluidize the sand beds. We noticed that the surface filter was difficult to backwash in this way, even at very high pump speeds, because of the large flocs that had settled on the surface (which occurred in most of the experiments but not all as described further below). These large flocs remained close to the surface of the sand column and did not get flushed out of the filter. Surface washing, the method of using a high velocity jet to effectively clean the surface or scraping off this top layer of floc build-up in the filter, would be necessary to thoroughly clean the filter. The subsurface filter had no visible signs of any large flocs or substantial floc build-up and it had no problems with the backwash method used in the experiment, which suggests that surface washing the SRSF should not be necessary. |
Our data suggested that there was no significant difference in the measured effluent turbidities of the two filters. Both filters also show linear relationships in measured head loss over time. However, one advantage we found to using the SRSF design is that the head loss increases at a slightly slower rate over time than the normal surface filter. We also noted that at a higher filter velocity, the surface filter did not show hardly any particle build-up on the surface of the sand column, which suggests that velocity is a determining factor in whether or not depth filtration occurs.
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