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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.
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This semester, the SRSF Theory team ran several experimental tests on the redesigned filter column and experimental apparatus. PACl dosages were varied between tests in order to analyze the trends in filter performance and head loss in relation to time and the amount of mass being added to the system. Progress towards developing a mathematical model to relate the maximum amount of head loss and the minimum sand pore diameter were in progress during the semester, but were stopped in order to reanalyze a previous hypothesis on the expected trend of head loss. Due to experimental data, several theories were proposed to explain the linearly increasing head loss after filter failure and are currently being assessed for its validity.
Fall 2014
The EStaRS StaRS Theory team will designed experiments to test the hypotheses on water flow through the filter and the resulting head loss. The team will also analyze the data from previous semesters and combine the data with the results from the new experimentsanalyzed the effluent turbidity and head loss data from Spring 2014 and found that while head loss depends on coagulant dosage, the results vary greatly. The experimental apparatus was rebuilt from the details of the previous semesters. The sand from the filter was removed and sieved so that sand stratification was reduced within the filter column. With the sand outside of the filter, the team tested for head loss across the mesh of the inlet and outlet pipes for the stacked rapid sand filter. In certain experiments, head loss was significant, though the collected data was rather inconclusive. A motion towards a stacked rapid sand filter without slotted pipes, as well as an accompanying experimental model, was recommended.
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