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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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