The Philadelphia Suburban Water Company replaced the support gravel and media in four of the dual media filters at the Pickering West Water Treatment Plant in Phoenixville, PA. The support gravel and sand was replaced with the same size and quality in all of the filters. The anthracite was replaced with the same effective size and quality in all of the filters, but with different uniformity coefficients (one at 1.6 UC, one at 1.5 UC, one at 1.4 UC and one at 1.3 UC). Data gathered over a one-year period indicates substantial differences in the filter run times and water quality. The lower the anthracite uniformity coefficient, the longer the run times (up to 50% longer), the fewer the backwashes (up to 33% less) and the better the water quality (up to 38% less 2-5 micron counts). This paper will present the methodology and results of the full-scale operational test.