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Typical pilot studies may slightly underestimate clean‐bed head loss in full‐scale filter beds. The weight of media per unit of filter area cannot be directly measured in a full‐scale filter bed as it can be in a pilot‐scale filter column. Thus the porosity of a full‐scale bed of filter media cannot be readily calculated, and determinations of the porosity in full‐scale filters are rarely reported. In this study the porosity of one full‐scale monomedia filter bed was estimated by comparing the head loss in a full‐scale filter with the head loss in a pilot‐scale filter, when both used the same anthracite media. The full‐scale filter contained uniform anthracite media with an effective size of 1.5 mm loaded to a depth of 6 ft (1.8 m). The gradual termination of the backwash flow produced more consistent porosities in pilot‐scale filter beds. The porosity of the full‐scale filter bed was about 48 percent, somewhat lower than that expected in pilot‐scale columns. Thus, full‐scale designs based on pilot data may underestimate clean‐bed head loss.

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