Various types of modified filters were investigated to replace greensand filters which clogged when removing iron from ground water. A properly designed uniform-grain sized filter can increase the filtration time more than ten times that of ordinary sand or greensand filters. The filter medium was obtained by passing commercial filter material between two standard sieves of a close size range, so that the resulting medium was of a uniform size. The head loss rate on such a medium was independent of the filter depth and was inversely proportional to the almost 3/2 power of the grain size. On the other hand, the filter depth was almost linearly proportional to the time of protective action. The effects of the grain size, filter depth, and filter material on the filter run were evaluated with a synthetic iron water; and optimum filter depths for each unisized material were determined. At identical filtration conditions, anthracite had a 70 to 110% longer filter run than the sand medium, and it was attributed to the greater porosity of the former. The performance of unisized filters on permanganate-treated ground water was much better than that of fine-grained greensand. The validity of the filtration theory and equations developed by Heerje and Lerk was tested in regard to the relationship between T(1) (time of protective action) and grain size, and head loss rate and grain size. (Author)