Experiences in the Use of Porous‐Plate Filter Bottoms

Abstract

During the past 20 years, most water utility men have heard or read something about porous‐plate filter underdrains and know that the use of such construction has been extensive. Progress in its use has been based on the benefits derived through elimination of graded gravel as a filter medium support, the chief benefit being that porous‐plate filter bottoms make it impossible for a bed to be upset. The first experimental studies took two courses. The first material tried was the smallest size of the usual supporting gravel, bonded with portland cement to form a porous slab. This proved unsuccessful in a comparatively short time, because the meager, semisoluble cement bond was completely inadequate to hold the gravel granules together. The second material tried was ceramically bonded porous plates. It was believed that a glassy bond fired in a high‐temperature kiln is both strong enough mechanically and sufficiently insoluble to provide permanent construction. Two independent tests were made of ceramically bonded plates, the first at the University of Texas and the second at Providence, R.I. These were made, respectively, with silicate‐bonded silica granules and clay‐bonded, fused aluminum oxide granules. The granules of these materials were relatively small compared with the pea gravel previously tried. The test plates were of the grade then used for air diffusion of small bubbles into sewage, and the available pore opening was considerably smaller than the interstices between ordinary sand particles. The Texas experiment proved that the basic idea was mechanically sound, but the raw water was not treated before filtration, and it was not possible to establish overall performance over a period of time. In contrast, the Providence experiment paralleled operation in the city's water treatment plant. As might have been expected, the underdrain plates eventually became clogged by the small solids that were able to pass through the filter medium. It was recognized that it was necessary to support filter media on a material with pore spaces larger than the interstices between the filter sand or anthracite granules. Thus, it can confidently be said that porous‐plate filter bottoms will not clog if the selection is correct in the first place, and if the treatment plant is operated normally. Several special situations that can result in clogging are discussed in the article.