Expansion and Headloss Dependencies for Flowrate and Fluidization during Backwashing of Sand, Anthracite and Filtralite® Expanded Aluminosilicate Filters

Abstract

The backwash expansion rates and headloss evolution of single- and dual-media granular filters of Filtralite® expanded aluminosilicate clay were compared with fine and coarser sand, as well as anthracite. Filtralite is manufactured in Norway, Årnesvegen 1, N-2009 Nordby. Abbreviations used for Filtralite is; N = Normal density, H = High density, C = Crushed. Each material had different particle densities and grain size distributions. The scope of the investigation was narrow: a clean-bed test was executed once for each parameter on single samples. As temperature affects the viscosity of water, tests were carried out within two temperature ranges (13–17 °C and 21–26 °C), and the effect on the fluidization of the materials was observed. The trial established that although the three types of materials have different physical properties, the expansion behaviors generally correlate with the grain sizes and particle densities of the media. To reach the expansion target of 15%, sand 1.2–2.0 mm (particle density 2656 kg/m3) required a flow rate of 67 m/h, Filtralite HC 0.8–1.6 (1742 kg/m3) required 34 m/h, and anthracite 0.8–1.6 mm (1355 kg/m3) required 15 m/h. The headloss peaks that indicate fluidization were found to correspond with the onset of expansion with increasing flow rate. This was for the example observed by fluidization of 0.4–0.6 mm sand (particle density 2698 kg/m3) at 0.94 m/m, fluidization of Filtralite HC 0.5–1 (1873 kg/m3) at 0.46 m/m and anthracite 0.8–1.6 mm (1355 kg/m3) at 0.21 m/m. Tests of dual-media filters of two types of Filtralite, i.e., Mono Multi and Mono Multi Fine, were also included. The backwash column used for the experiment consisted of extruded acrylic pipes with digital pressure sensors, an electronic flowmeter, a stepless pump and a water cycling system. A laminar water flow was provided by a mesh and a diffusor fixed above a single nozzle. No air was used. The trial was comparative, and its purpose was to shed light on the required water flow rates needed to fully expand different materials, and hence indicate requirements for performing proper filter backwashes.