• Lund, Majbritt Deichgræber
• Ramsay, Loren Mark
• Søborg, Ditte Andreasen

Abstract

In Denmark, the use of dual media in rapid sand filters for drinking water treatment is widespread. In<br/>gravity filters with groundwater sources, the typical construction is a 50-100 cm layer of sand (granular<br/>quarts) overlain by a 30-50 cm layer of granular anthracite. The anthracite has a larger grain size than the<br/>sand, which purportedly reduces clogging and the required frequency of backwashing.<br/>Filter backwashing can influence the layering of filter material. According to the textbook explanation,<br/>backwashing removes trapped iron oxides from the pore volume and returns the grains to their original<br/>layered position with the anthracite on top. This study examined this explanation in detail, using filter<br/>media samples from 10 waterworks. The study identified potential causes of stratification or lack thereof as<br/>well as the variation of selected physical filter media parameters with depth.<br/>Large amounts of data were collected from various depths in the filters at 10 waterworks. Measured<br/>parameters included grain size distribution, grain coating, porosity, particle density, grain shape, bed<br/>expansion, iron mass balance, media age, media growth, and backwash procedures.<br/>Results showed that almost every physical parameter that was measured varied greatly from one<br/>waterworks to another. Strong correlations between certain parameters were found. The take home<br/>message is that a new explanation regarding the effect of backwashing of dual media rapid sand filters<br/>must replace the existing textbook explanation. The new narrative is closely bound to the backwash<br/>procedure, the age of the filter media, the density of the granular material and other parameters. This<br/>understanding has wide-reaching consequences for the operation of drinking water treatment plants.

Topics

• density
• impedance spectroscopy
• pore
• grain
• grain size
• layered
• iron
• porosity