Stratification of physical parameters in dual media filters at 10 waterworks

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