There are basically three different types of filter media currently in use: monomedia, dual media, and multimedia. Each has its own characteristics. Following is a brief overview of each type.
Monomedia
One of the original monomedia designs was the "rapid sand" filter used
early in this century. Installed with several feet of sand, these filters
operate successfully in the range of 1 to 2 gpm/sf as long as the solids
loading is low. However, as the solids loading increases, sand filters
plug up quickly. Since the finest grains of sand are at the top, the tendency
is for most of the filtration to occur at the surface. As solids accumulate,
the incoming water tends to compress them into a thin layer which causes
high headloss and short filter runs.
"Deep beds" of coarse monomedia are occasionally used to lengthen the filter runs. Anthracite, sand, and gravel media in the range of 1.5 to 3 mm are often used. The coarser media permits solids to penetrate into the bed for greater storage and, therefore, longer filter runs. Coarser media also requires greater depths to retain good removal efficiency.
The ideal monomedia configuration would consist of completely uniform grains, exposing the incoming water to the same removal efficiency at any depth. Unfortunately, when a fluidizing backwash is used, any single layer of actual media will have the smallest particles on top and the coarser ones at the bottom. The sieve analysis resembles an inverted funnel, or cone, as shown in the accompanying illustration. Consequently, any solids that penetrate the upper part of the media have a progressively easier time passing through the entire bed. Therefore, it is mandatory that monomedia have the lowest possible uniformity coefficient. The steeper the sieve analysis curve, the more uniform it will be and the better it will operate.
Dual
Media
The use of anthracite over sand retains the removal efficiency of the
sand while providing greater solids storage.
The anthracite is placed on the top where it can remove the heavy solids and protect the sand from plugging. The fluidization step of the backwash cycle pushes the anthracite to the top and keeps it there because of its lower specific gravity.
Dual media beds usually consist of 10 to 12 in. of sand and 18 to 24 in. of anthracite. The depths of dual media filters are much less than monomedia filters because of the smaller anthracite grain. In order to be most effective, dual media like monomedia, also needs to have a low uniformity coefficient.
When using more than one layer of media, an important consideration is their interface. The greatest increases in headloss are in the top few inches of a given media layer. The sudden change from open channel flow to granular media causes a large increase in headloss at the surface. This headloss increase is compounded by the fact that the finest media grains are at the top of a given layer. A corresponding increase occurs with the sudden change from coarse to fine media.
To mitigate the interface problem, the size of the anthracite and sand is chosen so that the two intermix at the interface. The size of the sand is usually in the range of from 0.45 to 0.55 mm. The size of the anthracite varies from 0.9 to 1.2 mm, depending on the application. The heaviest grains of anthracite, being on the bottom of its layer, should mix with or sink into the upper level of the sand where the sand is the finest. The anthracite must not be too large or it will sink completely into the sand.
If intermixing is not desired, coarser sand is used. Filter runs are longer, but the effluent quality is much poorer. In some cases, polymers can be used to improve effluent quality. Intermixing permits a smooth transition from the anthracite to the sand with lower headloss. In addition, the solids storage capacity of the anthracite allows dual media to run as much as six times longer between backwashes.
Multimedia
While the addition of the larger anthracite particle to the sand
in a dual media filter improves headloss characteristics as well as filter
run times, it also reduces the overall surface area.
Adding a third layer to the dual media filter improves its efficiency. This third layer is finer than regular sand in order to increase the surface area while it is heavy enough to remain below the sand.
Garnet or ilmenite with the following characteristics are used for the third layer:
- Specific Gravity: 3.8-4.2
- Effective Size: 0.25-0.3 mm
- Uniformity Coefficient: <1.6
A typical multimedia configuration is 30 in. deep, consisting of 16½ in. of anthracite, 9 in. of sand, and 4½ in. of garnet/ilmenite. Uniformity coefficients in the range of 1.5 to 1.6 are normally used because of the relatively shallow layers and the height of the intermixing zones.
The ideal concept of the multimedia filter is one that is tapered, with the coarsest particles at the top and the finest at the bottom, as shown in the accompanying illustration. The larger solids are trapped in the anthracite. The sand removes solids that penetrate that layer. The garnet/ilmenite layer provides the final polishing. The ever-improving influent stream is presented with ever-increasing filter efficiency. The result is a media design that can provide long filter runs as well as high-quality water.
© 2001-2007. ITT Corporation, F.B. Leopold Company. All Rights Reserved.
