• Support Gravel
• Support Plate

Gravel is required to support the media and to keep the underdrain from clogging. The gravel design is very important since the life of the media depends on it remaining intact. The gravel design is especially important when an air wash cycle is used.

Even a uniform distribution of air can upset ordinary support gravel designs where the finest gravel is ordinarily on top

The hourglass gravel design overcomes this problem. As the accompanying illustration shows, the hourglass gravel design has two layers of coarser gravel on top of the finest layer. The larger gravel has less headloss per unit weight and, therefore, provides stability to the other layers. This design has been used successfully over air/water underdrain for more than 15 years.

Porous plates applied to the top of underdrain block or in false-bottom filter configurations are used to prevent filter media from leaking downward into the underdrain system.

One type of porous plate is made of granular mineral products. While the very small pores prevent the passage of filter media into the underdrain, they are susceptible to plugging due to chemical deposition. During backwashing granular mineral porous plate may clog with rust or debris. The brittle nature of this type of porous plate also makes it susceptible to breakage. Mineral product porous plate also exhibit poor flow distribution in large filters. This can lead to poor cleaning and the potential for fouling the media.

Another type of porous media support plate is made of plastic beads sintered together. The pores of the plastic support plate are small enough to prevent passage of the filter media into the underdrain, but the smooth surface of the plastic reduces the potential for plugging. The plastic plate is lightweight and easy to handle.

Porous media support plate offers a number of advantages over support gravel. Porous plate eliminates the possibility of upset gravel. Media changeout is easy because there is no need to worry about gravel disruption and displacement. And since the need for support gravel is eliminated, the freeboard is increased. More freeboard can be used for a deeper media load or to increase the distance from the top of the media to the bottom of the trough.