Underdrain Designs: Why The Internal Cone Is The Most Economical Choice

Granular activated carbon (GAC) filters are deceptively simple systems. Many engineers who specify or design drinking water filtration systems may understand the basic idea of how a GAC filter works, but do not necessarily know some of the more technical nuances of the filters and their operation. In particular, the design and function of the vessel’s underdrain is often overlooked.

The underdrain is located at the bottom of the carbon vessel. In the simplest of terms, the underdrain supports the media bed. Water empties through the underdrain via outlets called septa after passing through the carbon bed. The design of the underdrain is key to the function of the filter. A good underdrain design will optimize media performance and reduce changeouts, which can save time and money. However, a poor underdrain design can create inefficiencies and inconsistencies, including uneven pressure drops and dead zones where water flows slowly or not at all.