By Mark Gans & Troy Wallace
When considering the collection of water, whether it be sewer systems over a hydrologic land area or underneath an over lying holding device, the collection system is often referred to as an underdrain. In the case of water treatment, underdrains are located beneath the filter media used to remove suspended solids from the water. The underdrain serves a multifaceted purpose: first, it supports the filter media above it; second, it serves as a means of collecting the filtered water; third it provides a means of backwashing the filter media; and, finally, it acts as a barrier so that the filter media will not pass into the next stage of the treatment process (Reynolds and Richards, 292).
The oldest and most basic underdrain system is the pipe
lateral which is displayed in the top animation (Animation
courtesy of Mark Gans). This system utilizes perforated piping
laid beneath the supporting gravel surface. The perforated pipes
then feed into a central larger drainage pipe that carries the
clarified water to the clear well. Although this type of
underdrain has been used successfully in the past, it has been
phased out over the years in newer plants do to the fact that it
doesn't provide a "perfectly" uniform collection of the
filtrate. The collection pipes are usually set at 6-12"
center to center (Hoehn, 207) and the dead spaces in between the
collection pipes can allow small amounts of water to lay stagnant
and remain uncollected. This "dead space" is also a
concern when the 
backwashing
cycle is initiated (Leopold Universal Underdrain,
3). The dead spaces are not washed when the water is forced up
through the filter because the water cannot reach these spaces.
Please see the diagram (courtesy of F. B. Leopold, used with
permission) to better understand the difference between a pipe
lateral and a tile underdrain. One common solution to this
problem is to perforate the pipes three or four times around the
pipe circumference.
The Triton manufacturing company has provided their own enhancement to the conventional pipe lateral underdrain. Rather than using collection pipes perforated only at certain points along the pipe, this proprietary device is constructed from a patented "Vee-Wire" screen. This allows water to pass through the pipe no matter where it comes in contact with it. The fine micro-screen also eliminates the need for support gravel in the media bed; the openings in the screen are small enough that sand cannot pass. The elimination of support gravel creates a deeper bed of filtering media that allows a higher filtering load over the same depth and reduces the risk of media bed upset during backwashing. For more information please visit the US Filter's homepage.
In an attempt to move away from the pipe lateral system, the Leopold or tile underdrain was developed to collect water beneath the filter media bed. This system hopes to ensure uniform collection over the entire bed bottom by creating a "false bottom." This is done by laying sections of clay that have eighteen 1/4" perforations every square foot across the entire top surface. Beneath the 11" wide top surface, the square, as it appears in cross-section, is divided into quadrants. The top two quadrants collect the water and pass it to the lower quadrants by means of two perforations 25/32" in size every square foot (Hoehn, 207). These collection spaces also provide the operators with a means of forcing an air/water mixture up through the filter upon backwashing. Please see diagram (courtesy of F. B. Leopold, used with permission) for a better idea of what the crosssection looks like.
The tile
underdrain system does not, however, eliminate the need for
gravel support media. The Leopold company has a new variation of
the tile underdrain called the Universal Type-S underdrain. This
new thermoplastic underdrain is used along with the IMS filter
cap, and can dismiss support media from the filtering scheme. The
cap acts as a porous rock that will not allow filtering sand to
pass with the filtrate (Leopold IMS Cap, 2). The
cap is installed on top of the Universal Underdrain with
self-tapping screws as shown in the diagram on the left (courtesy
of F. B. Leopold, used with permission).
To see the entire Type-S underdrain demonstration, click here.
For more information on the Leopold underdrain system, please see F.B. Leopold, Inc.
A dramatically different approach to water collection by means of an underdrain is the Wheeler bottom. The Wheeler system requires supporting gravel media so that the fines will not be allowed to pass into the collection bed beneath the Wheeler bottom. This design uses cells containing five ceramic balls across the entire bed bottom. The ceramic spheres lie in an inverted square pyramid that is open on the underside. One larger ball measuring 3" in diameter is surrounded by four spheres 1-3/8" in diameter (Hoehn, 209). The water passes through the balls as it does the sand and supporting gravel above it - it finds spaces to flow through (Edwards, 12/10/97). During the backwashing cycle, the water is forced through the balls by the specified backwash pressure, and the water creates an uplift force on the balls (a bouyancy force). The uplift creates small spaces between the balls that allows water to be forced up through the media bed. The forces that the water exerts on the balls as it passes may cause the balls to spin. The balls are in contact while spinning and they can be worn down over time due to the friction that is created during the backwash cycle (Edwards, 12/10/97). They may need replacement over the lifetime of the filter.
The system usually comes in panels measuring two square feet. The panels are groove and tongue on the sides so that they may interlock upon installation. There are nine cells per unit on an 8" center line with a 1" perimeter around the outside and 2" separating the cells from each other; the cells are 6" x 6" across the square unit (Hoehn, 209). The filter bed area must be designed to accommodate the two square feet panels. Please see diagram for visual reinforcement of both the plan and section view (sketches courtesy of Troy Wallace).
PLAN VIEW________________________________ SECTION VIEW
Hoehn, Robert. "Class Notes: Water and Wastewater Design." CE 4104. Virginia Tech, Spring Semester, 1997. p. 206-209.
Reynolds, Tom D. and Richards, Paul A. Unit Operations and Processes in Environmental Engineering. Boston: PWS Publishing, 1996. p. 292-293.
Edwards, Marc. Personal Interview. 10 Dec. 97.
F.B Leopold. Universal Underdrain. Zelienople, PA: 1987.
F.B. Leopold. Leopold IMS Cap: Integral Media Support. Zelienople, PA: 1990.
F. B. Leopold. Leopold Superblock II. Zelienople, PA: 1991.
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