Underdrain Types
 

The purpose of an underdrain is to support the filter media, collect the treated water, and channel it to the clearwell. Then, as the filter media becomes dirty, the underdrain is used to evenly distribute backwash water used to flush out the solids. In any discussion of underdrain types, the primary consideration is for the backwash mode due to the higher flow rates.

The inaccessibility of an underdrain when installed, and the major part it plays in total filter efficiency and operating cost, make the underdrain selection a very important decision for the designer and owner. Basic design, materials of construction, structure requirements, ease of installation, gravel needs, experience, field results, reliability, and maintenance costs are all of major importance in selecting a filter underdrain. All are factors to be considered when evaluating designs.

 
Dual Parallel Lateral (Two Pass) Underdrain
The dual parallel lateral underdrain was developed by Leopold to solve the flow distribution problems of other systems. It consists of a feeder lateral (central) and two compensating laterals (one on each side) which are parallel with each other. Control orifices open from the feeder lateral directly into the compensating laterals. Washwater is admitted to and flows through the feeder lateral, and rises through and discharges from the control orifices into the compensating lateral. Any unbalanced flow from the feeder lateral creates an opposing flow, forming a directly compensating velocity pressure gradient which puts the compensating lateral into a uniform hydraulic pressure condition throughout its length. This uniform pressure services the dispersion orifices, which discharge from the compensating lateral into the filter box.




Hydraulics dictate that, neglecting friction, the amount of water that will flow from a lateral through a particular orifice is a function of the orifice coefficient and the hydraulic gradient in the lateral at the entry to the orifice. In a lateral having equal sized and shaped orifices along its length, the orifice furthest from the point of admittance will deliver the most water. These discharge variances exist in the feeder (central) lateral of the Leopold bottom, as in any single lateral. The function of the compensating (outer) laterals is to provide compensation and balance of these variances. This system provides an automatic and complete equalization of pressure prior to discharging the washwater from the filter bottom, resulting in complete, uniform discharge along the length of the entire row of blocks.

The unique dual parallel lateral design enables the Leopold universal underdrain block to accomplish difficult-to-achieve uniform distribution without introducing high headloss (pressure drop). The universal underdrain introduces less than 16 inches of headloss when backwashing at a 15 gpm/sf (24 inch rise) in a lateral 20 feet long. An additional benefit of the universal underdrain is its unique ability to meter and uniformly distribute air. Widely spaced nozzles release large volumes of air creating disruptive "explosions" propelling media to the surface where it can be washed away. The triangular shape of the primary lateral collects incoming air uniformly along its length. Small dispersion orifices then distribute the air evenly to all the discharge orifices.

Even a uniform distribution of air would upset ordinary support gravel designs. The finest gravel would ordinarily be on top where it is susceptible to disruption. The "hourglass" gravel design, used by Leopold, 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.



A case history on filter underdrain gravel mounding provides a gravel profile from an old Leopold installation. (See References.) The underdrain at this site is a dual parallel lateral design using hydraulic backwashing. Note that the gravel is largely intact after 28 years of operation. The continued successful operation of this plant demonstrates the operating efficiency of the dual parallel lateral concept.


The installation of the Leopold universal underdrain is simple. The filter compartment is constructed like a box with a plain flat floor. The design requires no special substructure such as supporting piers or beams, nor added filter depth for a plenum or false bottom. The Leopold blocks are set in grout directly on the flat filter floor with minimum anchorage required.

The blocks are placed end-to-end in rows so the laterals are continuous through the length of each row. Rows of blocks are placed on 12-inch centers across the width of the filter. The space between the rows is filled with grout, locking the blocks together and making a flat and level tile floor that is an integral part of the filter box.
 
Header Lateral (Single Pass) Underdrain
In the header lateral type of underdrain, the backwash water comes into the filter through a pipe manifold or pressurized flume. Laterals then branch out to distribute the flow through a series of orifices.

Flow distribution is dependent on headloss for uniformity. Due to the velocity of the incoming water, the static pressure in the manifold is usually highest at the end. As a result, the laterals at the end get the most water, while those in the front get the least. This maldistribution can be more than 20 percent. In order to get more uniform flow distribution, the orifices must have a high headloss (usually 6 to 8 feet of water). The orifice headloss must be far greater (200 to 300 percent) than the incoming velocity head plus the headloss in the manifold and laterals to achieve equal flow distribution. If high headloss is provided, pumping cost will be more expensive.


Another problem is that there are no orifices in the manifold. With no flow there, a dead area is created limiting the filters performance.
 
Nozzles/Strainers (Single Pass) Underdrain
Nozzle or strainer systems consist of a false bottom penetrated by the nozzles or strainers, usually on 8 to 10 inch centers. The false bottom typically sits on piers or short walls which create a large plenum area.
Strainer systems use fine openings to eliminate the need for support gravel. Slots or orifices small enough to retain conventional sand media will also "strain" any suspended solids or bits of sand or gravel that may be in the backwash water. (See Reference.) As the strainer gradually plugs with fines, the water pressure increases until the false bottom ruptures or nozzles fail. Many strainer or nozzle suppliers recommend a standpipe or other means to monitor and relieve pressure in the plenum. Cleaning plugged nozzles is extremely difficult. It requires access to the plenum area or the removal of any media that enters the plenum.


The strainers are widely spaced, usually 8 inches on center. This wide spacing yields many dead spaces in the media, reducing its effectiveness. The bottom 6 to 8 inches of media is largely ineffective, as a result. These dead spaces receive little or no backwash flow, and thus are prone to stagnation. Odors and septicity may result.


A false bottom is inherently susceptible to structural failure. During a backwash, water pressure acts upwardly on the entire underside of the plenum. This upward acting force is significant, especially if the underdrain is partially plugged.

Over time, repeated backwash cycles can cause structural failures due to flexure of the false bottom.

Nozzle or strainer systems have the same flume problems as the porous plate type. The velocity of the incoming water is such that the static pressure, and therefore the flow, is greatest at the inlet and around the sides unless very large plenum and flumes are used. Poor backwash flow distribution results in poor cleaning of the media, subsequent plugging, and potential structural failures.
 
Wheeler (Single Pass) Underdrain
The Wheeler underdrain is a false bottom type constructed of pre-cast or cast-in-place reinforced concrete supported on concrete sills. Each system contains uniformly spaced inverted pyramidal depressions. Unglazed porcelain balls are placed in the depressions to distribute flow. Each depression is filled and leveled with large gravel before placement of overlying gravel layers. During installation, placing the balls and first layer of gravel is very time-consuming.

The Wheeler underdrain is typical of the false-bottom, open-plenum, single-pass type of underdrain. There is not enough headloss in the nozzles or inverted pyramids to generate good flow distribution. The low headloss of the Wheeler underdrain is not adequate to control the media.

Once a filter of any type is in place, the life of the filter media is usually dependent on the support gravel. If the gravel begins to shift or "migrate, the filter bed will become upset resulting in loss of media and poor performance.

Following are excerpts from a case history on filter underdrain gravel mounding. (See References.)
 


What is Gravel Migration?
The support gravel is usually placed in layers of 2 to 3 inches thick each, with a total media support bed of 10 to 14 inches in filters using support gravel between the filter bottom and the filtering media. Gravel media migration occurs when these layers are displaced both vertically and laterally. This displacement is frequently described as gravel mounding.

What Causes Gravel Migration?
The cause of filter media migration and mounding is almost always associated with maldistribution of filter backwash water, frequently made worse by excessive backwash rates. Maldistribution of filter backwash water can be tied to three interdependent filter underdrain hydraulic characteristics:

  • Filter plenum hydraulics
  • Filter underdrain lateral hydraulics
  • Filter bottom block hydraulics

A problem frequently found was a notable maldistribution of water from the central filter backwash plenum to the underdrain laterals. There was a nozzle-like tendency for water entering the central plenum to shoot to the far end of the plenum and impact against the end wall where velocity head would be converted to pressure head.


This increase in pressure along the length of a lateral or plenum described above is the same hydraulic problem which occurs in most single lateral systems or false floor underdrains with low headloss.




 
Teepee (Single Pass) Underdrain
Teepee underdrains consist of a series of triangular-shaped laterals made of concrete or formed stainless steel sheet. A large number of orifices on the inclined surfaces point toward the center of the depression between the laterals. An air distribution pipe is usually located in the depression also. Some teepee designs use as much as 24 inches of gravel to cover it and distribute the flow. However, the ¾-inch gravel used is not large enough to allow much lateral flow.


Since the teepee design has no orifices in the center, dead spots will be created by the lack of flow in these areas. Dead spots will result in poor cleaning of the media and subsequent plugging.

Some teepee manufacturers claim even flow distribution with only 2 inches of headloss. That is possible, but only if the lateral or plenum and entrance nozzle are so large that the velocity head is negligible. Low headloss systems are more expensive to construct because of the very large flume and piping required to achieve the desired maldistributions. Even then, a low headloss underdrain would be incapable of controlling the media or insuring proper distribution. 

© 2001-2007. ITT Corporation, F.B. Leopold Company. All Rights Reserved.