Rehabilitating Gravity Filters

A dual parallel lateral configuration in a media filter underdrain offers advantages in effective backwashing and efficient filtration.
Rehabilitating Gravity Filters
Flat-bottom flume

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Granular filter media (effective sizes from 0.5 to 1.5 mm) generally remove particles larger than 7 to 10 percent of the smallest grains. Most water treatment applications try to remove color and all solids greater than 10 to 20 microns. Smaller particles are removed with the help of coagulating chemicals.

In gravity filters, the conditioned raw water is introduced at the top of a basin, flows down through the media bed, and is collected by the underdrain system (Figure 1). The driving force is the distance from the water level over the filter to the first air break, which is usually into a clearwell or control weir.

As solids accumulate in the media, the headloss increases until either the flow cannot be maintained or solids are driven through the filter. At that point, backwashing water or a combination of air and water scours the media and restores the driving force. Backwash effectiveness is measured by the resulting media cleanliness and costs of power and water used for backwashing.

Several studies have shown that an air/water backwash system provides a cleaner media and uses less backwash water than alternative methods.

The dual parallel lateral

The dual parallel underdrain was developed to solve flow distribution problems by providing a primary lateral and two parallel secondary compensating laterals (Figure 2). Control orifices open from the primary lateral directly into the secondary laterals.

Backwash water flows through the primary lateral, rises and discharges through the control orifices into the secondary compensating laterals. Any unbalanced flow from the primary lateral creates a directly compensating velocity pressure gradient, which puts the secondary lateral into a uniform hydraulic pressure condition throughout its length. This uniform pressure services the top-deck dispersion orifices that discharge from the compensating lateral into the filter box.

In a lateral with equal-sized and equal-shaped orifices along its length, the orifice farthest from the point of admittance delivers the most water. Discharge variances exist in the primary lateral of the dual parallel lateral underdrain; the secondary laterals provide balance.

This system provides uniform discharge along the entire length of the lateral. Dual parallel laterals can be sized up to 50 feet long with a backwash maldistribution of less than ±2 percent. An additional benefit of the dual parallel lateral is its ability to meter and uniformly distribute air to facilitate backwash.

A baffle in the bell end of the secondary laterals ensures that the air and water metered into each 4-foot section are discharged from that section. Without the baffle, the backwash water tends to push the air up and down the lateral, causing pulsations.

When rehabilitating existing filter systems, many factors must be considered. These include an adequate backwash flow based on the existing backwash system, configuration of the backwash flume, depth and shape of the filter box, and characteristics of the proposed media. In the case of dual media using anthracite and sand, the usual design backwash flow rate is 20 gpm per square foot at 70 degrees F.

Flume arrangements

Flume arrangement has a significant impact on rehabilitation cost. The configuration of the backwash flume is one of the major contributors of maldistribution in a filter, along with the underdrain lateral type and the media.

Front flume

Figure 1 shows a typical installation of the dual parallel lateral in a filter with a front flume. The backwash water enters the flume, travels into the dual parallel laterals, is dispersed up into the media, and finally is collected by washwater troughs. The air header piping in the flume uses J-risers to distribute the air to each dual parallel lateral. The air riser’s J shape is used to flush any water that accumulates in the bottom of the air header piping as the air fills the header.

Flat-bottom flume

The flat-bottom flume (Figure 3) is ideal for replacing existing plenum underdrains with an end feed inlet configuration. This design improves maldistribution characteristics while allowing for deeper, more efficient media beds.

Other flume designs are available for rehabilitation, depending on the existing infrastructure design. Figure 4 shows the arrangement used to rehabilitate a false-floor underdrain system. The old floor is removed and the dual parallel lateral underdrain is placed directly on the floor. The water enters the new underdrain layout at the end of the lateral and air is being added using drop pipes mounted on the filter wall.

The configuration of the filter box in a filter rehabilitation influences decisions such as lateral length, flume and underdrain configuration, media depth and selection, and backwash air delivery methods. The designer must fit the backwash flume, the underdrain, the media support, the media, and sufficient freeboard to the backwash outlet to prevent media loss.

Another method of adding media depth or increasing freeboard into an existing filter box is the use of a gravelless support plate instead of graded gravel. Media, such as sand, anthracite or granular activated carbon, can be placed directly onto the surface of the plate. This has the added benefit of eliminating gravel disruption and simplifies media change-out, such as for granular activated carbon reactivation.


The dual parallel lateral has been successfully used in thousands of new and rehabilitated water filter plant installations. The compensating secondary lateral has demonstrated excellent backwash characteristics versus other types of single-pass underdrain systems. The closely spaced orifices of the dual parallel lateral improve distribution of backwash air and water, providing better filter media cleaning.

The dual parallel lateral can be easily used to rehabilitate existing filter boxes. A low-profile block coupled with gravelless support plates enables designers to provide more filter depth and improve operating performance, while meeting ever more stringent regulatory requirements.

About the authors

Thomas M. Getting, P.E., BCEE, is principal engineer - filtration; and John Geibel, P.E., is senior product engineer - filtration; both with Xylem.  


  • Amirtharajah, A.; McNeily, N.; Page, G.; and McLeod, J. Optimum Backwash of Dual Media Filters and GAC Filter-Adsorbers With Air Scour. American Water Works Research Foundation, 1991.
  • Kleiner, M.; Snoeyink, V.; Horsley M.; Mayhugh, J.; and Cummings, C. Comparison of Filter Backwash Using Air Scour and Surface Wash at Decatur Illinois. Report, August 1989.
  • Beverly, P., and Morando, T. Filtration Training Manual. F.B. Leopold Co., 1997.


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