By Matthew Wirth
If the water industry has known as far back as the 1800s that air-enhanced backwash improves filter function and saves water (Image A), why is this backwash process so seldom used in applications other than municipal and industrial filtration? It is not the intent of knowledgeable water professionals to squander water, one of our most precious resources. System designers seek improved functionality and predictable quality results. So why is air scour seldom used in filter design for commercial and residential applications? There is no simple answer, but here are some ways it is applied, reasons why it is not commonplace, and recent discoveries that may make air-enhanced backwash a best practice for systems of all sizes.
Explanation of Air-Water Backwash
The practice of introducing air as part of the backwash process is described in several ways:
Air lancing—the practice of introducing air down into the media bed with pipes/laterals (lances).
Air scrubbing—more appropriately used in defining the cleansing of air but sometimes used to describe the practice of “scrubbing the media with air.”
Air scouring—practices that use air to loosen the collected debris trapped in a filter.
A Closer Look at Air Lancing
Air-lancing technology utilizes pipe laterals (Image B) or downward-directed lances to introduce air into the bed, commonly near the surface where most of the filtration happens, to loosen the debris cake and trapped solids and allow the water backwash to lift them up and out to drain.
Air scouring with air-lancing technology is primarily used with gravity bed filters. The standard operating practice (SOP) with piped air scour is to partially empty the gravity filter, leaving only a few inches of water above the surface of the media. At this point, the air-scour sequence injects air into the filter bed and allows it to pneumatically lift the top of the bed and begin the cleaning process.
After the air-scour cycle, the SOP is to stop the air introduction and begin the water backwash. In some cases, the unwanted solids exit through backwash troughs (Image C) and in others through a simple spillway that uses water to liquify the bed and lift the material to drain. The technology is sound, simple, and a best practice with large municipal and industrial gravity filter beds.
Air and Air-Water Scour
In a pressurized filter, the SOP is to introduce air from either the underdrain or from an air lateral placed below the media service level. With a lateral system, the filter is shut down and the pressure is released from the filter. Air is then forced through the lateral to begin the scour process by bubbling the upper layer of the media with air. The lateral is placed in the upper region of the bed because most of the filtration/separation happens in the top of the bed.
As explained by the Minnesota Rural Water Association, “[t]he upper six to ten inches of filter media remove most of the suspended material from the water.”1 A water-only backwash and subsequent fast rinse occur after the air sequence. It is not uncommon to have a hold time between the cycles to allow the system to repressurize and the media to partially reclassify before rinse begins. This SOP for pressurized filters will also add the air through the underdrain to scour the entire media bed.
Air scour with a combination of air and water introduces air from the underdrain while using a low-flow water backwash to lift solids from the bed while the air is tumbling the media. An example of this is offered in the operating specification for filter media GreensandPlus.2
Example:
Air-water scour: using 0.8-2.0 cubic feet per minute per square foot (cfm/sq ft), or 15-37 meters per hour (m/hr), with a simultaneous treated water backwash at 4.0-4.5 gallons per minute per square foot (gpm/sq ft), or 9.8-11.03 m/hr.
In this practice, water is flowing at 4.0-4.5 gpm/sq ft. The balance of the media cleaning is accomplished by the introduction of upflow air at 0.8-2.0 cfm/sq ft. The water conservation component is that GreensandPlus requires a minimum backwash of 12 gpm/sq ft at a water temperature of 55 degrees Fahrenheit. This practice saves approximately 60 percent of the water used in an all-water backwash.
Challenges Related to Introducing Air
Introduction of anything foreign into a potable water system requires consideration. If using an ambient air source for scouring, ensure the air is clean and safe from hazards. If the air is pumped through a compressor, the compressor must be of a design intended for use with potable water.
Bubble size determines how much air is being added into the media bed. Smaller bubbles have a greater surface area to hold oxygen, and they rise more slowly than larger bubbles, which allows the bed to actively scour; large bubbles simply escape into the freeboard. In addition, due to their size, smaller bubbles are more efficient at delivering oxygen into the water.
The size of the bubbles matter when adding air to water for scouring practices. Bubble size determines how much air is being added into the media bed. Smaller bubbles have a greater surface area to hold oxygen, and they rise more slowly than larger bubbles, which allows the bed to actively scour; large bubbles simply escape into the freeboard. In addition, due to their size, smaller bubbles are more efficient at delivering oxygen into the water. Large bubbles will collapse on themselves due to their size and surface tension, but smaller bubbles are easier to control and more predictable.
Image provided by Federal Screen Products Inc.
Benefits of Air Scour
It has long been established that using air scour to clean media beds improves filter operation and function and saves water. Simply put, filter beds cannot function effectively if they are dirty. When water supplies and flows are questionable, using air instead of relying on water backwash allows the system to operate without the need for high flows for fluidizing the bed. In the example, GreensandPlus can function with 4.5 gpm/sq ft water backwash when using 2 cfm/sq ft air scour. Otherwise, the Greensand bed needs 12 gpm/sq ft backwash using water only.
Another positive result of using air scour with the manganese dioxide (MnO2) catalyst is it provides O2 during the scour. This assists the manganese dioxide in staying active. Manganese dioxide is readily able to change oxidation states between the Mn+2 and Mn+4 valences by absorbing and releasing oxygen atoms.
During use, oxygen atoms are gradually depleted from the catalyst, and a ferric oxide crust forms on the surface of the media. Before all the oxygen is depleted, a cleaning backwash cycle is initiated to remove the accumulated ferric oxide and to replenish the manganese oxide with fresh oxygen atoms.3
The air scour/water backwash breaks the iron oxide crust away from the surface of the media and provides the oxygen (from the air) to replenish the manganese dioxide.
Advancements
in Air Scour
Two reasons air scour is not common in smaller systems are cost and deliverable platforms for introducing air into smaller equipment. In addition, air-scour systems are not “boxed” systems and require a higher level of application knowledge. It is just easier to use water backwash, but this thinking does not advance the move toward water conservation and best practice.
Recent system designs and controls with native relay drives make introducing air into filters for resident well water applications (Image D). Several new media offerings benefit from the introduction of air through the underdrain air-water interface. Providing O2 to media catalyst in this way offers the chemistry to perform without the need for a captured air head at the top of the tank—and all the service issues that presents.
Another development in the use of air-water interface in the underdrain is the ability to introduce low-concentration ozone to assist in media cleaning. Introducing the O3 while the bed is fluidized allows O3 to move throughout the media bed. In a downflow scenario, the O3 is being forced down into the bed. Air does not move freely downward through water, and the O3 is consumed in the freeboard without having an effect on the overall media bed.
Final Thoughts
Water professionals are not water wasters. Hopefully, they design with sound science and engineering to deliver effective, efficient, and functional systems to their customers. If building systems that utilize air-water backwash using air-scour techniques makes for better systems, the industry will find a way to implement these design advantages. We must be stewards of the world’s water, embracing the knowledge of the past and making it applicable to the earth’s future.
References
- Minnesota Rural Water Association, “Filtration,” accessed July 11, 2023, https://www.mrwa.com/WaterWorksMnl/Chapter%2018%20Filtration.pdf.
- Inversand Company, Greensand Technical Data Sheet, Suggested Operating Conditions, pg. 3.
- P. Meyers, email correspondence to author, 2020.
About the Author
Matthew Wirth is a water professional with 42 years of experience working at multiple levels in the water industry. The scope of his experience includes heavy industrial and commercial systems to public and private well applications, both customer direct and nationwide distribution. In addition to his work as frontline field support (including design, application, and service troubleshooting), Wirth is an approved trainer for several industry organizations and state CEU programs and an author for trade periodicals. He holds a Water Conditioning Master license in the state of Minnesota and a bachelor’s degree in organizational management and communication from Concordia University in Saint Paul, Minnesota. He received his engineering training at the South Dakota School of Mines and Technology in Rapid City, South Dakota. Wirth is the general manager of the Pargreen Sales Engineering-Water Division in Chicago, Illinois. He can be reached at (630) 433-7760.