TONKA WATER TECHNICAL BULLETIN

FILTRATION SYSTEM MINIMIZES MEDIA LOSS, MAXIMIZES CLEANING

By Thomas D. Davis, P.E.

THE SIMUL-WASH™ SYSTEM INCORPORATES A BAFFLED TROUGH THAT IS A LOW PROFILE DESIGN ALLOWING INSTALLATION INTO SHALLOW FILTERS. THE MEDIA-RETAINING DESIGN ASSURES LESS THAN 1” PER YEAR MEDIA LOSS UNDER COLLAPSE- PULSE BACKWASH CONDITIONS.

Baffled troughs that allow operators to take full advantage of the collapse-pulse action of sustained air/water backwashing can improve filter cleaning and prevent media loss.

One such system, currently being market ed by Tonka Water, a U.S. Water Brand, under tl1e Simul-Wash™ brand name, allows the length of the simultaneous air/water backwash step to be extended beyond tl1e filter’s geometric rise limitation. This ability allows operators to control the length of collapse -pulse backwashing for optimum adjustable filter cleaning. It also helps minimize wash water production and prevent mudball for mation.

Filter Backwash Research

In the 1990s, seminal filter backwash research was conducted by Amirtharajah, et al, at Georgia Water Tech under an AWWA Research Foundation (AWWARF) grant. This research was sum marized in an AWWARF publication enti tled Optimum Backwash of Dual Media Filters and GAC Filter-Adsorbers With Air Scour. The study determined that back washing filters with simultaneous air plus water at sub-fluidized rates provided improved filter media cleaning.

What was surprising was tl1at using high simultaneous rates of air and water (i.e., water at or near partial fluidization combined with air) did not significantly increase tl1e amount of debris released from the bed.

Reprinted with revisions to format, from the November 2004 edition of Water World magazine.

Simul-Wash™ Advantages:• Promotes optimum performance

• Lowest profile design

• Meets Ten States Standards

• Long, clear spans

• Good for “vertically challenged filters

• Mudball prevention

• Achieves superior media cleansing

• Exceptional hydraulic capacity

• Ability to utilize with multiple equipment types including retrofit applications

• Hundreds of proven installations

What showed the best results was simultaneous air and water at sub-fluidized rates, typically water rates on the order of 3 to 5 gpm/sf of filter bed area, coupled with air at 2 to 3 cfm/sf. This finding revolutionized the art of filter backwashing.

Collapse-Pulse ActionA mathematical model was developed by Amirtharajah, et al, to determine tl1e opti mum air/water rates, using fluid and soil mechanics theories. This theoretical model was later verified with experimental testing. The most surprising discovery was that under the action of simultaneous air and water, tl1e filter beds undergo a “collapse-pulse action”. The pri mary variables for attaining this col lapse-pulse action were found to be media gradation and water temperature. The collapse-pulse action can be described as fol lows: the air bubble exits the air deliv ery device (orifice) and expands under the weight of the media. When the air bubble expands, the media expands slightly within the vicinity of the bubble, and tl1e bubble collapses and re-forms just above its original location. This collapsing is due to the weight of the media. The bubble re forms above its original location because tl1e media is only partially expanded. Just prior to collapsing, high local water velocities occur

at tl1e perimeter of tl1e bub ble. Simultaneous to bubble collapse, media particles rush together and collide in a vio lent scouring action. This creates a “pulsation” in tl1e bed. The bubble travels on upward, expands, collapses and re-forms again, and repeats the process several times as it passes through the bed. Eventually the bubble reaches the surface and bursts to atmosphere. This collapse-pulse action was verified using endoscopic video cameras dur ing the latter stages of the research, and appears as a violent pulsation throughout tl1e depth of tl1e media.

Filter Geometry

The only limitation of tl1e AWWARF research was that the time of tl1e backwash was assumed to be limited by filter geome try. Most existing rapid sand gravity filters are designed for water only backwash with maximum 50% allowable room for media expansion below tl1e backwash troughs. This means that to practice simultaneous air and water backwash, water needs to be drained down to 6” above tl1e surface of the media, air scour applied, and then water introduced simultaneously until the water surface rises to the bottom of the backwash trough. At this point, the air must be turned off and water only backwash must com mence. If the air is not turned off, media loss will occur. Collapse-pulse action is so powerful that it expands media all the way to the water surface.

TONKA WATER TECHNICAL BULLETIN

Proven Design

Sustained Simultaneous Air/Water Backwash In tl1e late 1970s, research was begun on baffled backwash trough technology suit able for sustained simultaneous air and water backwash. That research led to the develop ment of tl1e baffled trough design offered by Tonka Water.

The Simul-Wash™ system incorporates a baffled trough that is a low profile design allowing installation into shallow filters. The media-retaining design assures less than 1” per year media loss under collapse-pulse backwash conditions. The trough is fabricated from stainless steel, providing corrosion resistance. Added characteristics of the system include a high moment of inertia for the baffled trough, with the ability to span approximately 25’ without requiring intermediate support. This is a key advantage in retrofitting large shallow filter cells with the system.

Tonka Water GuaranteeTonka Water provides the best custom manufactured water treatment systems in the industry. Our people will deliver excellent service and support for your project from conceptual and cost-effective design, to construction and commissioning; and throughout the system warranty and operational life of the project.

Thousands of quality water treatment installations since 1956.

Conclusions Typical backwash rates include 3 to 5 gpm/sf of water with air at 2 to 3 cfm/sf applied simultaneously. This sub-fluidized water flow rate reduces backwash volume by up to 50% ver sus conventional backwash sys tems. Because a re-stratification step is required, a dual rate or variable rate backwash control must be furnished . PLC con trols are required to ensure that each filter is backwashed exactly the same way each time backwash is initiated. Maximum water conservation is only attainable when collapse-pulse action can be sustained. The Tonka Water technology has been incorporated in pressure filters, and it includes all necessary features of a properly designed sustained simultaneous air/water backwash system for potable water applica tions.

About the AuthorThomas Davis, P.E. – Consultant

After over 25 years of experience with Tonka Water, Davis now serves as a consultant for the company. He holds membership in the American Society of Civil Engineers, the American Waterworks Association, and the Water Environment Federation.

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763.559.2837www.tonkawater.comTonka Water, a U.S. Water Brand

Filtralator™ gravity filter with Simul-Wash™ collapse pulse