By Matthew Wirth
Backwash is critical to the functionality of granular media filters and ion exchange systems that include an upflow bed expansion during the regenerating cycle. Without it, they will likely plug, channel, foul and eventually fail. While it is a fact that proper bed expansion is a necessity, it frequently is an unknown answer to diagnostic questioning related to inadequate filtration results. Volumetric bed expansion, drain line flow and system operating pressure during backwash is often misunderstood but cannot be overlooked or unknown. This article will explain the reason for backwash, show where to find operating specifications that identify proper bed expansion and review proper operating pressures for backwash. In addition, the reader will learn to calculate backwash rates with respect to media vessel diameter and look at different types of backwash techniques to improve backwash efficiency.
What does bed expansion mean and why is it critical to the discussion of backwashing?
Bed expansion is defined as the increase in the volume of a bed of ion exchange or filter media during upflow operations, i.e., during backwashing. It results from lifting, separating and stratifying the media bed (Image A). The left vessel depicts service flow and the right vessel represents bed expansion during the backwash cycle. As a rule, bed expansion is expressed as the percent of increase in bed depth. Once the media bed exhausts (such as a water softener) or reaches its design pressure drop (such as an iron filter), it is standard industry practice to clear the bed of collected solids using an upflow backwash cycle. This flushes particulates out of the bed in addition to breaking up filter cake and flushing it to drain. An effective backwash stratifies the media bed using an even flow of water flowing from the lower distribution apparatus. This expands the bed and lifts to drain the suspended particulates trapped in the bed. Each type of media has a prescribed bed expansion percentage. This is the value specified by the media manufacturer necessary for adequate flushing the product to prepare it for the next service run.
Why do systems need backwash?
Feed source water carries varying degrees of suspended solids. These solids lodge and/or build up when captured in the void spaces between the individual media substrate, i.e., resin beads or granular filter media. As the solids accumulate, they can allow channels to develop in the bed. In addition, if they build into a cake layer at the upper surface of the bed, this layer creates pressure loss (across the system), reducing flow and/or even stopping flow. Filter cake is a deposit that forms on the surface of a filter bed as particulates become trapped by the media. Eventually, the buildup can prevent functional operation of the filter due to increased pressure loss (and reduced flow). In extreme cases, the filter cake breaks apart during the service run, due to hydraulic forces and the filter dumps the cake material into the service flow resulting in discolored water (Image B). In all these cases, the system’s bed fails to work properly and poor water quality results.
Where to find backwash requirements
Most media manufacturers publish a cut sheet outlining Conditions for Operation and media specifications. Within the conditions of operation, one should find the bed expansion percentage recommended for that product. It is important to note that the flowrate necessary for the percentage expansion is expressed in gpm/sq. ft. Bed volumes are expressed in cubic feet and bed expansion flowrates, again, are expressed in square feet. In the Conditions for Operation for Anthracite (Image C) there is a bed expansion call out of 20-40 percent and corresponding gpm/sq. ft. backwash flowrates.
Calculating backwash rates
Calculating backwash rates for various diameter tanks for a given media. First one must determine the diameter of the tank.
This equation is:
With most math equations, there are simplified ways to get to the desired answer. Because the industry standard for expressing tank diameters is stated in inches, one needs to do several additional steps to get a resulting answer in square feet. Here is a way to get the answer in a single step. Use this equation D2/183. D = the tank diameter in inches. Example for a 12” diameter tank. 144/183 = 0.78 sq. ft. In case you are skeptical about D2/183:
Solving for D with π = 3.14 and expressed in inches: Area in ft2 = 3.14r2/144 = 3.14(.5D)2/144 = 3.14(.25)D2/144 = .785D2 /144 = 1/.785(.785)D2 ÷ 144(1/.785) = D2/144(1/.785) = D2/183.439
Area in ft2 ≈ D2/183
Before using the graph in Image C, we first need to know the feed water temperature. Water becomes more viscous (thicker) as it gets colder. The chart shows three different expansions based on water temperature. Colder water, therefore, creates more lift. Looking at the graph, a 30-percent bed expansion needs 12 gpm/sq. ft. of bed area at water temperature of 40°F. At 60°F it takes 15 gpm/sq. ft. Sixty degree Fahrenheit water has less lift because it is less viscous. Once the flowrate value is determined, multiply it times the square foot of bed area within the tank to get the system backwash flowrate.
Feedwater pressure and pressure loss
Feed water pressure and pressure drop also apply to establishing system backwash. Without pressure, there is no power to lift and stratify the bed to the required expansion percentage. Simply looking at the gpm of a supply line is no guarantee of backwash flow. While the source flow may be there, multi-port controls contribute loss through their internals and the DLFC (drain line flow control.) Drain line size and run length add to the mounting number of factors that slow backwash flow. Be sure to consider if the controls offer the needed gpm DLFC. Factor in all the related to the proposed system and its installation piping. Then answer this question, “Will the source deliver the needed backwash flow at an industry recommended 30 psi?” If not, consider other system options. Pay attention to the pressure drop during backwash for a particular controls valve. Larger controls offer backwash rates at lower pressure loss. Avoid using controls that show backwash rates that meet the require flow, but only at maximum pressure loss, i.e., 25 psi Delta P.
Types of backwash cycle sequence
Ion exchange systems use backwash to expand and loosen the bed ahead of brining to flush out impurities and illuminate possible channeling. Compared to media filtration, flowrates for backwashing ion exchange resins are fairly low – in the 4.5 to 5 gpm/sq, ft. range.
In sediment and iron filtration, there are generally two types of filter set-ups: pressure vessels and gravity beds. Pressurized filters are the standard application for residential, commercial & industrial and smaller municipal plants (Image E). These pressure systems backwash with line pressure, and use a free board for bed expansion and avoid media loss during backwash.
Large scale applications frequently use gravity bed filters. Their backwashing sequence involves several steps. First, the filter is taken offline and the water is drained to a level just above the surface of the filter bed. Next, compressed air from immersed laterals diffuses up through the filter media (Image F) causing the bed to expand breaking up the compacted filter bed and cake layer forcing the accumulated particles into suspension. After the air scour cycle, clean backwash water is forced upwards through the bed continuing the filter bed expansion and carrying the particles in suspension into backwash troughs suspended above the filter surface.
In some applications, both pressure and gravity filters use air and water simultaneously to enhance the backwash prior the downward flow rinse cycle. This is common in filters using heavy media such as pyrolusite or wide tanks needing extra assistance starting the bed expansion process. In addition, air enhanced water backwash is part of new technologies designed to reduce water waste and eliminate the need for double-digit square foot backwash flowrates.
It is a fact that media filters designed to backwash will not function properly without adequate bed expansion to clear the cake, the particulates and free solids from the filter material. Inadequate backwash enables solids to collect within the filter, the controls and on the media substrate – allowing this plugging material to eventually cause system failure. Before system design and sizing starts, get the source water flow details for the site – including the drain size. You cannot backwash 15 gpm down a drain that only takes 10 gpm. Do your homework on this most critical cycle and then decide on the application. Technologies are changing and if traditional high-backwash systems will not fit the site requirements, there are low water use options – options that function with a fraction of the water used with traditional filters. A little more time on the front side of a project will help prevent time-wasting service failures in the future.
About the author
Matthew Wirth is an industrial water specialist with 40 years of experience working with large process systems, including ro, iex, process filtration and media filtration. He is the General Manager of water at Pargreen Sales Engineering in Chicago, IL. Wirth received his engineering training at the South Dakota School Of Mines and Technology in Rapid City, SD, and earned a Bachelor’s Degree from Concordia University in St. Paul, MN. He can be reached via email, email@example.com or phone (630) 628-1330.