By C.F. ‘Chubb’ Michaud, CWS-VI
In Part 5 of this series, it was pointed out that in order to achieve uniform plug flow, laterals have to be engineered for distribution and full media bed utility. This usually introduces a slightly elevated pressure drop in the system, so one must think ahead to provide sufficient flow capabilities to handle the backwash needs by either sizing up the pipes or blocking other uses of water during backwash and/or regeneration. In Part 7, flow through media filters themselves will be discussed and the importance of retention time (empty bed contact time or EBCT) will be explained for adsorption filters. The concept of half-length adsorption times will be introduced.
Particulate filtration (media filters)
Particulate filters can be barriers, such as a fine-mesh screen or media such as sand or multi-media. But media filters work differently, and here hydraulics comes into play. Figure 12 illustrates the open channel size of a typical media filter. How small is the flow channel that represents barrier filtration? I’ve done the math and it’s about 15.5 percent of the size of the particles in the filter. That means that if we select a 1 mm (1,000 micron or 1,000µ) sand, it will block any particle larger than about 155µ. Typical 12 x 40 mesh filter media have an average particle size of about 550µ with a resulting barrier size of about 85µ. How is it that we can filter down to 25µ or less with these filters? The answer is that we take advantage of hydraulics and the Nernst layer discussed in Part 2.
As water passes between the media particles, there are areas (Nernst layers) where the flow is very low or zero. Particles from the solution will come to rest the same way a sand bar builds up on the inside bend in a river. The slower the flow, the thicker is the dead zone and the better the filtration. At higher flows, these layers become very thin and the ∆P between particles is high enough to force particles smaller than 155µ through the opening and down the line. Keep in mind that a backwashed and classified bed of filter media has its smallest particles on the top of the bed. The spaces get bigger after that. Any dirt particle that makes it through the top few inches of the bed will pass all the way through the bed and show up downstream. Filter beds of this type are made to run at flows of 4 to 5 gpm/sq ft and achieve 20 to 30µ particle removal. If you run even more slowly and the filtered debris begins to restrict the flow channels, they work even better.
The simplest type of media filter is the sand filter, which utilizes a layer of #20 sand at the top, supported by a layer of #12 sand. This, in turn, is supported by layers of graded gravel. The sand layers can be 12 to 24 inches deep. The typical sand filter is a roughing filter intended to do course filtration in the 30 to 50µ range. They run at 5 to 6 gpm/ft2 and backwash at 15 to 17 gpm/ ft2. A poorly designed bottom distributor can roll the bed during backwash, resulting in uneven layers when the filter is returned to service, leading to channeling and poor filtration efficiency.
Multi-media filters are also called depth filters. The top two layers, usually a fine anthracite coal supported by a fine mesh garnet, are supported by courser garnet and gravel. The top garnet is a finer mesh than the anthracite layer, so the filter works through more depth than does a sand filter. They run at 4 to 5 gpm/ft2 and can filter to 10 to 15µ or even smaller. These filters backwash at 16 to 20 gpm/ft2 to release captured debris. Reduce the top layer of garnet to 60 mesh (about 250µ) and the filter will take out 90 percent+ of 7 to 10µ particles. Backwashing at 15 to 18 gpm/sq ft will expand the bed.
GAC (adsorption) filters
Although GAC filters are often used for particulate filtration to clarify water, their primary purpose is for organic adsorption and chlorine removal. In this mode, GAC filters require a certain retention time (EBCT—more in Part 9) for the adsorption reaction to complete. The efficiency of an adsorption filter, therefore, depends on the kinetics of the media. Hydraulic design assures that flow distribution is as uniform as possible to allow maximum utility of all of the media and provide for adequate cleaning and bed reclassification during backwash. Too low a backwash rate will lead to a clogged bed and high pressure drops; too high a rate can stir the bed and force media to escape during backwash.
GAC beds are often run at flows of 1 to 3 gpm/ft3 providing EBCTs of 2.5 to 7.5 minutes (explained in Part 9). The slower the flow/ft2, the deeper the bed can be. Surface flow should not go below 4 gpm/ft2 to prevent channeling. An upper flow limit should be 10 gpm/ft2. Backwash for GAC filters will range from 10 gpm/ft2 to 12 gpm, depending on GAC density.
Over time, I have received numerous calls from customers asking why their iron filters have stopped working. They were very conscious of sizing the filter properly to reduce the flow per square foot but have iron bleed through. They were correct in that the cross-sectional area of the media bed had to be large enough so as not to overrun the filter, but fell short by using too small a feed pipe to allow the filter to properly backwash. Backwash flowrates on particulate (sand, multi-media and zeolite and oxidation filters) are up to six times higher than service flows. Eventually, precipitated iron works its way down to the bottom of the bed and slips downstream in service. This is very problematic when twin parallel systems are used. When one is pulled offline for cleaning, the other unit takes the full service flow and often passes particles. The best way to reduce particle leakage is to make sure the beds are thoroughly cleaned during backwashing.
How big is too big?
My preference in designing media filters is to use multiple tanks of smaller filters rather than a single large one, so they can be properly backwashed one at a time. A 63-inch-diameter (20- ft2) media filter runs at 100 gpm and backwashes at 400 gpm; it needs a 4-inch inlet and a 4-inch drain to handle the backwash flow. This has to be a 4-inch pipe all the way to the water source to accommodate the backwash demand. Twin 42-inch units present the same amount of surface area and backwash at half the flowrate. Three 36-inch units can be put online and backwash at one-third the flow while maintaining continuous flow. It is really best to design in sets of three for commercial considerations, with two on and one offline, to prevent having to increase the feed to the on-line units when the exhausted bed is in backwash. For residential systems, two units would run in parallel, with the backwash time set for late at night when there is less likelihood of high water demand in the home.
Pressure drop through media beds
While the use of finer mesh filter media will improve the filtration efficiency of a system, it also presents its own set of ∆P issues. In going from a typical 550µ (about 30 mesh) particle to a 250µ (60 mesh), the volume of each particle is decreased by a factor of eight. Volume is a cubic function (V = 4/3πr3); there are eight times the number of channels that are two times smaller (but eight times longer). Fortunately, the flow per channel drops by a factor of eight, but length will increase ∆P by eight. A clean bed will show a ∆P of 10 to 15 psi instead of 2 to 3. Run big and run slow. Also consider shallower beds (24-inch) to decrease total pressure drop since media filters only utilize the top few inches for filtration. Table 6 lists recommended flowrates for service and backwash for several common media filters as suggested by their manufacturers.
What role does hydraulics play in filter design?
Filter media (sand, GAC or ion exchange resin) have a huge impact on flow and pressure; their efficiency is all about hydraulic distribution and flowrate. The media creates thousands of small flow channels that are very long and tortuous. Nernst layers are relatively thick compared to the total flow channel, and turbulence is high. The ∆P through a media bed may be 1 psi or more per foot of media depth at 5 gpm/ft2 of bed area. Trying to push 10 gpm/ft2 through a filter will increase the ∆P from the bed alone by a factor of four. In addition to restricting flow, this is hard on the media. It also wastes energy and poses the risk of early equipment failure from stress. High flows may reduce the efficiency of some filters due to the hydraulic relationships already discussed. In addition, higher flows may not present sufficient retention times to allow adsorption filters to do their intended jobs.
Even an inexpensive sand filter needs a first-class distribution system. Often the backwash piping may be twice the size of the inlet/outlet. Think the whole system through before you commit. Make sure the piping leading to the system has sufficient flow available for backwashing and that the drain can handle the flow as well. Barrier filters, such as sand or multi-media, will dump if the flow is suddenly increased. Using multiple filters to share the total load can often be the best solution.
- Michaud, C.F. The Dynamics of Media Filtration, WC&P International, February, 2011.
About the author
C.F. ’Chubb’ Michaud is the Technical Director and CEO of Systematix Company of Buena Park, CA, which he founded in 1982. He has served as chair of several sections, committees and task forces with WQA, is a Past Director and Governor of WQA and currently serves on the PWQA Board, chairing the Technical and Education Committees. Michaud is a past recipient of the WQA Award of Merit, PWQA Robert Gans Award and a member of the PWQA Hall of Fame. He can be reached at (714) 522-5453 or via email at AskChubb@aol.com