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Backwashing Filter Sizing
May 15, 2008 |
Automatic backwashing filters use coarse media to treat water problems such as iron, taste & odor and even to balance pH. Because these filter media are so coarse, they do not allow for the easy passage of water, thus making the rated flow for filter media in general much less than water softener media which is spherical, smooth and slippery. Often times the flow rate for a given media is lower than the required backwash rate. For example, a 10-inch diameter filter media tank containing activated carbon will provide a flow rate of 3 to 4.5 gallons-per-minute (gpm) but has a backwashing flow requirement of 5gpm. So your water supply will need to have a maximum flow rate of 5gpm to backwash the carbon properly, but when the filter is in operation, your 5gpm flow rate will be restricted to 4gpm.
Yet another consideration is the amount of media to use which directly affects the size of the media tank. If you are removing 2 parts-per-million (ppm or mg/L) of chlorine with your carbon filter, then at least 1.0 cu/ft of granular activated carbon should be used. And if you are a heavy water user with several people in the household, then you may want to consider doubling that capacity. But you will first need to make sure that your water system’s flow rate is high enough to handle the 7gpm backwashing flow rate requirement. The more media you use, the larger the diameter of the media tank. The larger diameter of the media tank, the higher the service flow rate, but again, the higher the backwashing flow rate requirement.
There are some filter media that have as high of a service flow rate as the backwash flow rate, but for the most part, filter media needs a high backwash flow rate. The various filter media and their flow rates in different size media tanks can be found at our Filter Media Flow Rate page. Keep in mind that water pressure and temperature can also affect flow rates.
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