By Gene Franks
Backwash and flow rate statistics are given below for many of the media most commonly used in backwashing filters. This is information you should have if you're considering a backwashing filter.
The difficulty in sizing backwashing filters is that most filter media require a significantly higher backwash flow than the service flow they support.
The amount of flow and the amount needed to backwash the media are determined by the surface area of the media—depending on the tank diameter—not on the total area (cubic feet) of media.
A 12" diameter filter tank supports considerably greater gpm (gallons per minute) flow than a 10" diameter tank, but it also requires considerably more backwash water. If your well or city water supply does not supply adequate water flow to supply the backwash requirement, the filter will not work properly.
There is a tendency among filter buyers to assume that their water supply will support any filter and that, therefore, bigger is always better. Not so.
For most residential situations, a 10" or 12" diameter tank is the maximum you should consider. If more service flow is needed, using two smaller filters and installing them in parallel rather than in series is usually the best solution. (Parallel installation means that the water flow is split so that half flows through one filter and half through the other.)
For example, if your residential well has pumping capacity of 6 gallons per minute and want to install a Birm filter for iron removal, here's how you use the chart below.
Go the the Birm chart by clicking here. You'll note that a 12" Birm filter would require 8.6 gpm of backwash flow. Your well pump is only capable of 6. You can buy the filter and it will work—for a time at least — but it will probably eventually become overwhelmed with iron because it is simply not getting enough backwash water flow to clean itself out. If you choose a 10" filter you have just enough flow for an effective backwash, but your service flow rate is only about 2 ⅓ gallons. The filter will work well if you keep your flow rate down, and even if you exceed the recommended rate on occasion the worst that will happen is that some iron may leak through your filter. You're much better off with the 10" filter than the 12".
If you want a greater service flow rate, the best solution is to install two 10" filters side by side. This gives you a healthy and effective 4.5 gallon service rate. You set the timers on the filters so that they backwash at different times, then you have an adequate backwash potential of six gallons for each filter.
You will note from the chart that the density of some media makes them impractical for most residential situations. A single 10" Pyrolox filter, for example, would require 15 gallons per minute of backwash capacity to stay clean. That's rare in a residential situation where only 2.7 gallons per minute of service flow is needed
IMPORTANT NOTE: Figures in the chart below are based on average recommendations from media manufacturers. For example, if an 8 to 10 gallon backwash per square foot is recommended, we figured the rate for 9 gallons per minute. The figures are rounded "ball park" estimates. They are not intended as a guarantee of performance. Maximum flow figures can be exceeded, but performance efficiency will decrease.