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Metered Automatic Backwashing Filter?
July 17, 2007 |
Why not put a metered (on-demand) control valve on an auto-backwashing filter system? That way the filter system would only backwash when a certain amount of water has passed through it and not on a regular schedule.
It sounds good at first, but metered control valves are designed for and used on water softener media (ion exchange resin). Ion exchange softener resin is made up of some very small beads that have slippery surfaces which allows water to easily flow through the media bed. Flow rates on even small softeners often exceed 10 gallons-per-minute, where flow rates for auto-backwashing filter systems exist around 5 to 6 gallons-per-minute. This is because filter media (carbon, greensand, filter Ag, birm, etc.) is made up of small coarse particles which are not smooth at all, which restricts the water’s flow due to friction. These coarse particles that make up the media must be coarse to trap contaminants such as iron or sediment particles.
With that in mind, picture water flowing through one of these filter media tanks. It’s going to meet much more resistance from the filter media than it would from softener media. As is true with most things in nature, water is going to take the path of least resistance. Once several hundred gallons of water have passed through this filter tank, it’s going to start carving channels in the media just like a stream or river will cut through the earth’s surface. These channels will provide much less surface area for the water to come in contact with. As a matter of fact, much of the water will flow in the center of these channels and not come in contact with the filter media at all.
Also, most filter media is designed to trap microscopic particles such as oxidized iron. A backwash is required to release these trapped particles from the media and rinse them down the drain. If the filter media is allowed to completely load up with these particles, it will be ineffective. There is also bacteria that thrive on these iron particles and if they are not rinsed out, they can grow inside the media tank. For any of these reasons, your auto-backwashing filter system needs to be “stirred up” on a regular basis.
One story comes to mind. A customer on the West Coast had a local water treatment dealer set up a Fleck 9000 twin tank system for her with activated carbon in the tanks. Now the Fleck 9000 control valve has only one configuration, and that is metered. It is designed for water softener resin and nothing else. This customer called me and wanted my help in troubleshooting her carbon filter problem. It seems that after less than 3 years, the jury-rigged 9000 system was not allowing water to flow through it anymore. And because water would not flow through, it would not backwash either. What happened was that her carbon filter media clogged up with whatever-it-was-filtering-out of the water, and “cemented” up. It was useless and ruined.
The extra precaution of backwashing your filter media is well worth it in the long run. Metered control valves are designed for softeners and should not be used with filter media.
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