There is a lot of old discussions about backwashing medias... it is a little more complex than most people realize.
Simply stated, granular carbon is an irregularly shaped media. Resin beads are round.
Imagine a box of cereal, it settles significantly during shipping as the flakes find new ways of compacting everytime they are moved around. Cocoa puffs... they don't settle since round medias don't have any way to settle.
Silly sounding but hopefully it is making sense.
As the carbon media continually compacts it is also grabbing dirt and acting as an effective sediment filter. This is why carbon works well as a ferric iron filter (though technically it is not rated for it), and resin is ineffective as a ferric iron filter. The packed irregular shaped granular media effectively stops sediment (ferric iron is rust particles).
As this granular media is packed down over time, and the open pathways become clogged with sediment, the flow rate can be affected.
Intermittent backwashing reclassifies the carbon bed, gets rid of fines and removes most of the sediment it has collected.
If a carbon system is not backwashed, water may start to find paths (channel) around the media and it can become les effective. Water flow may also be impeded resulting in a loss of pressure.
Most municipal supplies are fairly free of sediment so backwashing a GAC system can be limited to once a month in most residential cases.
Resin based medias collect very little sediment, and they don't settle so backwashing them is less critical for non regenerated resins ie: ferric coated arsenic reduction resins or single use DI resins. Regenerated resins tend to need backwashing to assist with purging out broken resin beads that occur due to normal wear and tear.
Upflow carbon may seem like a good answer but it has its own unique problems and most of our more experienced and knowledgable customers would rather use a non backwashing downflow carbon system rather than deal with the higher potential for problems associated with upflow designs.
Class dismissed.
