Can anyone shed some light on carbon rebedding? I currently oversee four facilities with different carbon tank configurations. I have backwashable tanks that I re-bed every six months. I also have in and out tanks that I re-bed every three to four months. Even if my chloramine levels are remaining safe I realize that a carbon tank is a safe haven for bacteria. This is the reason I change my tanks out at this rate. I have recently heard a horror story (or maybe not) of a facility letting carbon go for two to three years because of the fact that they had backwashable heads. Please shed some light on this subject. Am I spending to much time and money on carbon exchanges ??????????

First of all how big are your backwashable carbons? I too have backwashable carbon tanks. Our policy states that they will be rebeddded once every 2 years or as needed due to breakthrough. We currently culture after each of our backwashable carbon with min. bacteria growth. I would say that as long as your tanks are backwashing every day or every other day you should be fine as bacteria goes. And our clinic would rebed once every 2 years or at breakthrough.

As for you portable exchange tanks we replace the carbon in our tanks every 90 days to keep the bacteria growth down.

So I would say that you are spending too much money rebedding your backwashable carbons but right about where you need to be with your portable exchange carbons.

What do you think Chuck? Am I right or wrong?

Matt, I agree with you.

How often you rebed your carbon tanks will depend on how much carbon you have and how much water flows through it. Rule of thumb is, each cubic foot of carbon will treat 1,000,000 gallons of water with a TOTAL chlorine level of 1.0 ppm. If your chlorine levels are lower they will treat more water and if they are higher they will treat less.

Example:
You have 2 carbon tanks with 8 cf of carbon in each.
Your RO produces 5 gpm at 75% recovery, your total flow is 7.5 gpm.
If the RO runs for 12 hrs/day, 6 days/week your annual water usage will be 1,684,800 gallons (if my math is right).
Your TOTAL chlorine level is 1.0 ppm.

Based on this, you should not see chlorine breakthrough from the first tank for 4.7 years (though this calculation does not account for water used during backwash).

Once you have breakthrough from your first tank, move the second tank into the #1 position, rebed the exhausted tank and place it in the #2 position and you should be good for about another 4 years or so.

If the tanks are backwashed daily (even on days that the facility isn't running) you should not have a bacteria problem which would cause you to have to rebed the tanks early.

As far as exchange tanks go, we use two tanks each with 1/2 cf of carbon on our portable acute machines and they get replaced every 3 months (sooner if there are high colony counts).

Chuck

[This message has been edited by Chuck W (edited 03-31-2003).]

Got a question... I understand the point of the debate about exchanging or rebedding carbon tanks every 6 months to keep bacteria to a minimum. I wouldn't do it myself, but I understand the reasoning. But why would you do that when the RO removes it anyway? And with regular cleaning and disinfecting of the RO, is it really a concern? Besides, carbon isn't cheap by any means. I've never had a problem w/ bacteria in the past. If my thinking is wrong, feel free to explain it to me because I would like to understand it better. Thanks!

A 6 month Max run for portable exchange or non-backwashing Carbon tanks has been common for years. The biggest problem of course, is bacteria and their endotoxins. The tanks are wet, dark, and the removed organics provide food. As bacteria multiply they form the dreaded bio-film through their activities. The dead layers can and do produce endotoxins. When left in use for long periods of time they can even promote channeling of the filter bed and produce a bypass circuit with immediate Chlorine/Chloramine breakthrough.

It is true that the RO membrane is the ultimate barrier and as with any barrier, is not perfect. Once overwhelmed, as with T.N.T.C. bacteria from an old carbon tank coming in, the membrane will begin to pass the intruder. No barrier is 100% sad to say. The big problem encountered in the past is the actual time that the passage will occur. Scheduled rebedding has been found to be the answer.

Overwhelming an RO membrane with bacteria also causes a 2nd problem. As the troops have found in Iraq, when the enemy is minimal in number they are easier to both overcome and control. Disinfection chemicals and RO disinfection technics are used to eliminate normal (isn't that a great word) bacteria colonization. Once overwhelmed, the membrane will quickly rebound and problems occur with down-stream pathways.

I have seen situations where the only fix was Carbon rebed, membrane replacement, and the total repiping of the distribution loop. All after hours of labor, chemicals, freight, and lab studies. The carbon cost was the cheapest.

Guess that's all I have to say.

Take care
Life is GOOD!!
Willie B.