Carbon Pretreatment Problem

[carbontank]

I have a 14x65 Carbon unit prior to a 5 GPM Reverse Osmosis Unit. Whenever I rebed the carbon the rejection rate drops from 97-98% to 88% or even lower.

Anyone know why this happens?

 

[waterexpert]

I'm guessing soluble salts in the carbon are being flushed out.

Sean

http://www.expertise-limited.co.uk

 

[carbontank]

Thanks for replying. Will the membrane rejection rate recover over time?

 

[bimr]

How are you able to quantify in such precision the rejection with such a small system? It does not make any sense that you are able to do it.

Are you actually measuring the flow rates? How do you go about calculating the rejection?

Is it possible that you are just seeing more flow through a clean carbon filter?

Anyway, the bottom line is that the carbon filter should not effect the RO unit in any significant manner.

 

[carbontank]

How are you able to quantify in such precision the rejection with such a small system? It does not make any sense that you are able to do it.
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Are you actually measuring the flow rates? How do you go about calculating the rejection?
..
Yes I have flowmeters, permeate, reject, recycle
feedwater TDS, divide by product water TDS, multiplied times 100, subtract result from 100 for rejection rate.
.....................

Is it possible that you are just seeing more flow through a clean carbon filter? <<<NO, REJECTION IS LOWER>>>

Anyway, the bottom line is that the carbon filter should not effect the RO unit in any significant manner.
IT IS EFFECTING THE RO MEMBRANES

This has been a common problem on many of the RO Units I have in the field. I'm trying to find the solution.

 

[waterexpert]

Without wishing to be disagreeable, I can think of two ways in which new AC cartridges might directly affect downstream RO, as well as a number of ways in which they might seem to do so.

Some grades of AC have residual contaminant salts, and virtually all have solid fines generated in transit or otherwise. Either of these have the potential to increase rejection rates.

In either case, flushing to drain with fresh water a couple of times for ten minutes at a time with a one hour gap in between before connecting in the RO stage should solve the problem.

If RO membranes are already contaminated, a backflush should hopefully sort them out, though it would be better not to contaminate them with solids in the first place.

There is also the possibility that air bubbles are getting into the system when you change the cartridge, lowering the effective membrane area.

Regards
Seán

http://www.wastewatertreatmentplantdesign.co.uk

 

[carbontank]

Sean, I'm not trying to disagree with anyone here, just a plain water guy seeking some clues.

The carbon unit prior to the RO is 14"Wx65"H .. has 60lbs of 1/4 gravel in it and 3cft of acid washed carbon.

The rejection rate always drops for some odd reason, Ran water through the carbon unit and RO for a full night to drain.

Rejection rate 88%

Instead of 97-98% prior to rebed

I appreciate all the input here.

 

[carbontank]

Just wanted to add, there is a 20" 5 micron pleated cartridge between the carbon unit and RO as well ....

 

[carbontank]

The membrane rejection rate this morning is 92-93% and climbing. It was definitely the carbon fines that were creating the problem.

Doesn't matter what type of carbon you use. I have used both Centaur and Westates and have had the same rejection rate failure after a rebed.

FLUSH FLUSH FLUSH ....

Thanks for the responses here.

 

[waterexpert]

I wasn't disagreeing with you, carbontank. Our replies crossed in posting.

So the fines were the problem?

All the best
Seán

http://www.seanmoran.co.uk

 

[bimr]

>Is it possible that you are just seeing more flow through >a clean carbon filter? <<<NO, REJECTION IS LOWER>>>

You seem to be confusing terms:

Recovery is defined as the percentage of the feed that is converted to permeate and it is usually expressed as percent. Flux is the rate at which the permeate passes through the membrane per unit of membrane surface area. Rejection is the ability of the membrane to restrict the passage of dissolved salts into the permeate, and is related to particular salt species.

I would think that your rejection rate is stays constant, but your recovery rate is dropping as your membranes foul.

 

[carbontank]

You seem to be confusing terms
.........................................................

Not to disagree with you but, Here's the definition of Rejection: Material not being allowed to pass through a membrane. This is what a reverse osmosis unit does: it "rejects" contaminants and does not allow them to enter the permeate, or product water.

If you employ the mathematical equation as I have stated above, you will get the rejection rate of the membrane(s)

In this instance, the permeate, reject and recycle readings taken from the 3 flow gauges all remained the same. Yet the rejection rate of the membranes dipped, according to the resistivity meter, So one would have to surmise that there was some sort of leachable material from the Carbon rebed that effected the membranes.

This is something I have been trying to rectify for years. I did some experimental testing several years ago using muriatic acid. Introducing small amounts into the feed stream both proir to and after the carbon unit, prior to the RO unit. My research showed when I introduced the acid prior to the carbon unit the resistivity of the water dropped significantly, however, when introducing the acid after the carbon unit I could actually bring the membranes back to life, Thin film membranes.

Perhaps I am confusing you with my terms. When I say the rejection rate I am talking of the ability of the membrane to reject salts and such.

 

[quark]

When you said your rejection rate dropped, do you mean your permeate rate increased?

If carbon fines are a problem then your permeate rate should decrease, rather.

Waht about the feed flowrate in all cases?

Excuse my ignorance. What is the meaning of rebed? Is it backwashing or topping up?

 

[waterexpert]

I think the confusion here is between rejection rates, which are to do with percentage removal of solutes, and recovery/permate rates, which are to do with the ratio of permeate to "rejected" feed. Calling the water which does not pass through the membrane the "reject" stream is probably at the root of this.

It is well-known that high levels of incoming solutes can reduce the rejection of other solutes. Hence my original suggestion that soluble residues from GAC preparation might reduce rejection of other species. Acid washed GAC does have such residues.

I could also see a number of ways in which fine GAC solids could reduce rejection rates. I would expect this to be associated with increased trans-membrane pressures at constant permeate flow rates. Does pressure rise?

Seán

http://www.seanmoran.co.uk

 

[carbontank]

When you said your rejection rate dropped, do you mean your permeate rate increased? NO - NO INCREASE

If carbon fines are a problem then your permeate rate should decrease, rather. NO DECREASE

Waht about the feed flowrate in all cases? FLOWRATE STAYED THE SAME

Excuse my ignorance. What is the meaning of rebed? Is it backwashing or topping up? I PHYSICALLY EMPTIED THE 14"W x 65"H CARBON UNIT (a backwashable unit) THEN REPACKED IT WITH 60 LBS OF 1/4" GRAVEL AND 3 CFT OF CARBON
........................................................................................................................

I think the confusion here is between rejection rates, which are to do with percentage removal of solutes, and recovery/permate rates, which are to do with the ratio of permeate to "rejected" feed. Calling the water which does not pass through the membrane the "reject" stream is probably at the root of this. THAT IS CORRECT - THE WATER GOING TO DRAIN IS IN FACT THE REJECT

It is well-known that high levels of incoming solutes can reduce the rejection of other solutes. Hence my original suggestion that soluble residues from GAC preparation might reduce rejection of other species. Acid washed GAC does have such residues. OK THEN MY UNDERSTANDING OF THIS STATEMENT SHOULD BE; THERE ARE CERTAIN ELEMENTS IN THE FEEDWATER AFTER THE CARBON REBED THAT WILL CHANGE THE CHARACTERISTICS OF THE FEEDWATER BEING INTRODUCED TO THE RO MEMBRANES?

I could also see a number of ways in which fine GAC solids could reduce rejection rates. I would expect this to be associated with increased trans-membrane pressures at constant permeate flow rates. Does pressure rise? NO THERE WAS NO INDICATION OF ANY CHANGES WHATSOEVER - ACCORDING TO THE FLOWMETERS THE PERMEATE GAUGE REJECT GAUGE AND RECYCLE GAUGE ALL RECORDED THEIR ORIGINAL READINGS - THE ONE THING I HAVE NOT DONE WHICH I FEEL RATHER STUPID FOR NOT DOING AT THE MOMENT IS RECORDING THE CHARACTERISTICS OF THE INCOMING FEED WATER THROUGH THE INLINE MONITOR - I WILL GET THOSE READINGS AND POST THEM HERE -

THANKS FOR ALL THE INPUT FROM EVERYONE THIS IS HELPFUL

 

[bimr]

What you are seeing is the results of a partial ion exchange reaction in new activated carbon. Over time, the ion exchange sites are exhausted and the partial ion exchange reaction ends.

The capacity of activated carbon to remove specific substances and mixtures depends on the nature of the specific chemical compounds, their concentration, conditions of adsorption (temperature, pressure, contact time, etc.), and how the activated carbon is applied to the problem

The partial ion exchange results in a difference in solution pH in the effluent of the carbon filter. Variations in feed pH to the RO (as well as a difference in ions), then affect the RO's rejection of ions.

Basic to membrane separation is the effect of feedwater chemistry and the benchmark is conductivity. Dissolved gases such as carbon dioxide (CO2) can affect permeate conductivity, and these constituents cannot be effectively dealt with in their original state by membrane technology alone. In the case of CO2, however, it is possible to force a conversion to bicarbonate (HCO3) and carbonate (CO3) ions by raising the feedwater pH. These ions are well rejected by RO membranes whereas CO2 (and carbonic acid) are not rejected at all.

Note that acid washing does not totally alleviate the problem with the activated carbon.

http://www.reskem.com/pages/carbon-selection-chart.php

http://www.resintech.com/pdfs/0501ph.pdf#search=" acid washing activated carbon calgon"

http://www.reskem.com/pdf/calgon-phrise1of5.pdf