high pH in RO water

[cheme6540]

Recently experiencing high pH in plant RO water system. Normally pH runs 6.5-6.8 range, for past several weeks around 9-9.2. Conductivity normal, about 15 microS/cm. We also test for Cl and NO3, those both normal as well. Any thoughts on what would be causing pH to go high?

 

[ashtree]

Do you know what the feed water pH is and what it has been before?
If a feedwater is under about pH8 it will go more acidic after RO treatment. But if the raw water is greater than pH8.2 it will tend to a higher pH after treatment. At 15 microsiemens the water has almost no buffering capacity so when you add an alkali like sodium hypochlorite the pH will go up.
So perhaps the feed water pH has changed slightly.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[cheme6540]

Feed/raw water pH has consistently been 9.2-9.4, conductivity 250-300 microS/cm.

 

[ashtree]

Without knowing the exact chemistry of the water or your process i would think if the raw feed water is normally around pH9.2 then it is perfectly normal for the permeate pH to be pH9.2 as well.
I think you should ask yourself why is it normally pH6.8.

Do you inject chemicals?
Has there been any detectable changes to the feedwater other than pH?
What are all the parameters that you test for?
Do you have a full water quality analysis?

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[cheme6540]

To answer your questions:
* This is a battery manufacturing plant. My focus is on quality control of incoming and process acid, but the RO water is what is used to dilute the concentrated acid. We have an outside vendor that monitors and services the RO system.
* The feed water/raw water is city water. No chemicals added to feed or RO water.
* Within the scope of my testing, I haven't seen any changes to the feed water. I don't have a full array of tools to test water, so for water I can do elemental analysis by ICP-OES, Cl- and NO3- by UV-Vis, TOC by titration, and pH/conductivity by laboratory benchtop meter. For anything else, we have a good outside lab that also handles the water testing for the municipal area we are in. My next move may be to send them samples for whatever water tests they recommend.

I have a smaller, independent RO water system in my laboratory (using the same city water as feed) that in turn feeds my DI water system. The pH of that RO water is consistently 6.7-6.8.

Water treatment is not my specialty, but my understanding from the literature I have read is that RO water, being fairly pure water, typically has a pH that is near/somewhat below 7. CO2 easily dissolves in this water, creating carbonic acid and thus driving the pH below 7.

 

[ashtree]

In high purity conditions as you describe , the DI water will likely be around pH7 and have negligible buffering capacity. On exposure to the air CO2 is absorbed and will reduce the pH.
It is interesting that you are measuring your city water at pH9.2. Normally city water would be less than 8.5 and more likely between 6.8 and 8.0, because of the need to manage corrossion and to ensure disinfection efficiency.
I would also be interested in the thinking behind the testing capability that you have and what the typical results for all those parameters are.

It is possible that the city are changing what chemicals they are using to manage pH and alkalinity at the water treatment plant. In periods where you get low pH it is possible that a greater amount of carbonate/bicarbonate chemicals are being used. When you see high pH perhaps only hydroxide chemicals are being used to manage the pH.



Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[IRstuff]

It's possible that this is the time of year when the water supply changes characteristics, such as having a much higher portion of runoff and snow melt, as opposed to aquifer water. I used to visit a semiconductor plant in the Northwest, and every, around late spring, we would get yield crashes, that were attributed to the DI water changing because the input water was changing.

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[shahyar]

Did you check your alkalinity of the feed?
If alkalinity drops, the pH of permeate will drop too.

 

[bimr]

If your incoming water has a high pH, then the RO effluent will be high. High pH influent lacks CO₂. With a lower pH, the water has CO₂ which will go through the RO membrane and result in lower pH RO effluent.

 

[ropoint]

Thanks for sharing useful information
Commercial Ro System