pH of 4.0 in deionized water ?

[dobit]

We have two resins (Cation and Anion) beds in our facility. The feed water (into the beds) from RO unit has pH of about 8.0, conductivity of 35.0 and 0.0 ppm of silica.

Does anyone know why the final water out of these beds should read pH of about 4.5. In the past, pH used to be about 7.5. There is no problem with conductivity which runs between 0.5 after regeneration and 3.0 when bed shuts itself off.

Could this problem be attributed to resins going bad, concentration of acid and NaoH along with the resident times during regeneration? I know we changed our membrane filters not too long ago.

Thanks in advance for inputs.

 

[aspearin1]

Did you flush the membranes? Many membranes are made under acidic conditions, and if improperly rinsed, will contain acid salts that eventually go into solution. These salts could have killed your anionic bed.

 

[dobit]

Aspearin1, thanks for the input. Loaner RO membranes (Hydronautics) were put into service while our regular RO membranes(Triseps) were cleaned. I am not sure whether the loaner ones were properly flushed before they put inservice.

I know that it is not from our regular RO membranes 'cos the drop in pH were noticed 24 hours after the loners were inservice and we're still having the low pH problem after the loners were replaced with cleaned old membranes.

Do you know what need to be done apart from having to replace the resins?

 

[smile1]

I cannot comment on the RO membrane contribution, but before you invest in new resin, make absolutely sure that there is no leakage around the anion bed. A pH of 4 is trivial in low conductivity water, and may be a miniscule amount of untreated water bypassing the anion bed. Physically break and isolate valves which may be contributing to water bypassing the unit. You may be pleasantly surprised.

 

[sludgeking]

Ultrapure water will also take on a small amount of carbon dioxide from the air, which will cause the pH of the water to drop via hydrolysis. Our RO membrane permeate typically will run pH 5.0-5.5, and can be raised to ~6.8 by sparging with nitrogen.