High pH levels and RO systems.

[Hermit]

My city water report gives 'typical' readings for a year. pH 10.81

http://meanderwater.com/Portals/MeanderWater/Typical Water Analysis - January 2012.pdf

This is the best information I've been able to find on high pH and RO. Anyone have better they can point me too?

http://www.membranes.com/docs/paper...ing High pH - High Temperature Wastewater.pdf

"At high pH, the membrane retains its negative charge. However, as pH increases and the hydroxyl ion concentration increases, the carboxyl groups on membranes surface are repelled or opened and the membrane “swells”. This “swelling” effect increases the passage of certain ions. Hydroxyl ions (OH-), for example, pass through the membrane more readily than chloride ions (Cl-). This is evidenced by a negligible decrease in pH in the permeate of a produced water RO systems when running at high pH. At one site running at pH greater than 10.5, the pH in the permeate of the RO system actually increased to more than 11.

Along with the high passage of hydroxyl ions, an increase in the passage of sodium is also seen. The sodium acts as the counter i on to the hydroxyl ions in order to maintain charge neutrality in the permeate."

I just put in a new system. I had a pH meter and got a TDS meter when I got the system. The system is new and supposedly the TDS will decline for a while, but I'm getting about 80% reduction now. I'm not sure it is going drop that much more. pH drops some but I haven't done too much with it yet. I'm thinking I might have to drop the $35 or whatever Ward Labs charges now to see what I have in a month or two.

 

[mabrungard]

If your water is similar to that report, it would be fine for brewing excepting for the alkalinity. The alkalinity is easily neutralized with an acid addition. The RO treatment does not appear necessary for most brewing. If you are achieving 80% rejection, that does raise a question. However the raw water is not that mineralized to begin with and the modest rejection rate is still OK.

While I'm not familiar with the effects of high raw water pH on membranes, the study you cite is not very representative since it studied the effect with very high TDS brines from well drilling. I suppose it could be the cause of the low rejection rate.

I would not concern yourself too much with ending up with an elevated RO water pH. That is also easily treatable with acid. Given the apparent modest rejection your system achieves, sending a sample of the treated water to a lab does appear prudent. Then you would have a better idea of what the water quality is. While you are at it, send a sample of the raw water too. See if it is similar to the report.

 

[Hermit]

This is reservoir water so 'typical' can be all over the place. I did appliance repair in this area for many years. In the spring when you had high run off rates you knew you were going to get calls about washing machines leaking. Most turned out to be over sudsing through the overflow because the water was softer. The point of the RO system was to have a known starting point but it looks like the high pH is going to still leave me in the dark unless a lab report gives me a better idea on what is being rejected in what ratios I guess.

If the report I saw holds then I'll probably have little chloride and lots of sodium. I'm guessing the anions will be blocked and the cations will mostly still get through. Again, IF this report translates to my situation, then this still isn't a bad thing. If the chloride and sulfates are almost totatly blocked the other ions aren't too bad so I could still dial in the levels on those pretty well.

 

[Hermit]

This may all be moot. I've been out of town weekdays for a while so I didn't know that the local water department is upgrading their facilities. SCC/ Recarbonation Improvement Project I don't know the details but it looks like recarbonation at least has the potential to lower pH from what little I've read so far.

 

[ajdelange]

80% rejection in TDS isn't very good performance for an RO system and the cause is, as you have surmised, the OH ions. They don't get rejected as your reference paper indicates. Remember that a good part of your alkalinity is hydroxyl alkalinity so you can't expect very good effect against that. The answer is pre treatment to lower the pH to closer to neutral and the obvious way to do that (on an industrial scale) is recarbonation (adding carbonic acid) though you probably wouldn't want to install facilities for that. Apparently citric acid is commonly used on a smaller scale. This would still be a pain to deal with - you would have to mix the chemicals into the tap water in a large vessel and then pump the mix to the membrane. Obviously the best case will be if the city does the carbonic injection. I can't believe that any municipality is delivering water with pH this high. I would have thought that some regulatory body would have gone after them by now (and perhaps, indeed, that is what happened).

 

[Hermit]

pH is only a "guideline" for the EPA at this point because there is no known health hazard. (I did find some people advocate the drinking of alkaline water as a health benefit. They think it goes through the body scarfing up free radicals because of the charge, kind of oblivious to what happens when it hits the acid in the stomach)

I haven't found a completion date on the project as of yet though. I saw a blog post indicating the construction is already underway. I'll probably just have to call the number listed and ask. It would be nice if it was by the end of summer at least.