RO Equipment Question

[coffutt]

I am in the process of purchasing a water filtration system. A vendor I am working with put together a system that included a tradional water softener prior to a 4 stage RO. If the RO system is rated to remove, say 90-95% sodium, would there be a significant amount of residual sodium in the final water due to the softening?

Also, is there any advantage of a carbon filter on the softener if the RO has a stage or two of carbon filtering (I suppose one would be before RO and one after)?

Alkalinity CaCO3 65.8 ppm
Aluminum 0.092 ppm
Calcium 27 ppm
Chloride 120 ppm
Conductivity 1300
UMHO/CM Corrosivity 0.22 Langlier
Magnesium 14 ppm
Metolachlor 3.3 ppb
Manganese 7.7 ppb
Nickel 0.002 ppm
pH 8.6
Phosphorus 0.33 ppm
Potassium 16 ppm
Silica 5.6 ppm
Sodium 200 ppm
Sulfate 380 ppm
Total Hardness 120 ppm
Total Dissolved Solids 800 ppm
Total Organic Carbon 4.9 ppm

Does a 9:1 water ratio seem to keep me on board with the primer?

 

[Nateo]

Water softeners replace Ca and Mg with Na. Your combined Ca and Mg are 41ppm. If the RO system gets rid of 90% of it, that'd leave 4.1ppm. So, not much sodium would make it through the RO system.

 

[ajdelange]

Your supplier is smart to propose a softener in front of your RO system. That will prevent it from becoming fouled with calcium carbonate. Because this is commonly done RO systems are designed to have good sodium rejection. I'd press your man on that but look for 98 - 99%.

You have total hardness of 120. That's 120/50 = 2.4 mEq/L calclum plus magnesium. A softener will get nearly all of this (down to 1 - 2 ppm as CaCO3) and so add about 2.4*23 = 55 mg/L sodium so the sodium added by the softener is small compared to that in the water already. You'd have a total of 255 mg/L. 99% rejection of that would have you at 2.55 mg/L which is fine. Even 95% would put you at 12.75 - still OK.

Even 9:1 dilution will leave sulfate at 38 ppm which is too much for some beers but OK for others. For lagers I'd just go with pure RO plus salts per the Primer's recommendations.

The key issue with pre-carbon filtering is to get chlorine/chloramine out of the water before it hits the membrane which can be poisoned by it. Nearly all units have a carbon filter prior to the membrane(s) for this reason. A second pre-filter would confer some comfort as part of a 'belt and suspenders' system and, as they are pretty inexpensive. may be worth it for the peace of mind. If you are on a well and have no chloramine/chlrine added then forget about the second filter.

 

[mabrungard]

A water softener is not needed for that water since the calcium and magnesium are already fairly low. If anything, that water has the appearance of having been run through a softener already.

The 90 to 95 percent removal quoted is pretty poor. Even a monovalent ion like sodium should be removed at a rate closer to 95% or slightly better. Exceeding about 97% sodium removal is tough to do.

Including a second activated carbon filter in a RO system is kind of worthless in my opinion. The prefilter HAS to remove all the chlorine compounds prior to the membrane. That prefilter should get everything stripable from the water. If anything exists in the water that the post-filter gets, then its too late for the membrane.

 

[ajdelange]

Even a monovalent ion like sodium should be removed at a rate closer to 95% or slightly better. Exceeding about 97% sodium removal is tough to do.

The RO UltraTec TFC-HF1 membranes are rated for minimum NaCl rejection of 96%, nominal rejection 98.5%

 

[coffutt]

Thank you for the responses. I did double check the sodium rejection numbers, and they were correctly indicated in the original post--however, these were simply nominal numbers and not specific to the recommended system.

I am sure the answer to this question is a short one, but are there any advantages on using DI and UV in a light commercial application? The cost of these add-ons is relatively low, so I suppose the question should be, are there any disadvantages?

 

[coffutt]

A water softener is not needed for that water since the calcium and magnesium are already fairly low. If anything, that water has the appearance of having been run through a softener already.

The thing that has me confused is the company only asked for (as far as hardness) Total Hardness to make a system recommendation. According to A.J. deLange in Understanding Alkalinity and Hardness - Part I, he states, "The term “hardness” refers to the concentrations of calcium and magnesium, and only these two elements, in a water sample. This leads to some confusion because one can argue, and some do, that other elements such as strontium which is found at fairly high concentration in some wells should be included because it behaves very similarly to calcium and magnesium and will be measured by most simple tests. Nevertheless, the definition of hardness excludes all but calcium and magnesium."

My question regarding is, what makes up the rest of my Total Hardness with Calcium and Magnesium being relatively low? Could the vendor have another reason to recommend a softener?

 

[mabrungard]

Hardness is actually the result of all divalent metal ions, but the convention in water treatment is to count only the contributions of the calcium and magnesium ions. Those are typically present at much higher concentrations than the other metal ions, so the typical water treatment convention is not too bad.

For users with high iron or other metal ions, salt-based ion exchange is a suitable treatment alternative. That could be a reason the ion exchange softener was recommended. I see that the manganese is also high, but there are other treatment alternatives for that.

 

[ajdelange]

are there any advantages on using DI and UV in a light commercial application?

It depends on what you are doing here. I assumed that this was for home brewing in which case you need neither DI nor UV as RO water is low enough in everything that going even lower doesn't buy you anything. But now it seems that this is for another use and there may be advantages or even requirements imposed by regulation. The disadvantages with DI are that the resins must be regenerated from time to time (or replaced) and with UV the electricity must be paid for and the bulbs replaced from time to time. Also keep in mind that RO and to a even greater extent, DI water are corrosive and must be run around in non metallic pipe.

Now we need to go back to the question of the water softener. Your water is saturated with respect to calcium carbonate and thus lime scale will form on your membranes unless action is taken. The obvious approach is to soften the water but if you take that approach the softener must lower the calcium content to below 2 mg/L. At that level calcium carbonate is still your limiting salt and you must restrict recovery to about 20% (4 gal concentrate down the drain for each gal of permeate). If the softener can keep the calcium at 1 mg/L then it is possible to have recovery as high as about 45%. At 3 mg/L recovery would have to be less than 5%.

Typical home systems have recoveries of about 20% but as you mentioned commercial use the amount of waste may be at issue.

The problem isn't so much the hardness of your water but its high pH. At that pH even with modest alkalinity like yours, it doesn't take much calcium to saturate the water. In situations where the pH is high, like yours, RO feed water is usually pre-treated with acid to lower its pH and solve this problem. Were you able to do that here and get the pH down to 7 then while calcium carbonate would still be your limiting salt (this is the salt which is most likely to precipitate and thus limits the allowable recovery) you could operate your system at a recovery of 50%.

Bottom line: If you can acidify your feed then a softener is not necessary. If you cannot then the softener is absolutely essential and its regeneration and performance must be closely monitored to insure its ouput is at less than 2 mg/L Ca++.

 

[ajdelange]

You guys were typing while I was typing.

Hardness is formally ("Standard Methods") defined as the sum of the calcium and magnesium ion content. If one were to measure all the metals by AAS or ICP one could calculate the actual formal hardness and a well equipped lab can do that. But a smaller lab or the likes of us just adds chelant to a sample until an indicator changes color and reports the mEq/L of chelant used to suck up all the di and trivalent ions including strontium, iron etc that EDTA (the usual chelant) will grab. Some labs will use a separate chelating agent for calcium or precipitate out all the magnesium before using EDTA and thus get separate values for calcium and magnesium hardnesses by subtracting the calcium hardness (magnesium precipitated) from the total (magnesium not precipitated). Fancy labs seem to run the EDTA test and call that result total hardness and then do AAS or ICP for calcium and magnesium and report those as the metal. It should not be a surpise that 50*[Ca++]/40 + 50*[Mg++]/12.15 = 122.47 (for this water) does not equal the reported total hardness of 120.

If you add up all the ions the sum won't be 800 mg/L either.

The reason the company only asked for total hardness is that total hardness is sufficient to allow them to size the unit and determine how often it must be regenerated. 1000 gallons of water of total hardness 120 mg/L is going to pull out (120/50)*3.78*1000/1000 = 9 Eq of sodium. If the resin can only deliver 4.5 Eq per charge it will have to be recharged every 500 gal. (have no idea if those are reasonable numbers).

You provider should be very concerned about the calcium hardness of your water as he should be making (with greater accuracy than I did as he know more about the system he is selling) the limiting salt calculations I sketched out in my previous post.

 

[mabrungard]

I am sure the answer to this question is a short one, but are there any advantages on using DI and UV in a light commercial application? The cost of these add-ons is relatively low, so I suppose the question should be, are there any disadvantages?

There is no reason to include deionization (DI) or Ultraviolet (UV) radiation in typical brewery use. RO removes far too much mineralization as it is for typical brewery use and enhancing that removal with DI is just a waste of resources. The UV treatment is a disinfection measure that may be needed in biologic or health laboratories, but not in a brewery.

No advantages and the only disadvantage is the loss of capital that could go to a more useful end. (AKA: Don't waste your money!)