Understanding a UV term
Q:After reading an article you published in Water Technology^®, here is my question: What does the centimeter² term refer to in UV (ultraviolet) treatment? Is it the area of the bulb?

— California

A:The whole term is microwatt-seconds per square centimeter. It’s often written as: mW-s/cm². You are correct. The cm² refers to the effective (light-emitting) area of the lamp or the surface that the UV rays are striking. These two ought to be the same.

UV/chlorine interaction
Q:There is data showing that THMs (trihalomethanes) are produced when chlorinated water passes through a UV system. We have two concerns. Our drinking water division installs UV systems on drinking water from lakes and rivers for seasonal RV parks. We use a 1-micron prefilter and GAC, which provides suitable water for UV application. We do not chlorinate prior to the UV.

However, in our swimming pool division where the pool or spa is chlorinated, we do not install UV out of concern for the THMs and their increased levels, especially in the morning when the swim team arrives.

Everything we read says that UV and chlorinated water are not a good combination.

— Ontario, Canada

A:Disinfection byproducts (DBPs), including THMs, can be produced in any chlorinated water with or without UV, although I have read that their production can be enhanced with UV.

The reactions involved with UV, free chlorine, combined chlorine, DBPs, and THMs are more complex than your comments imply, though.

Also, as a side note, keep in mind that UV treatment is used as a dechlorination technique, in which free chlorine becomes hydrochloric acid. Discolored water after softening

Discolored water after softening
Q:I recently replaced the resin in my water softener, since it was about 7 years old and not softening very well. The 10 x 44 tank requires approximately 1.25 cubic feet of resin.

I purchased 1 cubic foot of amber resin and used the remainder of some leftover black resin. It appears to be softening, but I have discolored water (light yellow). I’ve run the softener through two regenerations and still have the same problem with discolored water. I never had a discoloration problem with the old black resin.

The water analysis is as follows:

Total hardness = 44.21 gpg (grains per gallon), or 756 mg/L (milligrams per liter).

Ca = 396 mg/L
Mg = 360 mg/L
Mn = .07 mg/L
Estimated TDS = 1,031 mg/L
SO₄ = 416 mg/L
pH = 7.13
Cl = 150 mg/L
SiO₂ = 17.76 mg/L
Fe⁺² (ferrous) iron = 6.27 mg/L (Note: There is an iron filter upstream of the softener.)

A:Since you describe the “black” resin as your original softening resin, we can assume it is a strong acid cation resin (standard softening resin). The newer “amber” resin (whose name you provided but which I’ve omitted here) is also a softening resin, so we know that you mixed two standard softening resins.

The mixture of the two resins would be no problem, and not a factor here. The amber resin is not made in the United States, so I’m unaware if it was rinsed sufficiently after manufacturing. Insufficiently rinsed resin could give off a color, but only for a short while.

I’d be more concerned about iron leakage from the iron filter. Check the iron concentration before and after the softener. If you have an iron filter that oxidizes and also filters the iron, there may be some iron leakage that would not be removed by the softener but would foul the softener resin.

After your resin replacement, the iron, if not all removed by the filter, might be disturbed and “loosened” within the softener bed. This might cause the iron to pass through the softener, causing the color you’re finding in the softener effluent as Fe⁺³ (ferric iron).

Unless there is ferric iron in the filter feedwater, most softeners will remove ferrous (the more common species) iron up to about 10 mg/L. So, you might also try bypassing the iron filter to determine how well the new softener resin works at removing the hardness and the iron.

There are also “resin cleaners” that will strip iron from the resin if it happens to become fouled by iron. These are usually comprised of sodium hydrosulfite and sodium bisulfite. Other cleaning options are citric acid or other acids. Any acid will convert ferric iron into ferrous iron that will be rinsed off the softener resin and sent to the drain.

Ozone and UV placement in bottling system
Q:I presently have a small water bottling facility with a capacity of 3,000 gallons per day through a 1-inch pipe, and I use ozone to purify.

I do not have a problem with the water quality, but, to be perfectly sure that I never have this problem, I want to add a UV lamp. My question is: Where should the UV be placed? Is it before the ozone or after? And what dosage should I use on both of each?

A:Usually ozone is used last. Since UV has no residual effect, it has no ability to destroy any microbial matter in the bottled water.

Ozone, on the other hand, stays in the bottle, at least a short while, with the ability to destroy whatever microbials that may remain, if any, after the UV treatment.

Salt ‘bridging’ in humid climate
Q:I find your column very clear and helpful. Thanks!

Regarding your recent comment in this column (“Pressure loss after softener,” November column) that “It’s normal to fill [a brine tank] all the way to the top”:

That’s not necessarily true in our hot, humid market in Houston. We never fill the brine tank to the top. If we did, the salt would bridge.

We always advise our customers to only fill it about 1/3 full, and we have been doing so for almost 20 years. Very often, when a customer calls with salt bridging issues, it’s because they filled the brine tank to the top.

— Houston, TX

A:Thanks for your input. I’m sure that you’re correct about the “bridging,” and I hope the tip will be useful to other readers who have your same climate.

As another alternative, I know that dealers from other humid parts of the US have good outcomes with different grades of salt.