Ultraviolet (UV) light is that part of the electromagnetic spectrum that falls between x-rays and visible light. For decades, UV light has been used for disinfection in many industries. The unusual light next to the jar of combs at the barber shop, that strange light emitting from the cracks where the handrails creep out of the floor on escalators, and those glowing lights around a swimming pool are all examples of UV in action.
UV vs. Chloramines in Pools & Spas
For more than a decade, medium-pressure UV has been viewed as a reliable way to enhance efforts to eliminate the chloramines (combined chlorine compounds) that so often cause eye burning and irritation, especially in busy indoor swimming pools, through a process called photo-oxidation.
UV has also been unfortunately promoted to provide a second tier of disinfection (after traditional chlorine products). Yes, UV will inactivate pathogens, however, with that being said, it is vital that operators understand that UV (of any type) does disinfect, however it is an "outboard" disinfection and oxidation process (it does not happen in the pool, but in the mechanical room) that is totally dependent on the body of water's "turnover" rate. Gage and Bidwell's "Law of Dilution" clearly illustrates that even the best UV cannot treat all the water in any pool until near five full turnovers are accomplished (in a pool that is approaching 30 hours!).
As these systems gained acceptance and popularity, many of us in the aquatics industry began to assume that the installation of a medium-pressure UV system would lead to a reduction in chlorine use. Imagine our surprise, then, when more than one client called and told us they were actually using more chlorine after they installed medium-pressure UV units. We had to get to the bottom of this.
The Chlorine/Chloramine Conundrum
We set out to find answers by creating a testing environment at three separate indoor facilities where medium-pressure UV systems had been installed. After drilling and tapping the PVC pipes on either side of the medium-pressure UV contact chamber located in the pool mechanical room, pet cocks were installed so that water samples could be drawn and water testing completed.
With the mystery of excessive chlorine use in mind, we performed tests with the UV system on and off. The results were surprising! When the UV system was on, by using a typical colorimetric free chlorine test, we found a significant drop in quantitative chlorine (PPM) from before and after the UV chamber. We confirmed the readings using both a slide colorimetric comparator and a digital test unit. In one case, the difference was greater than 40 percent. After we tested three other units at other locations, the average loss in quantitative chlorine (PPM) was just over 28 percent.
The medium-pressure UV system was destroying a considerable amount of the actual chlorine residual as it passed through the UV chamber. This resulted in the chemical controller feeding more chlorine to replace what was lost and maintain code-required residuals. So we were getting the chloramines destroyed—but at what cost?
Dollars and Sense
At one facility, a busy swim school, the average daily use was just over 11 gallons of sodium hypochlorite. This facility was losing more than 3.5 gallons of chlorine daily at a cost of just over $6.60. Adding in the acid to neutralize the high pH of that chlorine injected downstream of the UV unit brought the cost up to around $7.50 per day. That may not sound like much, but do the math: It adds up to $225 per month, or more than $2,700 annually.
Then, if you add in the cost of the medium-pressure UV lamps (which require annual replacement), the financial impact of adding medium-pressure UV is around $3,500 each year. Let's say that again in a different way: The system specifically put in place to lower the pool's operational costs was actually increasing costs every year.
The Low-Pressure Solution
After doing more research, we found there might be a better way. Low-pressure UV has been used as a disinfection method for years, though its benefits haven't been as widely touted and it hasn't been universally adopted.
Additionally, what we found is that there are different "grades" of low-pressure UV. An amalgam, high-intensity lamp puts out all the UV spectrums and intensity necessary to destroy the same pathogens (bacteria, viruses and protozoan cysts) that medium-pressure UV does. It will also destroy "mono-chloramines"—but only mono-chloramines.
It is important to understand that other forms of chloramine: di-chloramine and tri-chloramine, do not form if mono-chloramine is not present.
Now the really good news: Amalgam, high-intensity UV lamps do not destroy the chlorine compound (the stuff we want and need for both disinfection and oxidation in the pool). Further research revealed that low-pressure UV units are less expensive to operate, can often run on single-phase power, and the high-intensity lamps last twice as long (and are less costly to replace when you finally need to).
But… You Knew There Was a But, Right?
There are some caveats. First, (this is important enough to repeat) like any other "outboard" water-treatment system, (filtration, ozone, etc.) all UV systems can only treat water that is passing though the pump room. They are totally dependent upon recirculation and turnover rates. How confident are you that water is being properly recirculated in your pool?
Second, there are many low-pressure UV systems that do not have the spectrum or intensity of UV light we need for mono-chloramines to be destroyed. Just looking for the term "low-pressure UV system" may not be the path to success, you'll need to ask some questions and dig around for details.
Lastly, the chambers in which the UV system is housed should be constructed from polished stainless steel so that the UV light is reflected and refracted around the chamber as the water passes through it. If not, the results may vary significantly. Of course, we should be looking for both UL and NSF listing on all pool mechanical equipment.
UV & Your Pool
If you're looking at UV, you owe it to yourself to do some research. Consider asking other pool operators how their chlorine usage changed after they installed UV systems. The results may surprise even them.
Your decision to go with a low-pressure UV system has the potential to save your facility a bunch of money over other systems, but keep in mind, the main reason for installing UV is for water quality. The main savings is in the cost of chlorine, acid and labor to complete superchorinations (breakpoint chlorination for you CPOs and AFOs). Make sure you are basing your decisions on facts and not just marketing materials.