Manufacturers of swimming pool Ultraviolet (UV) disinfection systems often disagree on which type of UV system is best for a given swimming pool. Specifically, the marketplace has conflicting information and opinions about low pressure UV vs. medium pressure UV. So let's discuss both.
This article is a continuation of our comparison of secondary disinfectant/oxidation systems for swimming pools. UV is most often compared against Ozone, because both are effective secondary disinfectants for water.
We know that both types of UV can help water chemistry, yet both types of UV are limited when it comes to improving indoor air quality. They are both point-of-contact systems without residual sanitizing ability. But to be fair, every secondary system has its drawbacks. So let's get started by comparing the two types of UV systems.
Related: Pool Chemistry Resources
Low Pressure vs. Medium Pressure
Pressure, in this context, refers to the pressure of gases inside the UV light bulb. It has nothing to do with flow rates or water pressure that moves through the UV chamber. The higher the pressure, the more UV radiation can be released into the water. Both UV systems inactivate pathogens by disrupting the DNA of the cells, preventing their reproduction, and often killing the cell itself (according to some sources). And according to this source, both low and medium pressure UV kill bacteria, viruses, and protozoan cysts in a single pass. That's good to know, beacuse it gives us a baseline for comparison: anything beyond that basic sanitization level.
Low Pressure UV
Because low pressure UV bulbs emit less energy, they produce less heat. This allows low pressure systems to be a better fit for intermittent flow, such as residential pools that do not run 24/7 circulation. Less heat means these systems–like the one pictured–can be in a plastic chamber. There are low pressure systems that are metal too, but not all of them have to be. It depends on the size of the unit and the flow rate.
Beyond killing germs and viruses in a single pass, low pressure UV can destroy mono-chloramine (NH2Cl) in one pass, through a process called 'photolysis', or 'photodegradation'. If monochloramine is destroyed, that monochloramine cannot progress and become dichloramine, or trichloramine after that. So low pressure UV manufacturers are proud to say they destroy monochloramine–which is true, but misleading.
UV is a point-of-contact system without a circulating residual, as mentioned earlier. Because of this physical limitation, UV cannot prevent all monochloramine from becoming dichloramine or trichloramine. These reactions can occur out in the pool itself, prior to ever circulating through the UV chamber. Water does not 'wait its turn' to be recirculated and pass through the pump room, filter and UV. Water circulates, gets treated, and mixes back in with the rest of the pool.
So while it is true that monochloramines are destroyed by low pressure UV, it is not true that the pool will have no dichloramine or trichloramine produced. These pools can still have indoor air quality problems. That being said, low pressure UV has been shown in at least one study to reduce combined chlorine levels, as pictured in this chart from this source:
Here again, this case study shows an improvement, but is misleading. If your swimming pool has 4.0 ppm or more of combined chlorine, you've got a major problem with nitrogen compounds contaminating your water. There is no excuse for levels this high in the first place. If your numbers are this high, from our experience, it is more than just swimmers peeing in the pool (which they will). This is usually from using ammonia-based cleaning products that get washed into the gutters or pool. To have combined chlorine this high, the ammonia has to be coming from somewhere.
Advantages of Low Pressure UV
For the sake of comparison between low and medium pressure UV, there are some advantages to low pressure. First and foremost is cost. Low pressure UV costs significantly less than medium pressure systems, both up front and ongoing. The lamps last longer, they use less energy, and require less frequent replacement of bulbs. All of those contribute to a much more affordable system compared to medium pressure UV.
Another advantage is that low pressure UV can better handle what manufacturers call "intermittent flow", as opposed to constant flow, like medium pressure needs. This means a few things, but foremost among them is less risk of overheating and replacement bulbs for low pressure UV. Chalk that up as more long-term cost savings.
The final advantage we will cover here is that low pressure UV does not have the wavelength of UV light that destroys free chlorine–or more specifically, Hypochlorous Acid (HOCl).
As you can see from that graphic, at 295nm, HOCl gets destroyed by medium pressure UV, but not low pressure UV.
Disadvantages of Low Pressure UV
While it can destroy monochloramine, low pressure does not destroy dichloramine or trichloramine, as discussed earlier. It also has a lower energy output, so if more power is necessary, it could take up a larger footprint in the pump room to get enough UV light to do the job. Another disadvantage is against sunscreen, a staple of outdoor pool contamination. Sunscreen blocks UV light by design, so naturally it can interfere with the efficacy of a UV system–especially if it builds up on the glass sleeve around the bulb itself. UV can only inactivate what its light actually penetrates, and if sunscreen is blocking that light, it stands to reason that less water is being exposed to the UV.
Medium Pressure UV
Medium Pressure UV systems are robust and made of metal, not plastic
While low pressure UV is more popular on residential pools, medium pressure UV dominates in the commercial pool market–especially indoor swimming pools. In an effort to remain brand-neutral, we are cautious in which sources we cite, because most of the information on medium pressure UV comes from manufacturers or dealers who sell it. And that's understandable, it's just important to recognize there are biases. So we are trying to only cite sources that are informational and not product driven.
Advantages of Medium Pressure UV
The main advancement that medium pressure offers over low pressure is the amount of energy output that a single lamp can put into the water. This is evident in how the units are built. Low pressure UV systems are often plastic chambers with a UV light inside, whereas medium pressure UV is thick stainless steel and robust in its construction.
Medium pressure has much more power, and therefore can inactivate more things in the same amount of water, faster. According to this source, this allows the system to have a smaller physical footprint with less lamps, whereas a low pressure system may need multiple lamps to equal the same rate of disinfection of medium pressure.
But the spectrum of UV light is also broader in what it can inactivate and disinfect in a single pass. Medium pressure UV can kill cryposporidium, giardia, and other dangerous diseases with ease. Here is a chart from that same source:
Source: Aquionics UV
The chart above does not show it, but the variation in wavelengths that medium pressure UV can produce destroys many things beyond just germs. As mentioned in the beginning of this article, both low and medium pressure systems inactivate most germs, viruses and other disease-causing pathogens. So this is talking about things beyond common germs. Things like chloramines and other disinfection byproducts (DBPs).
While low pressure UV can destroy only monochloramine, medium pressure destroys dichloramine and trichloramine (at least, while it's still in the water). It can also destroy trihalomethanes and other disinfection byproducts in a single pass. This means UV helps reduce combined chlorine.
Disadvantages of Medium Pressure UV
Medium pressure systems require constant flow, as the bulbs release a lot of heat. This usually means the bulbs have a shorter useful lifespan. Cooling is necessary, and if water is not constantly flowing, it can shorten the lifespan of the bulbs even more.
High intensity, broad-spectrum UV systems (also known as medium-pressure UV) reduce both free chlorine and combined chlorine compounds (chloramines) into easily removed byproducts.
Between the wavelengths 180 and 400 nm, UV light produces photochemical reactions that dissociate free chlorine to form hydrochloric acid. The peak wavelengths for dissociation of free chlorine range from 180 to 200 nm, while the peak wavelengths for dissociation of chloramines (mono-, di- and tri-chloramine) range from 245 to 365 nm.
And while destroying free chlorine and its byproducts can be beneficial for drinking water, we need free chlorine in our swimming pools as a primary sanitizer. To us, that seems like it's a conflicting strategy. Direct sunlight, of course, also destroys free chlorine. This is why cyanuric acid (CYA) is used in outdoor pools, but not indoor pools.
But chlorine can be replenished, and most commercial pools that have medium pressure UV also have chemical automation and chlorine feeders.
Perhaps a more important disadvantage to medium pressure UV is its cost. These systems are not cheap to purchase, and they are not cheap to maintain. Bulbs need to be replaced every year or so (depending on usage hours), and the bulbs themselves are expensive. And that's not the only cost associated with it. Another major cost to be considered is energy consumption. These systems take a lot of electricity to operate, and it's a constant. Operators we have spoken to have been shocked at just how much their power bills have gone up just from installing medium pressure UV.
Both low and medium pressure UV systems help water quality. They supplement chlorine as a secondary disinfectant, and both are very effective at killing pathogens like crypto and giardia. Medium pressure UV has an advantage in terms of the range of things it can deactivate in a single pass, such as dichloramine and trichloramine, whereas low pressure only destroys monochloramine. But that's a double edged sword, because low pressure tends to not destroy HOCl, which is the killing form of free chlorine–and we need free chlorine.
We remain brand neutral and just wanted this article to inform you of the pros and cons of both UV systems. They are both beneficial for commercial pools. The biggest differences are with costs. Low pressure is significantly less cost, both short and long term. Not only equipment and maintenance wise, but with energy consumption too. We do not have a strong opinion of one system versus the other, because as we said at the beginning, both help water quality, yet both are limited in their impact on air quality.
No matter how effective a UV system is, it is still a point-of-contact system, and is at the mercy of the circulation rate of the pool. It can only disinfect the water it sees. Meanwhile, chlorine is circulating throughout as the primary residual sanitizer, and it is creating DBPs in the process. Those DBPs, like chloramines, will eventually go airborne, and UV cannot do a darn thing about it. Sure, they can theoretically reduce the amount of chloramine in the water, but UV does nothing for the chloramines that go airborne before circulating through the UV chamber. It can't.
So when we talk about indoor air quality, UV plays a very small role, but it does help chlorine with sanitization, which makes water safer.