Ultraviolet disinfection can keep your water safe from any pathogens
Ultraviolet disinfection can keep your water safe from pathogens that might make you sick from a wide number of gastro-intestinal diseases. But if you do not properly pretreat your water before putting it through a UV reactor, you may be wasting your time. As we like to say in this industry, ‘It takes good water to make great water.’
What’s in the cup?
Now remember that the topic of the day is how to pretreat your water so that you can effectively disinfect with UV. We need to be concerned about many of the potential contaminants that could be in your water, but for now, let’s just focus on what can impede disinfection.
Water, the most amazing substance in the world, can contain many, many things. Because of its polar nature, it is an incredible solvent – in fact it is often referred to as the universal solvent because of the large number of things it can dissolve. Chances are, if water flows through or over something, it will pick up some foreign molecule. For this reason, we often say that there is no such thing as pure water.
In addition to the micro-organisms (that will eventually be deactivated by the UV), water contains a number of particles that will interfere with the disinfection process: particulate matter, dissolved minerals, and organic molecules called tannins.
Who cares what’s in the water?
The reason that pretreatment equipment is necessary is because we need to maximize the amount of ultraviolet light that penetrates the water so that the UV intensity is as high as possible. Think about getting a suntan – you get a better suntan in the Mediterranean than you do in Scandinavia because the sun’s energy is more intense in southern climates. Similarly, more intense UV light leads to a higher UV dose and safer water.
This schematic indicates that the UV light travels from the lamp, through the quartz sleeve and penetrates the entire water sample.
Filter out the grit
Water will contain particulate matter such as dirt, rust, or sediment. This must be filtered out so that particles do not shield potential pathogens from the UV. Back to the suntan analogy, the particulate matter acts like a barrier or shadow, protecting the pathogen from getting a good dose of UV light.
Best practice is to have a 5 micron sediment filter installed before every UV system. 5 microns is smaller than the smallest particle that the human eye can detect, but then again, micro-organisms that can make you sick are also very small.
Avoid sleeve fouling
Dissolved minerals will pass right through sediment filters, but can precipitate out on warm surfaces. Iron and hardness minerals such as calcium and magnesium are especially common culprits of quartz sleeve fouling.
An iron deposit on a quartz sleeve will completely block UV, leaving the water untreated.
Just because you cannot see it …
Finally, organic molecules called tannins can also interfere with UV disinfection. Tannins absorb the UV light, decreasing the effective ultraviolet transmittance (UVT) of the water. This means that the UV energy will be decreased and the light will not be able to penetrate the water.
The tricky thing about tannins is that they easily pass through sediment filters and cannot necessarily be seen in the water. They are like suntan lotion – you can’t see it once it is applied – but it will limit the UV intensity. Tannins must also be removed from the water before the water is passed through the UV reactor.
You are not going to get a good suntan if you slather yourself in sunscreen, sit under a big umbrella which happens to be in a shadow. Similarly, if you want to optimize the effectiveness of your residential ultraviolet light disinfection system, make sure that all of the proper pretreatment equipment is in place.