Ultraviolet light is a very popular method of disinfecting water. UV radiation is part of the electromagnetic spectrum that has incredible properties for the killing of microscopic organisms. While there are varying degrees UV resistance within microscopic organisms, not one has yet been able to develop a total resistance. Because UV disinfection systems are not chemical or biological they have an extremely long shelf life.
There is a wide variety of ultraviolet disinfection systems that range from the size of a pen to large banks of meter long light bulbs and many options in between. UV systems tend to be very simple to install and operate and UV leaves nothing behind and there are no disinfection by-products from its application. In fact ultraviolet radiation can break down some potentially harmful chemicals like chlorine and chloramine compounds.
The limitations of UV disinfection are; distance, time, turbidity and electricity.
Proximity is critical for UV disinfection, the water needs to be very close to the UV light source. The farther away the water is the more radiation is absorbed by the water. Meaning that with increasing distance you get weakening disinfection. Proximity becomes even more critical in hard water. Hard water sources leave a white chalky residue of calcium carbonate which covers the UV light bulb, making the radiation emitted significantly weaker.
Time is another significant limitation of UV disinfection. The amount of time pathogens spend in the UV greatly affects whether or not the pathogen is neutralised. This is similar to how people get worse sunburns the longer they are exposed to the sun. Time is directly related to the flow of the water, if the flow is too much, the water will not spend enough time exposed to the radiation and will not be disinfected. Slow moving water or even static water is best.
The efficiency of UV disinfection is greatly reduced by turbidity. Turbidity physically shields the organisms from the UV light. Exactly the way a beach umbrella shades people from the sun. This is called line of sight disinfection. There is no disinfection in the shadows when using UV radiation.
Electricity is another limitation of UV disinfection systems. They are limited in two ways by electricity. First by the fact that they are quite literally light bulbs placed underwater and secondly by fluctuations in the electrical source cause fluctuations in the UV radiation field emitted from the bulbs. Both these problems are easily overcome. By sealing the system in clear waterproof chambers can effectively keep the system safe from the water. Fluctuations in the electrical source can be minimized through proper system design and using fresh/charged batteries in battery powered systems.
Portability is a mixed blessing with UV disinfection systems. Smaller, pen-like devices are easy to transport, but are significantly less powerful. That means they need to be used on slower moving/still water and used for longer than larger UV systems. Another mixed blessing of ultraviolet disifection is the fact that there is no disinfection residual left in the water. Not having a disinfection residual is great if you are drinking the water immediately, otherwise recontamination can occur very quickly after the UV lamps are shut off. UV is not enough if you plan on storing the water for a long period of time.
Recirculating the water to be disinfected a second or third time will greatly increase the chances of proper disinfection. Remember that disinfection whether by UV or chlorine or any other method is one of the final stages of water treatment. Forgetting to filter the water first will make disinfection significantly more difficult. Regardless of the size of the of the system used, ultraviolet radiation can be used to supplement any water treatment process.