Electromagnetic radiation (light) covers an enormous range of wavelengths and energies, from very weak but long radio waves many miles in length to extremely powerful X-rays and gamma-rays with wavelengths only tiny fractions of a micron in size. The size of the wave is inversely proportional to the energy it carries, which means that light with the smaller wavelengths can do more damage. The visible portion of the spectrum has wavelengths that range from about 0.40 um or 400 nm for violet light to about 0.77 um or 770 nm for red light. Light with wavelengths shorter than violet (but greater energy) is ultra-violet, and light with wavelengths longer than red (but less energy) is infra-red. Thus, infra-red light is simply “heat,” while visible light can give you a suntan, and UV light can cause terrible bums and blindness.
It happens that UV light with a wavelength near 0.254 um or 254 nm is able to produce resonance effects in the DNA of living cells which cause breaks and kinks, leading to cell death. Therefore, UV irradiation is useful as a method of disinfection for water. The energy;s measured in watts (micro-watts) per cm2, and the total dosage is the wattage multiplied by the time: uWsec/cm2. Thus, a standard UV bulb that delivers 3800 uW/cm2 will produce a dosage of 38,000 uWsec/cm2 after ten seconds of exposure. That is the minimum dosage required to meet the NSF International standard for Class A “purification” of raw waters that may contain pathogenic bacteria and viruses. A lesser standard of 16,000 uWsec/cm2 can be NSF-Certified for controlling bacterial regrowth in the pipes-of systems in which the water has already been disinfected, and only non-pathogenic organisms are present. Standard UV systems are not powerful enough to kill protozoan cysts and oocysts and the larger parasites, so these still require
Physical removal. New pulsed – UV systems may prove to be effective against cysts, but they are not yet approved by any heath regulatory agency.
Advantages of UV treatment:
- Treatment is essentially instantaneous. It is not difficult to provide a 10-second contact time (an exposure vessel of less than a liter at a flow rate of 1 gallon/min. (3.79 L/min)) with modest resources.
- It’s all electrical, with no moving parts to break down, so the system can be relatively simple in design and maintenance.
- Because it’s all electrical, fail-safe measures are relatively easy to build in.
- Water clarity is extremely important, but fine-filtration to remove turbidity will also remove any protozoan cysts and larger parasites that would not be killed.
Disadvantages of UV treatment:
- There is no residual activity to protect the water against subsequent contamination, so the overall system design may be complicated by the need to place the UV last.
- Routine maintenance to clean the optical surfaces is mandatory; the fail-safe system should turn the system off if/when they get dirty.
- Special meters for monitoring the wavelength and intensity of the UV light are very costly, so complete systems cannot in expensive.
- Treated water should be kept in the dark for 30 minutes, because ordinary sunlight has the ability to activate repair enzymes in many bacteria, and as many of 2/3 of those previously killed may be revived after only a moment’s exposure.
- There is a special design problem with small systems intended for intermittent use: cold lamps require a warm-up to achieve peak efficiency, but turning the system on and off repeatedly damages the lamp, and leaving it on all the time heats the water and stimulates the growth of the few organisms that survive.
- UV treatment must not be used in recirculating systems because that can produce a “super-strain” of UV-resistant bacteria that might be dangerous.
- UV light is damaging to many plastics, so special shielding may be important.