Part I: UV Disinfection
Designing an effective system that works on various water sources and applications
Designing UV reactors to be used in a water or wastewater applications is rather complicated. Mathematical or theoretical dose rates can be calculated, but final product evaluation needs to be confirmed, tested and verified by laboratories. Various strains of germs, bacteria and pathogens react differently to UV radiation. Some require strong dose rates for longer periods of time and others only need a short burst of UV to disrupt the proliferation process. An assumed calculated dose rate may be viewed as more than enough to kill certain pathogens, but what happens if the fluence rate is altered by turbidity or other contaminants in the water? What happens if the protective sleeve, whether it is fluoropolymers or quartz, gets scaled and blocks the UV from entering the stream of water? UV systems, just like reverse osmosis (RO) and de-ionization, need to have pretreatment on many water sources. Iron and hardness minerals can cause premature failure of a UV system. All the varying parameters must be analyzed in order to properly design a system that will work effectively on a multitude of water sources and applications, whether the source water is fresh or waste. This month’s issue of Water Quality Products features Part I of this article, which will focus on fluence rates, design considerations and the effect different fluent rates have on various microorganisms.
Defining UV Disinfection
First, without trying to be redundant, what is Ultraviolet Light?
UV is the invisible wavelength of the light spectrum that falls between the 100-400 nanometer (nm) range. These can be broken down further into sub-ranges with the first being Vacuum Ultraviolet (VUV) at 100-200 nm, the next three ranges are as follows: UVC (200-280 nm), UVB (280-315 nm) and UVA (315-400 nm). The UVA range causes the change in the skin that leads to sun tanning. The UVB range can cause sunburning and is known to eventually induce cancer. The UVC range is known as the germicidal range and extremely dangerous since it is absorbed by proteins RNA and DNA, and can cause cell mutations and/or cell death. The UVC range is very effective in inactivating bacteria and viruses. The VUV range is absorbed by almost all substances (including water and air) and can only be transmitted in a vacuum.
The UVC or germicidal range is used in UV disinfection equipment design. The range is 200-280 nm, but 254 nm is the peak effective wavelength. The UV energy is absorbed by the DNA of the microorganism and is inactivated or destroyed, thus the reproduction of the organism eliminated.
Fluence rate is the preferred term over “UV Dose,” because the meaning of the term Dose is used to imply total absorbed energy, for example, the UV Dose to induce sunburn on the skin. Only a small fraction of the radiant energy is absorbed (about 1%), but the fluence rate represents the radiant energy induced on the micro-organisms. This term is expressed in mWs/cm2 or the preferred unit of mJ/cm2. UV reactors are measured and the fluence rates are determined and tested at levels that kill germs at different rates. NSF Standards rate units in several classes that protect the consumer and ensure proper water quality. A Class B certification requires a dose rate of 16 mJ/cm2 and a Class A certification would require a 40 mJ/cm2 dose or fluence rate. The Fluence Rate (UV Dose) = Intensity of the UV Bulb x (% transmission of protective cover) x Contact Time.
Proper designs will inactivate organisms to render them from photoreactivation. If the DNA is not completely disrupted, these organisms can reactivate or repair themselves and start to proliferate. Fluence rates must be high enough to keep all organisms that the reactor is designed to control from replicating downstream. Because UV does not add anything to the water these issues must be considered when designing a system.
Scaling issues of the protective sleeve must be addressed. Water sources have contaminants that can cause degradation of the UV intensity because the sleeves become fouled with debris and reduce UV radiance. These same contaminants can cause shadowing of the microorganisms, which prevent proper fluence rates from entering the stream for effective inactivation. Pre-treatment is always recommended on water with high hardness, iron or turbidity, or other contaminants that can cause premature failure. Softened water or even RO should be used prior to the UV system to ensure lower maintenance costs and longer run times.
UV sensors should be incorporated in order to alert the user that the system is not producing the effective amount of UV. These units should not just be light-out-type sensors, but sensors that measure UV intensity. This would warn the consumer that their system was compromised and should be serviced. The reactor should
use a bulb that can produce at least a
30mJ/cm2 fluence rate after one year of continuous use. Another consideration is warm up time. If a unit is designed to have the bulb shut off when not in use, the bulb should be allowed ample warm up to become effective. Some bulbs are not as effective at lower temperatures.
Flow rates are always a consideration in the design process. Too fast of a velocity through the UV reactor can render the system ineffective. Pressures and pipe size play a factor in this parameter. Be sure that the proper flow rate is calculated and measured in order to deliver the desired fluence rate to the water stream.
Consider Some Germs
Table 1 (page 13) shows how the
different fluence rates affect various bacteria, molds, protozoa, viruses and yeasts, therefore, one can understand how vigorous testing must be conducted to properly bring a UV system to market. Calculated fluence rates can be a basis for design, but testing is the only method to reveal real application success for inactivation. Some rates are very high and these microorganisms would never be candidates for a UV application, but for the most part, UV is a very acceptable treatment method.