Written by: Michael Annett, Sales / Business Manager UV Systems, Pentair Aquatic Eco-Systems
Originally published in International Aquafeed Magazine
View the full publicitation here: https://issuu.com/international_aquafeed/docs/iaf1604_w1/50
Pentair Aquatic Eco-Systems (PAES) is a leading manufacturer of UV water treatment systems for use in a range of industries. PAES has particular expertise in designing and supporting UV systems in aquaculture field applications. In addition to new installations, PAES also provides consulting and support for UV systems that are installed in existing aquaculture facilities. UV treatment has been used successfully for approximately six decades to treat and sanitize water in many critical applications. When designed, installed, and maintained properly, UV systems are extremely robust, reliable and effective.
In this article, we will describe the array of UV system configurations along with their associated lamp technologies, to serve as a high-level guide to important design and operating considerations for UV systems.
UV Process discussion
Ultraviolet or UV energy is located in the electromagnetic spectrum with a wavelength shorter than that of visible light, and longer than x-rays. When UV systems are deployed to treat water, the reactions are instantaneous, taking only a few seconds to occur, without the creation of disinfection by-products. Chemicals are not required, and the associated hazards of chemical handling and storage are not present. UV treatment does not alter color, odor, taste or pH. As such, UV processes are environmentally safe and are considered ecologically-responsible technology. The science behind UV technology is well-established; and its efficacy is well-proven. UV systems are trusted in a wide range of industries including drinking water production, aquaculture farming, and other applications.
UV light is comprised of electromagnetic radiation of wavelengths ranging from 100nm to 400nm.
- UV-A (long wave UV): 315-400nm
- UV-B (middle wave UV): 280-315nm
- UV-C (short wave UV): 200-280nm
- Vacuum UV: 100-200nm
When a UV system is used to treat water, UV light at the 254 nm wavelength penetrates the cell wall of microorganisms that are present in the treatment water. The amount of UV delivered to the organism is called the dose. The UV energy permanently alters the DNA structure of the microorganism in a process called thymine dimerization. The microorganism is not destroyed, rather it is “inactivated” and rendered unable to reproduce or infect.
Another useful way that UV can be utilized is in the destruction of oxidants that are present in post-treatment water in the form of residuals. Ozone (and some other oxidant chemicals) can be eliminated by the application of UVC radiation. UV energy in the wavelength of 254 nm has the ability to catalyze ozone into harmless oxygen and water. UV is very effective at destroying ozone (O3). O3 molecules absorb 254 nm UV radiation and this absorption causes decomposition of the O3 molecule. O3 concentrations at less than 0.5 mg/l can be eliminated with a UV dose of 60 mJ/cm2. 1.0 mg/l ozone concentrations require a minimum UV dose of 90 mJ/cm² for complete destruction.
UV Reactor Types
UV reactors for processing fluids are generally either enclosed or oriented in channels without an enclosure. UV reactors can be in-line in a pressurized piping system or fed by gravity without requiring pumps.
Enclosed reactors can be installed vertically or horizontally, in an “L” shape or “U” shape, or with the inlet and outlet on opposing sides. They are cylindrical and can be manufactured from stainless steel, various plastics and polymers such as PVC, HDPE and others.
UV Lamp Technology
UV lamps used in disinfection and photolysis applications are categorized into two basic types: low pressure and medium pressure. Both types of UV lamps utilize mercury which becomes vaporized when energized by the application of electricity. As a result, photons are generated in the UVC spectrum. The following are characteristics of both lamp types:
- Monochromatic Emission; 254 nm
- UV efficiency 30-35%, as portion of Input Power
- Medium Power UV-C Output (Input Power up to 320W to 500W)
- Long lifetime (8000-18000 hours)
- <5 mg mercury content on
- Mercury vapor pressure = 0.007 Torr
- Operating temperature 40-140 C
- Polychromatic Emission 220-280nm
- UV efficiency 11%- 13%, as portion of Input Power
- High Power UV-C Output (Input Power 3kw to 30kw or more)
- Medium lifetime (6000-8000 hours)
- Up to 400mg mercury
- Mercury vapor pressure 100 – 10,000 Torr
- Operating temperature 600-900 C (Have Marketing insert a Sita MP photo)
Low pressure lamp systems are more energy-efficient than medium pressure systems, but are much larger due to the lamp size and quantity required to apply the same dose. Medium pressure systems, however, generate much more heat, are more expensive to operate and have lower effective lamp life. Medium pressure UV systems have the advantage of smaller overall size and ease of maintenance due to the reduced numbers of lamps. Both systems are commercially-viable and application appropriate, and effective in terms of disinfection and photolysis performance.
UV Design Considerations:
There are several critical parameters which apply when designing or sizing a UV system.
- UV transmittance (or UVC light absorption properties) of the fluid being treated.
- The dose required to inactivate the desired target pathogen.
- System flow rate.
- Fluid temperature & salinity.
- Space requirement of the installation location.
- Power consumption.
- Heat load imparted on the process.
- Seasonal fluctuations in water quality.
All of these conditions should be considered prior to choosing a UV design. The most critical aspect of properly sizing a UV system is the UV transmittance (UVT) of the fluid being treated. Accurate knowledge of UVT is essential for ensuring that a system will perform properly after it has been installed. Unfortunately, UVT is often overlooked or incorrectly estimated; which may result in poor UV system performance. Always consult a qualified expert or use a good-quality UVT meter to assure that UVT is properly taken into consideration.
The amount of UVC light that is absorbed by the fluid being processed dramatically affects the amount of energy available to apply the germicidal dose required to inactive the target pathogen, or catalyze the target compound such as ozone. A UVT analyzer is used to measure the 254 nm UVC light absorbed across a 1 CM cuvette, providing a UVT measurement. UVT analyzers are easily obtained and are inexpensive, valuable tools. Table 1 illustrates the importance of providing an accurate UVT reading by showing the additional energy required at various UVT values:
Ninety percent UVT is a common UVT assumption for many UV applications. At 90% UVT, 36% of the UVC generating capacity is absorbed by the water being treated. If the UVT drops to 80% there is a corresponding loss of almost 57% of the UVC dose being absorbed. It is therefore essential to test the UVT at various times of year to ensure that the required dose is available at all times.
Water Quality Issues
Water quality affects the actual UV dose in several important ways. First, turbidity and suspended matter physically shields the pathogen from the UVC dose, rendering the UV process ineffective. As a general rule of thumb, pretreatment to UV systems is generally recommended, and defined as filtration down to 40 microns and 3 NTU turbidity.
Factors which have an impact upon UVT are:
- Shielding – (Turbidity)
- Chelating Agents
- Halogen Stabilizers
- Water Additives/Conditioners
- Minerals especially Iron and Manganese
In UV systems, the lamps are enclosed in quartz sleeves to allow UVC light transmittance, while protecting the lamps from treatment water. Minerals, hardness and organic matter can form on the outside of the sleeves, reducing the amount of UVC light transmitted to the fluid, thus reducing the dose. The UV system can be rendered ineffective if enough build-up occurs. Wiping systems can be employed to keep the sleeves clean. Periodic manual cleaning of the sleeves can also be done if wipers are not used. If a drop in UV intensity is noticed, cleaning is likely required.
End of Useful Lamp Life (EOLL)
Low pressure and medium pressure lamps have a useful lamp life. Lamp performance degrades over time as the mercury degrades within the lamp. The lamp degradation process is unavoidable and eventually requires the lamps to be replaced. Lamps have life ratings which vary by type and manufacturer. Depending on the lamp type and manufacturer, the degradation loss over the lamp’s service life varies between 15% and 40%. To ensure the design dose is applied over the complete life of the lamp, it is best to initially size the lamps for the 15 to 40% degradation loss. In this way, the lamps will actually produce a higher dose when they are new, and will degrade to the design level until the end of lamp life, ensuring correct dose for the entire lamp life period.
Pathogen Dose Response
Pathogens respond to UV differently and they are inactivated at a variety of dose levels. When designing for multiple pathogens, it is critical to design for the pathogen with the highest resistance to UVC. The amount of pathogen being inactivated is referred to as log reduction.
Log-1 = 90%, Log-2 = 99%, Log-3 = 99.9%, Log-4 = 99.99%, Log-5 = 99.999% reduction
Table 2 the dose required for various pathogens. If log reduction is not indicated, this is the generally accepted dose value for inactivation.
UVC dose is calculated using an “Ideal Reactor” referred to as a collimated beam. A collimated beam is laboratory apparatus that is used to deliver very precise UV doses to samples of fluid containing the target organism or chemical that is to be inactivated or decomposed. The UV beam is comprised of uniform UV light that is easily quantified. A petri dish is used that contains a thin layer of fluid with the target organism or chemical distributed within. This is intended to represent perfect hydraulic design. The dose required for log reduction is considered ‘Ideal’ and is the base comparison for reactor performance (efficiency) and used to determine inactivation rates of organisms or chemicals.
UV systems are effective, economical, environmentally responsible, and low maintenance tools for disinfection and photo catalysis for aquaculture facilities. Attention to design and actual operating conditions are critical to reliable performance. Proper dose, UVT and EOLL conditions are frequently overlooked or designed with an unrealistic UVT causing poor performance. It is therefore essential to consult with a reputable, experienced UV provider prior to installing a new system or retrofitting an existing system to ensure satisfactory consistent UV performance. UVT testing routinely is inexpensive and ensures that the system operates as intended; systems can be designed to pace the lamp output based on flow and UVT in order to obtain lower power consumption on both lamp types.