Ultraviolet (UV) systems are in use worldwide for effective secondary or supplemental disinfection and chloramine reduction in a range of swimming pool applications. The technology is one of the most sustainable forms of disinfection, as UV light effectively inactivates microorganisms such as Cryptosporidium without the use of chemicals. To explain more about its benefits and parameters, our top UV experts at Evoqua Water Technologies, answer five key questions on best practices for UV disinfection.
What is the Importance of Sizing UV Based on UVT?
When selecting a UV system for aquatics use, the flow rate is primarily the main sizing parameter considered. However, flow is only relevant to the water quality of the individual body of water. UVT is the other piece of the puzzle. UV transmittance (UVT) is a measurement to determine how much UV light passes through a sample of the water compared to distilled water.
Dissolved organics, suspended matter, color, and certain UV light absorbing metals can all absorb UV light and prevent the output from the UV lamps making it all the way through the water in the UV reactor. All UV manufacturers provide sizing sheets that show the maximum flow rate a particular UV reactor model can treat. All of these sizing sheets are based on an assumed UVT% value, however, this value is not necessarily the same from manufacturer to manufacturer.
UVT is an important measurement as the same reactor sized for 500 gpm at 98% UVT, may only be capable of treating 300gpm at 94% UVT. In this instance, sizing the UV system solely on flow rate would leave you with a UV reactor unable to perform at the design criteria. Remember, the UVT% of a pool is not set by the UV manufacturer, it is a measurable water quality parameter that changes from pool to pool. However most well managed pools with reasonable bather loads should have UVT values around 94%.
What is the Difference Between Low and Medium Pressure UV?
Low pressure and medium pressure both refer to the pressure of the gas within the UV lamp. Low pressure UV lamps are monochromatic and generate one usable wavelength of UV-C (254nm) and this type of UV lamp is generally used for disinfection. Medium pressure UV lamps are polychromatic and produce many usable wavelengths along the UV-C spectrum (200-290nm). Medium pressure lamps do more with the available output, in terms of both disinfection potential, as well as photochemical reactions such as destruction of chloramines.
Low pressure lamps are often sold as being more electrically efficient than medium pressure lamps as they are commonly produced in sizes ranging from 50-400w each, whereas medium pressure lamps commonly range from 1000-5000w each. However, efficiency must be looked at across the operation as a whole, as in certain applications it may take many low pressure lamps to equal the disinfection output of just one medium pressure lamp. This can have implications on UV reactor sizing and footprint, consumable volume, and maintenance times associated with the equipment. When considering chloramine removal, medium pressure polychromatic UV lamps are much better able to reduce dichloramines and trichloramines which is why they are preferred for aquatics applications over low pressure offerings.
What is the Importance of UV on Both Indoor and Outdoor Applications?
In aquatics applications, UV is generally used indoors to reduce/control chloramines in pool water. Chloramines can cause ‘red eye’, headaches, unpleasant odors, skin irritation and have been linked to asthma. Swimmers in pools introduce substances to the pool water such as sweat, skin cells and sometimes urine. These contaminants contain high levels of uric acid, urea, ammonia, histidine, and creatinine and can react with free chlorine in the water to form a number of by-products, notably chloramines. The polychromatic emission of medium pressure UV is extremely effective and reliable when it comes to chloramine destruction and changes to water and natatorium atmosphere should be noticeable rapidly.
Chloramines are perceived as a larger issue on indoor pools because patrons can recognize them once in the facility, however, they are just as common outdoors as well. While chloramines tend to dissipate quickly in the air above outdoor pool water, swimmers can still be exposed to chloramines while swimming in outdoor pools.
UV is also used for secondary disinfection for microorganisms such as Cryptosporidium. Cryptosporidium can be treated with chlorine in extremely high doses and long exposures. However, it is generally tolerant to the levels of free chlorine found in pools today. Evoqua’s ETS-UVTM Systems are proven effective in providing a 3-log inactivation (99.9%) of Cryptosporidium in a single pass. Only validated systems can prove inactivation of these microorganisms.
As more regions move towards UV as true secondary disinfection, they will undoubtably only specify equipment that has undergone third party validations that ensure UV systems are capable of the disinfection they claim in accordance with the USEPA Disinfection Guidance Manual.
Do all UV Systems Inactivate Cryptosporidium?
Yes and no. It is proven that UV-C output from both medium and low pressure UV lamps has the potential to damage the DNA of microorganisms in the water and render them non-viable or unable to reproduce. However, that is simply the science of the UV application and does not consider real world variables. While all UV systems have this disinfection output, applying the correct ‘dose’ of it to the water being treated is key to ensuring that the required disinfection is taking place. UV dose is the product of both the total output from the UV lamps and the speed at which the water is moving through the UV system (flow rate).
Comprehensive and accurate UV dose calculations are made possible by testing the UV systems through a wide range of flow and water quality conditions while attempting to inactivate surrogate microorganisms that respond to UV radiation similarly to that of actual Cryptosporidium. This in effect is what takes place when using a third party to independently validate the performance of a UV system. Once this is complete, the validated data is used in UV system controllers to calculate and operate the system to the same parameters that were effective during the validation testing, ensuring proper disinfection.
What Does NSF50 Mean for UV Systems?
NSF50 is a standard that is geared towards swimming pools, spas, and the equipment that is used to operate them. The NSF50 standard addresses many different types of equipment and can have different levels of testing for each type of equipment. To simply say that equipment is NSF50 listed, does not specify to what extent.
UV systems that are NSF50 listed meet quality and material standards that are set for the application and environment that they operate in such as natatoriums, interactive water features, therapy pools, etc. This is the base level of the NSF50 standard for UV that a manufacturer can test against.
NSF50 Annex H ‘Supplemental’ Disinfection Listing
NSF50 Annex H ‘Supplemental’ disinfection listings will include UV systems that have demonstrated the ability to produce a minimum of a 3-log inactivation (99.9%) of the bacteria enterococcus faecium and pseudomonas. The UV system must be capable of this level of disinfection in six passes through the UV reactor.
NSF50 Annex H ‘Secondary’ Disinfection Listing
NSF50 Annex H ‘Secondary’ disinfection listings will include UV systems that have demonstrated the ability to produce a minimum of a 3-log inactivation (99.9%) of Cryptosporidium. The UV system must be capable of this level of disinfection in a single pass through the UV reactor. These two methods of testing in NSF50 are not equivalent. The Secondary disinfection standard is much harder to achieve so responsible operators should inquire with their supplier on what testing has been done to demonstrate their equipment’s ability to inactivate Cryptosporidium in accordance with NSF50 Standards.
Unlike using chemicals for disinfection, UV systems provide no residual disinfection to the water they are treating. Regardless of the level of the NSF50 standard a UV system is tested against, it is commonly used in conjunction with chemical disinfection systems such as chlorine or bromine that are intended to provide residual disinfectant in the body of water.