Ultraviolet radiation is a secondary disinfection process, used alongside a primary disinfectant, usually chlorine, in swimming and spa pools. It is recommended by PWTAG, both for its capacity to reduce chloramines and kill microorganisms – including chlorine-resistant Cryptosporidium.
Its use can reduce the chlorine residual levels necessary to keep pool water healthy. It is important that UV is specified and installed properly, advises PWTAG. The UV system should be designed to treat the full water flow through the pool circulation system.
UV systems intended for the control of chloramines as well as micro organisms shall be equipped with medium-pressure lamps (broad spectrum between 200 and 320nm). Low-pressure lamps (254nm only) are biocidal, but do not deal with di and tri-chloramines and are not recommended for general use for pools. They use less power than medium pressure lamps but their lower output means that more lamps are needed. They have a larger footprint.
The system should be designed to achieve a minimum 3-log (99.9%) reduction in the number of infective Cryptosporidium parvum oocysts per pass through the UV system. UV systems should be third-party validated using the US recreational water standard NSF/ANSI 50, or other regulatory standards confirming 3-log reduction in the viability of Cryptosporidium parvum oocysts. The UV equipment should be provided with calibrated UV intensity sensors, which measure the output of all the UV lamps installed in a system. It should be able to display UV dose, expressed in units of energy per unit area. The UV dechloramination dose should be 60mJ/cm2 average, based on the actual circulation flow.
The chamber and all its components should be designed to withstand a maximum operating temperature of 40°C, but also occasional brief temperatures as high as 60°C. UV chambers should be fitted with high purity quartz sleeves/thimbles (which can be doped) to separate the water passing through the chamber from the UV source. The UV system should be designed to permit cleaning of the quartz sleeves/thimbles without mechanical disassembly.
The UV system should be installed post filtration, but before the heat exchanger, pH correction and residual chlorine dosing points. A by-pass should be provided to allow continuous pool operation during maintenance of the UV system. A strainer should be fitted downstream of the UV system to prevent any quartz shards entering the pool in the event of accidental breakage of the quartz sleeve.