Health and sanitation
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A dose of 30mJ/cm 2 achieved 99.999% (5-log) reduction in 20
minutes in a recirculating model premise plumbing system under
three different test conditions (non-turbid water at 25 degrees C
(77 degrees F); turbid water at 25 degrees C; and non-turbid water
at 43 degrees C (109.4 degrees F)). However, viable numbers of
L. pneumophila remained in the treated water despite six hours of
continuous UV light exposure. UV irradiation was not affected by
turbid conditions or increased temperature. (Muraca et al., 1987).
Usually, there are limited opportunities for exposure to light for
water treated and held in premise plumbing systems. However, if
there is a significant opportunity for light repair (repair of UV-
induced DNA damage using photo-reactivating light), such as
in water used in tubs, pools and baths, a higher UV dose should
be considered. At a UV dose adequate to achieve 99.9% (3-log)
reduction of L. pneumophila, subsequent exposure to fluorescent
light for one hour resulted in only a 68% (0.5-log) reduction
following initial inactivation by low pressure (LP) UV lamps and only
60% (0.4-log) reduction following inactivation by medium pressure
(MP) UV lamps (Oguma et al., 2004). Similar significant light repair
of legionella has been observed by others (Knudson, 1985).
Potential water quality issues
UV disinfection does not produce a disinfectant residual (USEPA,
2007). Also, when UV disinfection is applied to waters containing
a disinfectant residual, the residual may be diminished following
treatment with UV (USEPA, 2006c). Therefore, water treated
using only UV disinfection may be, in some cases, susceptible
to contamination at downstream points. More than one type of
disinfection or other control measure may be needed to protect the
treated water downstream of UV disinfection, between the UV lamp
and the taps and other water outlets (for example showerheads).
At UV doses typically used in drinking water, UV disinfection does
not support the formation of regulated DBPs (USEPA, 2006c). In
addition, UV disinfection does not change the pH or treated water
quality in such a way as to make it more corrosive to premise
plumbing (USEPA, 2006c).
Mercury can be released into the treated water when a UV lamp
breaks (Wright et al., 2012). The amount of mercury that could
potentially enter the water depends on the type of lamp and
operation. Vapor phase mercury can dissolve into solution and be
discharged downstream whereas liquid phase or amalgam mercury
would tend to settle in the UV reactor. The author recommends
developing a mercury mitigation plan (Wright et al., 2012). PA
UV is only effective at inactivating
legionella in the water that flows
through the UV reactor.
July 2017 Volume 23 I Number 5