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December 2009
Water System Design Manual
inactivation data and an applied safety factor of 3. Chapter 246-290 WAC does not specify a
minimum log inactivation requirement of any organism or organism class for groundwater.
In January 2006, EPA finalized LT2ESWTR. While the focus of the LT2ESWTR is on surface
water sources, the rule also established UV dose requirements for viruses. In November 2006,
EPA finalized the Ground Water Rule. Treatment equipment installed to comply with this rule
must
provide 4-log inactivation or removal of viruses. For UV disinfection, that means the
minimum required validated dose is 186 mJ/cm
2
. The reduction equivalent dose for a given UV
reactor will likely be greater because of the uncertainties inherent in the validation of UV
reactors.
Groundwater Sources That are not Required to Disinfect
DOH recommends that installations provide a UV dose of at least 186 mJ/cm
2
and that water
systems meet the same design criteria applied to sources required to disinfect. Water systems that
install UV treatment units that have not been validated will be required to install validated
equipment (or use another DOH-approved disinfectant) if disinfection is required in the future,
and may be required to conduct additional source water sampling for coliform bacteria as
required by the Ground Water Rule.
Surface Water
Surface water treatment regulations prescribe treatment techniques that, either in combination
with filtration or alone, must achieve identified levels of inactivation and/or removal of
pathogens. All surface water systems must provide a minimum of 3-log removal and/or
inactivation of Giardia lamblia and 4-log inactivation and/or removal of viruses, and control of
pathogenic bacteria. These requirements are prescribed in the federal Surface Water Treatment
Rule (40 CFR 141.70 through 141.75), and chapter 246-290 WAC, Part 6. The Interim Enhanced
Surface Water Treatment Rule (IESWTR) and Long Term 1 Enhanced Surface Water Treatment
Rule (LT1ESWTR) impose a 2-log Cryptosporidium removal requirement on water systems that
are required to filter. With the promulgation of LT2ESWTR, EPA formally recognized the
effectiveness of UV for inactivation of Giardia lamblia and Cryptosporidium. The LT2ESWTR
public health protection requirements are in addition to those required by the Surface Water
Treatment Rule, IESWTR, and the LT1ESWTR. The different UV disinfection standards for
disinfection of surface water sources are explained in more detail below.
Limited Alternative to Filtration
To meet the limited alternative to filtration (LAF) standards, the water system must provide
greater
removal or inactivation of pathogens for the surface water source than would be
provided by the combination of chlorination and filtration. Where UV disinfection is used to
meet LAF requirements, DOH has established that a minimum design dose of 40 mJ/cm
2
be
used.