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UV Biodosimetry
Using MS2 Coliphage
By R.C. Cooper
The use of
ultraviolet light as a means to
disinfect water and wastewater has
gained in popularity because of
limited production of disinfection
byproducts and its effectiveness in
inactivating pathogenic
microorganisms, particularly
Cryptosporidium sp. One of the
important operational parameters in
the design and evaluation of the UV
treatment is the cumulative UV dose
delivered as the water to be treated
flows through the irradiation
(disinfection) process.
The UV dose is a
product of the intensity of the
light at a given distance from the
lamp and the time of exposure. Using
photometric equipment, this dose can
be measured in the treatment process
only at specific points and at fixed
distances from the lamp; which,
because of hydraulic vagaries and
varying distance from the lamp(s),
gives only a limited measure of the
dose that a microorganism might
receive during its passage through
the UV treatment unit. In order to
overcome these limitations
biodosimetry is employed.
Biodosimetry is a method by which an
integrated value for the dose
received during passage through the
UV unit can be estimated using
microorganisms that have known UV
dose response characteristics.
At the
present time the microorganism of
choice for biodosimetry is the MS2
coliphage, a bacterial virus that
infects specific species of
Escherichia coli and responds to
doses of UV in a very consistent
manner. In order to measure the dose
produced by a treatment process the
water to be disinfected is seeded
with a known concentration of MS2
coliphage and passed through the UV
unit. After an appropriate flow time
has elapsed effluent samples are
collected and analyzed for MS2
coliphage levels. The degree of
virus inactivation is calculated as
the difference between the Log10
of the influent and effluent phage
numbers. The dose of UV required to
cause this magnitude of virus
reduction can then be estimated from
an understanding of how this virus
responds (is reduced in numbers) to
varying UV doses.
The dose
response characteristic (dose
response curve) of this virus is
determined by collimated beam assay.
In this analysis a device is used
that produces a beam of UV light of
uniform intensity, i.e. collimated,
on to a solution of known depth
containing the MS2 coliphage at a
predetermined concentration. Being
collimated the intensity of the UV
beam is uniform across the surface
of the irradiated solution. Knowing
the intensity of the beam, the UV
absorbency characteristic of the
water matrix and depth of the
solution the dose (intensity x time)
delivered after any given time of
exposure can be calculated. The
numbers of MS2 viruses surviving
after measured exposure time periods
are determined and a dose response
curve is generated such as is shown
in Figure1. As an example, using the
dose response curve, if a 4 log
reduction in MS2 numbers is seen
through the unit it would indicate
that a dose of 100mJ/cm2
was delivered by the UV treatment
system. This value is also referred
to as the RED for Reduction
Equivalent Dose. The
data from which this Figure was
generated were the results of dose
response values (181 data points)
collected from 45 different batches
of MS2 produced by BioVir
Laboratories. The curve displayed is
the result of a linear regression
analysis of the data. The R2
statistic for the curve "fit"
is 0.926 which is an indication of
the uniformity of the MS2 response
to UV254 doses.
BioVir can produce
multi Liter volumes of MS2 coliphage
at concentrations of 1 x 1011
plaque forming units (Pfu) per mL.
This concentration and volume of
phage allows for seeding UV systems
in water reclamation plants with
capacities of over 1mgd with phage of
known UV sensitivity. Since these
viruses have limited if any
environmental impact and are not
infectious to humans or animals they
are safe to use in these
applications.
Phage
Bacteriophage
(pdf.)
NWRI UV Guidelines
EPA
Method 1601/1602
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