By Christian Williamson, Ph.D. and Adam Festger,M.S.
The pilot facility at CFB Valcartier.
Even in water-rich Canada, there
is a pressing need to preserve
the quality of available water
resources. UV-oxidation (UV
combined with hydrogen peroxide) is
gaining in popularity as a multi-functional
part of a water treatment process
that can provide both disinfection and
treatment of chemical contaminants.
To achieve high-quality
water, many applications
require a variety of treatment
steps. This is true of
municipal, surface water
applications, but it is also
true of groundwater remediation
applications where a
variety of contaminants that
require different treatment
technologies are often present.
For example, some
water providers in Southern
California use a treatment
train that includes air stripping
for volatile organic
compounds, biological treatment
for the removal of perchlorate
and other ionic compounds,
and UV for the treatment of N-nitrosodimethylamine.
In addition, the need to disinfect
groundwater is underscored by the
United States Environmental Protection
Agency's proposed Groundwater Rule.
As stated by the EPA: "Although
ground water has historically been
thought to be free of microbial contamination,
recent research indicates that
some ground waters are a source of
waterborne disease."
The effects of this microbial contamination
can be especially serious
for sensitive subpopulations such as
the elderly or young children. One of
the conclusions from the report on the
Walkerton Incident in 2000 was that
E.Coli. bacteria entered the distribution
system though contamination of a
shallow well following a heavy rainfall.
Without adequate disinfection,
these deadly microbial contaminants
remained in the distribution system.
This tragedy underscores the importance
of disinfection of groundwater,
both primary disinfection and residual
disinfection.
Increasingly, chemical contamination
of source waters has become an
issue that many more water providers
have had to face. The
sources of chemicals in the
water supply are varied.
Watersheds are under pressure
from industry, agriculture,
animal feeding operations,
and wastewater discharge,
among many other
sources.
In a recent study, the
United States Geological
Survey found that in vulnerable
watersheds that were
downstream of urban areas
and animal production
facilities, one or more
investigated chemicals was
detected in 80% of the samples
collected. Examples of the chemicals
investigated include the steroid
coprostanol, the insect repellant N,Ndiethyltoluamide,
the pesticide diazinon,
and the non-prescription drug caffeine.
Similar studies with similar
results have been conducted in Europe.
Some contaminants are
difficult to remove
Canadian regulations require that
trace organic contaminants be treated.
For many contaminants, this can be
accomplished easily with carbon or air
stripping. However, a growing list of
contaminants cannot be treated with
these conventional technologies. For
example, the solvent stabilizer 1,4-
dioxane has been detected widely
across North America as part of existing
solvent plumes. This additive,
added to prevent the breakdown of the
solvent due to acids generated during
the degreasing process, has proved to be
a stubborn groundwater contaminant. It
travels farther and faster in groundwater
than the host solvent (often trichloroethylene
or 1,1,1-trichloroethane) due
to its low affinity for carbon materials
in soil. As a semi-volatile contaminant,
it also does not volatilize, making air
stripping ineffective. Therefore, an oxidation
process has become the preferred
method of treatment.
In treating chemical contaminants
with UV, there are two photochemical
processes at work: UV-photolysis and
UV-oxidation. UV-photolysis involves
UV light alone; UV-oxidation requires
the addition of hydrogen peroxide.
UV-photolysis is the process by
which chemical bonds of the contaminants
are broken by the energy associated
with UV light. When light is incident
on an object, the photons may be
reflected, transmitted, or absorbed.
When UV photons enter a medium
(water, for example), they are both
transmitted and absorbed by the medium
and its constituents (dissolved
species including organic and inorganic
substances). Photons that are
absorbed may initiate a photolysis
reaction. A contaminant molecule will
undergo the photolysis reaction if the
contaminant molecules in water are
capable of absorbing UV photons
(measured by the contaminant's molar
absorption coefficient) and if the energy
holding the chemical bonds in the
molecule together is less than the energy
of the UV photons absorbed.
UV-oxidation is a photochemical
process that breaks down organic constituents
in water by the process of oxidation.
The UV-oxidation reaction is
initiated by the UV-photolysis of
hydrogen peroxide (or another oxidant).
When UV photons are absorbed
by hydrogen peroxide dissolved in
water, hydroxyl radicals are formed.
Hydroxyl radicals are highly reactive
chemical species that attack the contaminant
molecule. Second only to fluorine
(a poisonous, corrosive and malodorous
gas), the hydroxyl radical is
the most reactive species known.
Some chemicals are preferentially
treated by the UV-photolysis process;
others are preferentially treated by the
UV-oxidation process. In most cases,
UV-photolysis and UV-oxidation act
simultaneously to break down chemical
contaminants. Applied in a treatment
plant, water moving through an optimized
UV reactor is both disinfected
and treated for organic chemicals. Thus,
UV has the ability to act as a multi-functional
part of a multi-barrier system.
In 1997, trichloroethylene (TCE)
was discovered on the property of the
Canadian Forces Base (CFB) Valcartier
in Loretteville, Québec. The aquifer
beneath the property is used for drinking
water. Iron and manganese are also
present in elevated concentrations. The
Department of National Defense has
elected to use UV-oxidation for the
treatment of TCE following a biological process that will remove iron and
manganese. UV-oxidation was chosen
as the treatment technology for TCE
due to its cost-effectiveness and its
ability to destroy TCE, rather than simply
transferring it to another phase
(such as the gas phase in air stripping
or the solid phase in carbon treatment).
To date, a demonstration system
consisting of a TCE injection system,
an H2O2 injection system and a
UVPhox™ Model 30AL50 UV reactor
from Trojan Technologies was installed
and operated for approximately two
months. This pilot has demonstrated
that the technology is effective for the
treatment of TCE. Full scale installation
of the UV-oxidation system is
scheduled for early next year.
Christian Williamson is Managing
Director, Environmental Contaminant
Treatment, and
Adam Festger is the
Technical Communications
Coordinator, Environmental
Contaminant Treatment, at
Trojan
Technologies Inc.
Contact e-mail: cwilliamson@trojanuv.com or afestger@trojanuv.com.
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