Evaluating disinfection by-product formation and
corrosion under simulated distribution systems
by W.J.Bayless, R.C.Andrews, J.Eisnor and G.A.Gagnon
The formation of trihalomethanes
(THMs) and haloacetic
acids (HAA6) from the
use of chlorine-based disinfectants
is of concern to treatment facilities
due to the associated toxicological
effects and strengthening of disinfection
regulations. To minimize the formation
of these disinfection by-products
(DBPs) treatment facilities may
consider alternative disinfectants,
chlorine dioxide (primary) or chloramines
(secondary). However, the
impacts on distribution system water
quality from switching to an alternative
disinfectant is not clearly understood,
nor are the relationships between
corrosion and DBP formation.
This project was undertaken to
evaluate the formation of DBPs when
using simulated cast iron distribution
system conditions and chlorine, chloramines
or chlorine dioxide. As there is
very little information in the literature
concerning the impacts of chlorine
dioxide, chlorine and chloramines
were examined to provide a comparative
baseline. To provide a comprehensive
approach, bench-scale tests were
undertaken using both annular reactors
(ARs) and cast-iron pipe loops.
In the pipe loop experiments, 6 and
12 hour retention times were explored
for each of the three disinfectants.
Additionally, a “high” and “low” disinfectant
residual concentration was
passed through the pipe loop to
observe the effect on DBP formation
and corrosion rates (Table 1). The corrosiveness
of the water in the pipe
loops was the highest when chlorine
and monochloramines were used as the
primary disinfectant. The pipe loop
which contained chlorine dioxide
demonstrated the lowest corrosion rate
(Table 2) when compared to other disinfectants.
When the retention time was
increased to 12
hours the corrosive
rate of the
water did not
demonstrate any
observable trends;
however the total
iron levels generally
increased
with the longer
retention times
(Table 3). The
total iron levels
were observed to
be highest in the
chlorine pipe loop,
followed by the
chlorine dioxide
and chloramines.
The average
THM formation was observed to be
highest in the chlorine pipe loop, averaging
12 µg/L for a total chlorine
residual of 0.7 mg/L. The THM formation
in the pipe loops being fed chlorine
dioxide and chloramines was
observed to be less then the method
detection limits over the course of the
experiment (Figure 1). This is similar
to published literature which acknowledges that chlorine dioxide and chloramines do not form
THMs in any significant level.
Figure 1: THM formation in cast iron pipe loops for
various disinfectants.
The HAA formation levels followed a similar trend as
was observed for THM formation. The use of chlorine as a
primary disinfectant resulted in the highest HAA6 levels
(Figure 2).
Figure 2: HAA formation for various disinfectants.
During the experiment there were no observable trends
between the corrosion rates and the DBP formation. In the
chlorine dioxide and chloramine pipe loop the amount of
DBPs formed was below detection limits. For the chlorine
pipe loop, the THM and HAA6 concentrations were not
observed to be related to the corrosion rate (Figure 3).
Similarly, no trend was observed between the total iron
levels and the DBP formation in the chlorine pipe loop
(Figure 4).
Figure 3: DBP formation related to corrosion rate.
Figure 4: DBP formation related to total iron concentration.
The information gathered through these experiments
indicates that DBP formation can be minimized while
water quality can be improved by changing primary disinfectants.
However, this is often a significant undertaking
and would only be advantageous for municipalities which
are treating a difficult source water, such as one with a
high THM formation potential or red water problems.
Furthermore, the introduction of more stringent DBP regulations
such as that proposed by the USEPA could force
some municipalities to look towards alternative disinfection
practices.
At the current time the Ontario MOE does not have
DBP regulations that are as restrictive as their American
counterparts; however the goal of water treatment facilities
is to provide safe drinking water which includes minimizing
the DBP formation.
W.J. Bayless and R.C. Andrews are with the Drinking Water
Research Group, Department of Civil Engineering,
University of Toronto. J. Eisnor and G.A. Gagnon are with
the Department of Civil Engineering, Dalhousie
University, Halifax. For further information, contact:
www.dwrg.ca, e-mail: graham.gagnon@dal.ca.
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