Chlorine dioxide trial as a post disinfectant in Wiarton, Ontario
by Dr. Robert Andrews,
Dr. Bryan Karney
and Dr. Georges Ranger
R101 Generator Process Diagram.
On June 20, 2000, the Town
of Wiarton, Ontario, began
a two-month trial where the
disinfectant used to treat the
tap water was changed from chlorine to
chlorine dioxide.
This project, headed by Sterling Pulp
Chemicals Ltd., Water Technologies
Division, was undertaken with the participation
of the Town of Wiarton, the
Ontario Clean Water Agency (OCWA),
the Ontario Ministry of the Environment
and the University of Toronto, Department
of Civil Engineering.
The Town of Wiarton, with a population
of 2300, is supplied with drinking
water from the Wiarton Water Filtration
Plant completed in 1993. The
plant is located on the shore of Colpoys
Bay and has a maximum day capacity
of 5137 m3/day. The plant operates on
demand from the city water storage tank
and cycles with a frequency of about 4-
5 hours. Raw water from Colpoys Bay
is pumped through the water intake.
From there the water is screened and
then enters the wet well. The water is
pumped into two flocculation tanks
where polyaluminum chloride is added.
From the flocculation tanks, water is
then filtered through two multi-media
sand filters. Water chlorination takes
place at the plant intake (dosage rate: ~
1mg/L Cl2) and at the filter outlets (~ 1
mg/L Cl2) via addition of 12% sodium
hypochlorite solution. Treated chlorinated
water is pumped from the clearwell
into the trunkmain, distribution system
and the water tower through two high
lift pumps.
The ERCO R101 electrochemical
generation system consists of a circulation
loop (anolyte loop) where the sodium
chlorite solution (anolyte) is re-circulated
through the anode compartments
of an integrated cell comprising the chlorine
dioxide generating and anolyte pH
control cells. Once generated, the product
chlorine dioxide disengages from the
anolyte as it passes through the donor
side of a perstraction module (Pall Corporation,
New York). Water is circulated
through the receiver compartments of the perstraction module where chlorine
dioxide and water vapor diffuse through
the hydrophobic membrane and are absorbed.
The resulting high purity chlorine
dioxide solution then leaves the system
and goes to the point of use. The
anolyte, depleted of sodium chlorite,
gets replenished with sodium chlorite
feed solution. Weak caustic solution is
separated from the hydrogen gas, which
is evolved at the cathode, and then used
or drained. The generator is operated
via a computer-controlled system, with
safety interlocks that allow it to operate
autonomously. Once started it will operate
unattended at its preset parameters
with very little intervention or monitoring.
The R101 system installed at the
Wiarton site had a ClO2 production capacity
of 10 Kg/day, but due to the low
demand of the plant, the unit was operated
at 6.5 Kg/day. All of the ClO2 solution
produced was sent to a storage tank
to provide a reasonable ClO2 solution
buffer capacity and a means of dealing
with possible fluctuations in disinfectant
demand.
Three metering pumps drew ClO2
solution from this tank, injecting it at
the raw water intake for pre-disinfection
and the control/prevention of zebra mussel
infestation, and prior to entering the
clearwell. On line monitoring of the
clearwell water chlorine dioxide residual
was accomplished by the use of a
ProMinent Dulcometer analyzer.
The responsibility of the SPC Water
Technologies project team was to monitor
the performance of the R101 generator
and peripheral equipment as well
as to provide the right amount of disinfectant
to the Wiarton drinking water in
order to maintain a residual value at the
furthest point in the town distribution
system. The Wiarton Water Distribution
System Monitoring Study was the responsibility
of University of Toronto
personnel. To assist in the identification
of suitable sampling locations a hydraulic
model of the network was constructed.
Initial modeling efforts were
directed at trunk watermains in the central
part of the town and major hydraulic
devices (e.g., reservoir, pumping station,
treatment plant, etc.).
Sampling Locations and
Study Timeline
Samples were collected from five
locations in Wiarton:
The water treatment plant clear well
The water tower effluent
House in Oxenden (west part of system)
Wiarton Hospital (east part of system)
The OPP (Ontario Provincial Police)
Station (southern most point of system)
The sampling sites in the distribution
system were selected on the basis of a
hydraulic model and were mainly located
in easily accessible public buildings.
EPANet 2.0x was utilized to hydraulically
model the Wiarton Water
Distribution System.
Sampling commenced two weeks
before an initial “ramp up” period. The
project was scheduled for 17 weeks with
a total of 23 sample collection days.
During the initial “ramp up” period the
hypochlorite used at the intake for predisinfection
and zebra mussel control
was replaced by chlorine dioxide. After
a period of approximately one week,
the post-filter addition of hypochlorite
was also replaced. Microbial protection
at all times during the study exceeded
that typically obtained when using chlorine
by a factor of 4 to 5.
Monitored Parameters
Throughout the trial, data was collected
at the plant and sampling sites to
assess the R101 generator performance
and the impact of chlorine dioxide on
water quality. Monitoring included:
R101 generator yield and chlorine dioxide
solution quality.
Disinfectant residuals (free and combined
chlorine, chlorine dioxide).
Total heterotrophic counts (direct microscopic counts using
acridine orange).
Heterotrophic plate counts on R2A.
Figure 1 - THM’s Concentration versus Contact Time for Chlorine and Chlorine Dioxide.
Figure 2 - HAA’s Concentration versus Contact Time for Chlorine and Chlorine Dioxide.
Results
R101 Generator Performance - The performance of the
R101 unit at the Wiarton Water Filtration Plant was essentially
flawless. The following is a summary of the generator
performance for the trial duration:
The unit produced well over 400kg of pure chlorine dioxide
solution at an average production rate of 6.5kg ClO2/
day, at an on time of over 96%.
The average generator yield based on sodium chlorite
consumed was 96 ± 1% within a 95% confidence interval
based on the data population.
The generator average chlorine dioxide yield (EPA formula)
was 99.4 ± 0.1%.
The generator average excess chlorine (EPA formula) was
0.5 ± 0.2%.
The R101 output chlorine dioxide purity as mass ratio
averaged 98.8 ± 0.3%, for an average output solution concentration
of 1200 mg/L ClO2.
In addition to generator parameters monitoring, water
samples were collected throughout the trial at several locations
within the treatment plant and at the distribution system
in order to confirm and control chlorine dioxide residuals
and disinfection by-products in the drinking water. In order
to maintain a residual disinfectant value of at least 0.1mg/L
ClO2 at the distribution system furthest point (Oxenden),
the chlorine dioxide dosage (based on water analysis) during
the constant period consisted of 0.4mg/L addition at the
plant intake and 0.8 mg/L ClO2 post-filter addition. The
resulting plant clearwell disinfectant and Disinfection Byproducts
(DBP’s) concentration averaged 0.6 mg/L ClO2,
0.5 mg/L ClO2-, and 0.06 mg/L ClO3.
Water Quality Monitoring - Over the monitoring program,
more than 500 samples were collected to determine the impact
of disinfectant change on the observed bacteriological
and chemical parameters. The results of this work provided
valuable insights into the phenomena that are involved when
introducing chlorine dioxide to a utility.
Disinfectant Residuals - Chlorine dioxide residuals were
consistently present above detection limits (and below
USEPA MCLs) throughout the distribution system. The lowest
residual observed was 0.1 mg/L.
Chlorite values throughout the system averaged approximately
0.7 mg/L; chlorate values averaged approximately
0.1 mg/L. Both of theses were also well within acceptable
USEPA limits.
THMs (Trihalomethanes) and HAAs (Haloacetic acids)
- USEPA limits for THMs and HAAs. have been set to 80µg/
L and 60µg/L respectively. Results from THM and HAA
monitoring in Wiarton are shown in Figures 1 and 2. It was
found that the change in disinfectant from chlorine to chlorine
dioxide led to an 85% reduction in THMs (i.e. from
30µg/L to 5µg/L) and a 60% reduction in HAAs (i.e. from
20µg/L to 8µg/L).
AODC (Acridine Orange Count) - AODC is a measurement,
based on a DNA stain, of the number of organisms in
water. AODC measurements in Wiarton tap water decreased
by approximately 60% following the switch to chlorine dioxide.
This suggests that chlorine dioxide was more effective
at killing microorganisms in the tap water than chlorine.
In addition, no coliform bacteria were detected in the
drinking water during the duration of the trial.
Conclusions
The trial undertaken at the Town of Wiarton with the joint
participation of SPC Water Technologies Division, the University
of Toronto, Department of Civil Engineering, the
Ontario Clean Water Agency and the Ontario Ministry of
the Environment clearly exceeded expectations for the
project.
Dr. Robert Andrews and Dr. Bryan Karney are with the Department
of Civil Engineering, University of Toronto.
Dr. Georges Ranger is with Sterling Pulp Chemicals Ltd.,
Water Technologies Division.
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