Automation of disinfection process using ORP reduces chlorine by 32%
by Max Rao, P.Eng.,
Indachem Inc. &
Christine Thibeault,
City of Greater Sudbury
Operator accesses the control system at the Sudbury Wastewater
Treatment Plant (below).
The City of Greater Sudbury,
with a population of 155,000,
is located 390 kilometres north
of Toronto. At 3,627 square
kilometres including water bodies, it is
the largest municipality in Ontario.
The main Sudbury Wastewater
Treatment Plant is an 80,000 m3/day
high rate activated sludge facility,
serving a community of over 85,000
residents and over 4,600 local businesses.
A future plant expansion will
see the plant capacity increase to
102,375 m3/day.
Chlorination is the final stage of the
treatment train before the effluent is
discharged into Junction Creek, which
is immediately upstream of Kelly
Lake. In order to achieve an E-Coli
discharge limit set in the facility’s
Certificate of Approval, year-round
chlorination is required. Manual control
of chlorine feed to the contact
chamber produced good results with
respect to E-Coli discharge compliance.
However, it was hypothesized that
chlorine was being overfed to ensure
compliance since the manual chlorine
feed adjustments could not keep up
with the frequent changes in demand
associated with a municipal wastewater
treatment plant. The City Plants
Engineer and staff at the Sudbury
facility decided to investigate methods
to automate chlorine feed to the contact
chamber to get better control of
the disinfection process.
Using a gas chlorinator, chlorine
gas is fed from tonne cylinders into a
wetwell directly upstream of the contact
chamber. Two 2150 m3 contact
chambers provide an average and minimum
detention time of 60 and 15 minutes,
respectively. In order to ensure
disinfection to the required level, historic
measurements have shown that
maintaining a total chlorine residual
between 0.5 and 1.0 mg/L at the outlet
of the contact chamber, prior to discharge
to Kelly Lake was adequate.
Effluent samples were taken three
times per week for total chlorine analysis.
Depending on the results measured,
the output from the chlorinator
was either increased, or decreased, to
bring the chlorine residual number to
within the acceptable range. Since
manual adjustments were only performed
a few times per week, adjustments
were not able to account for
changes in oxidant demand that continually
occured throughout the day. In
order to ensure proper disinfection
during peak demand, chlorine was
overfed. This resulted in proper disinfection
but high chlorine residuals discharging
into the receiving water body
during periods of low demand.
Flow Pacing
The first attempt to automate the
chlorination process was with flow
pacing. A 4-20 mA signal from the
plant flow meter was used to pace the
output from the gas chlorinator to the
effluent flow through the contact
chamber. It was quickly determined
that automatic chlorination control by
flow pacing was inadequate and often
resulted in chlorine overfeed and
underfeed. Although hydraulic loads
can increase significantly, coliform
levels do not increase proportionately.
Conversely, low plant flows do not
necessarily reflect low coliform levels.
Amperometric Chlorine Analyzer
An amperometric analyzer was
installed to automatically measure the
total chlorine residual in the effluent
exiting the contact chamber and adjust
the chlorinator output to maintain a
residual setpoint. Operators found that
the sensing electrode fouled very
quickly, which resulted in inaccurate
residual readings, and required frequent
cleaning. Furthermore, the analyzers
required a steady supply of
reagents to function properly and were
dependent on proper calibration procedures
being followed. Operators at the
Sudbury facility found that the amperometric
analyzer was too maintenance intensive to automate their disinfection
process.
Research also indicated that chlorine
residual measurement is not a
good predictor of disinfection.
Measuring residual only tells how
much chlorine is left at a given point in
time. Chlorine residual is not a measure
of the work value of the chlorine or
a measure of the chlorine demand,
which is continually changing.
Controlling with residual measurement
can, therefore, result in variable
E-Coli levels discharging to the environment.
High Resolution Redox (HRR)
Research by Danish virologist Dr.
Ebba Lund ("Oxidation Inactivation of
Poliovirus", Copenhagen, 1963) indicated
a direct link between the disinfection
rate of poliovirus and the oxidation
reduction potential (ORP). She found
that a given ORP held for a period of
time would produce a repeatable inactivation
curve. Lund’s work led to the
development of Lund’s Law of
Oxidative Disinfection: The log
decrease in microorganism activity is
proportional to the ORP maintained,
times the contact time.
Plant management decided to investigate
the use of ORP as the control
basis for the disinfection process.
ORP (also referred to as redox) is a
measurement of the electromotive
force (emf) generated when an oxidant
is present in an aqueous solution.
Measurable in millivolts (mV), the
strength of this force is directly proportional
to the oxidative strength of
the treated system. The higher the
concentration of the oxidant, the higher
the voltage. Conversely, the higher
the concentration of the reductant (i.e.
E-Coli, organic material, etc.), the
lower the voltage. A redox sensing
instrument, which detects this voltage,
can be used to monitor the chlorine
demand. The ORP of the Sudbury
WWTP’s effluent, at any given time, is
a direct measurement of the current
oxidant demand of the system.
A Strantrol® Model 890 High
Resolution Redox (HRR) controller,
manufactured by USFilter/Stranco
from Bradley, Illinois, was installed by
Indachem Inc. at the Sudbury WWTP
as part of a trial to determine if the unit
could effectively automate the disinfection
process. A heavy-duty submersible
electrode assembly, installed
within the contact chamber approximately
100 feet from the chlorine
injection point, measures the ORP
(oxidant demand) of the effluent. The
unique probe design, which comes
with a two year warranty, incorporates
a 99.999% pure platinum sensing electrode
and a porous Teflon liquid junction.
These features allow the HRR
system to effectively operate in a
wastewater environment and measure
1 mV ORP changes, which is required
for accurate control.
The Strantrol 890 combines plant
flow and oxidative requirements to
match chlorine feed rate to the changing
oxidant demand in the system and
to maintain a predetermined ORP setpoint
(mV). The SloLogic control
algorithm used by the Strantrol 890
was developed to mimic the actions of
an operator. Utilizing the flow signal
to calculate changes in lag time
between the chemical injection point
and the sensor location, the controller
looks at deviations from ORP setpoint
at a 10:2:1 time ratio. For example, if
the lag time is 10 minutes, the controller
looks at the deviation from setpoint
during the whole 10 minutes,
during the last two minutes, and during
the last one minute. Then the SloLogic
processor averages the verdict of all
three factors, determines how much to
change output and in which direction,
and then executes the change on the 10
minute interval.
Results
Upon the initial installation of the
demonstration unit, daily bacteriological
samples were taken for E-Coli
analysis. At the same time, total chlorine
residual of the final effluent exiting
the contact chamber was measured
using a colorimetric handheld analyzer.
These two measurements were
used to optimize the ORP setpoint of
the HRR controller to ensure discharge
compliance and to minimize chlorine
consumption.
The setpoint, determined to be 425
mV, was entered into the Strantrol unit,
which controlled the output from the
gas chlorinator to maintain the setpoint
or the oxidative strength within the
contact chamber. Furthermore, minimum
and maximum dosage setpoints
where established and inputted into the
Strantrol 890 to maintain chlorine feed
during process upsets. Maintenance of
the system simply involved weekly
cleaning of the submersible ORP probe.
By automating chlorine feed to the
contact chamber using the Strantrol
890 HRR unit, the amount of chlorine
fed and ultimately discharged to Kelly
Lake was significantly reduced.
During the three and a half month trial
period (May 15 to August 31, 2002),
4040 kg less chlorine was used compared
to the same period in 2001, a
32% decrease, adjusted for flow.
Furthermore, the average chlorine
residual measured over the same period
was reduced from 0.82 mg/L to
0.44 mg/L while in compliance with
E-Coli discharge limits under the
Certificate of Approval during the
entire trial period under automated
control; this proved that controlling
oxidant feed to meet demand can eliminate
the need to overfeed chlorine.
The Sudbury Wastewater Treatment
Plant purchased the Strantrol 890 HRR
controller with the automatic probe
wash option in September 2002. Plant
management estimates that chlorine
savings alone will help pay for it in
approximately 13 months.
See our home page on how to order your subscription. We regret we can
only accept orders from Canada and the United States.
