PRACTICE REPORT
PROCESS ANALYSIS
CHLORINE DIOXIDE
SC 100 / 9187 SC
New chlorine dioxide sensor for
monitoring disinfection
Wassergewinnung Essen GmbH (WGE), a subsidiary of Stadtwerke Essen
AG and Gelsenwasser AG, operates two drinking water plants with an annual
production volume of about 65 million cubic metres. The drinking water is
obtained by the costly treatment of surface water (Ruhr). To monitor
the water quality, water quality stations containing instruments that
measure chlorine dioxide are operated in the pumping works. On the
basis of the experience gained in the trial described in this report, one of the
old chlorine dioxide measuring instruments, which was no longer properly
functional, was replaced by a new HACH LANGE ClO2 sensor.
Author:
Bernd Schoemaker
- Head of Laboratory
- Wassergewinnung Essen GmbH
2
DRINKING_WATER_DISINFECTION
Core element of disinfection: the exact
dosage of ClO2 solution
Fig. 1: The purpose of all treatment steps is the
production of fresh, healthy drinking water
Extract from the 2001 Drinking Water
Ordinance (TVO)
§4 General requirements
(1) Water for human consumption must be
free of pathogens, wholesome and clean.
§11 Substances used for the treatment of
water; disinfection methods
(1) Only substances included in the list
published by the Federal Ministry Of Health
in the Federal Health Gazette may be used for
the treatment of water intended for human
consumption.
Extract from the Federal Health
Gazette (Federal Environment Agency
12/2004)
Part I c
Substances used to disinfect water
Name of substance: Chlorine dioxide (max.
0.2 mg/L ClO2)
Fig. 2: Water treatment at a glance
1 Surface water (side arm of the Ruhr)
2 Pumping station
3 Ozonation
4 Flocculation
www.hach-lange.com
The treatment procedure
The first treatment step (Fig. 2) is
ozonation, followed by flocculation.
Powdered activated carbon is then
added and the water is passed through
a rapid two-layer filter. Finally the water
percolates through a slow sand filter, so
that the groundwater in the subsoil is
artificially recharged. Horizontal subsoil
drains (5,500 m, DN 800-1300) gather
the recharged groundwater and channel
it through lift pipes to the pumping
works, where the final treatment steps
are carried out, i.e. neutralisation
with sodium hydroxide solution and
disinfection with chlorine dioxide.
Water quality stations
Water quality stations are operated
in the pumping works to monitor
the quality of the water (Fig. 3).
Instruments are installed in these
stations to continuously measure
turbidity, conductivity, oxygen,
temperature, pH and chlorine dioxide.
Alongside the pH measurement, which
is used to regulate the addition of
sodium hydroxide solution, the chlorine
dioxide measurement is extremely
important, as it monitors the exact
dosage of chlorine dioxide solution.
The meters are maintained and
calibrated by employees of WGE’s
accredited laboratory.
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6
7
8
9
Powdered activated carbon
Rapid filter
Intermediate storage
Slow sand filter
Well
Chlorine dioxide measurement
Two chlorine dioxide measuring
instruments are positioned at each
outflow of a pumping works (Fig. 4).
The measurements are carried out
immediately downstream of the point
where the chlorine dioxide is added.
The samples measured at the second
instrument are channelled through
a delaying loop, so that the second
measurement is carried out 20 minutes
after the first one. This corresponds
exactly to the time required to treat the
water. This arrangement enables the
following criteria to be checked and
monitored:
• Chlorine dioxide is being added.
• The added amount of ClO2
corresponds to the prescribed
dosage and the dispensing
systems are functioning correctly.
• The concentrations in the treated
water are within the limit values of
the Drinking Water Ordinance.
The values obtained from the first
(immediate) and second (delayed)
measurements enable conclusions
to be drawn about the depletion
characteristics of the pumped drinking
water. And if the first measurement
shows that the ClO2 concentration is
below a lower limit value, emergency
disinfection is automatically switched
on to ensure that no non-disinfected
10 Ruhr
11 Pumping works (water quality stations)
12 Water container
13 Consumer
3
drinking water is pumped into the
supply network.
Measuring instruments used
For more than 15 years, type CP1
chlorine process photometers from
Dr. Lange (now HACH LANGE) have
been used. These instruments work
photometrically. The measurement
method is based on the reaction of
chlorine and chlorine dioxide with
N,N-diethyl-1,4-phenyldiammonium
sulphate (DPD) to form a red
compound. The continuous
measurement method is based on
DIN 38 408 G 4-2.
When the WGE laboratory was
accredited in 1995 by the DAP
(Deutsches Akkreditierungssystem
Prüfwesen), the continuous
measurement of chlorine dioxide with
the CP1 was also accredited and
was validated in accordance with the
requirements of the standard.
Replacement of the existing
measuring instruments
In view of the age of the measuring
instruments, a new measurement
system had to be found to replace
them. Given WGE’s good experience
with the photometric method,
its search was at first limited to
photometric instruments with the
following characteristics:
• Continuous measurement of
chlorine dioxide concentrations
from 0 to 0.5 mg/l
• Reliable determination of a
concentration of 0.05 mg ClO2/l
• Fast response time
• Minimal maintenance and costs
• Long-term stability of the
measurement reading
Two commercially available instruments
were tested by the laboratory under
real conditions in a water treatment
plant. The results obtained were not
convincing. WGE therefore started to
look at amperometric instruments.
Comparison of 9187 sc chlorine
dioxide sensor and CP1
As part of the search for a replacement
for the CP1 chlorine process
photometer, WGE was provided
with a HACH LANGE 9187 sc ClO2
sensor (Fig. 5) in combination with
a SC 100 universal controller for
the purpose of carrying out test
measurements.
The probe functions amperometrically
and uses a chlorine dioxide selective
membrane to provide protection
against cross-sensitivities to chlorine,
bromine and hydrogen peroxide.
Operational test and results
The chlorine dioxide sensor was tested
for three months in parallel to the
accredited CP1 measurement method
under operational conditions in the
WGE pumping works. The output
signals from both instruments were
recorded with the help of an electronic
plotter.
The installation of the instruments was
easy, as they were supplied together
with a mounting frame for both the
probe, including the measuring cell,
and the controller.
After a short familiarisation period
and the calibration of the probe, the
comparative measurements were
started. The measured chlorine
dioxide concentrations were between
0.08 and 0.18 mg/l, depending on
the operational status of the pumping
works outflow.
The daily time curves of both
measuring instruments were almost
identical, showing a maximum
deviation of 0.01 mg/l chlorine dioxide
(Fig. 6).
Fig. 3: Water quality station with extensive
process measurement technology
Fig. 4: Two measuring points downstream of
the ClO2 dosage point
Fig. 5: 9187 sc chlorine dioxide sensor with
SC 100 controller
DRINKING_WATER_DISINFECTION
The 9187 sc chlorine dioxide sensor
corresponds to the requirements of WGE
9187 sc
CP1
Fig. 6: Typical section of the recorded data with
identical time curves from the two instruments.
The new sensor (blue) is unaffected by air
bubbles.
The outliers among the CP1
measurements are attributable to the
air bubbles that occasionally passed
through the measurement cuvette.
In the three month test phase, no
maintenance work had to be carried
out on the 9187 sc, and a recalibration
was first needed after more than four
weeks.
9187 sc replaces old instrument
The 9187 sc satisfied all WGE’s
demands on a chlorine dioxide
measuring instrument with regard to
the quality of the measurement results,
Time
;FJU
stability of the reading, rapid response
time and low maintenance costs. The
requirement for a simple changeover to
the parameter chlorine was also met;
replacing chloride dioxide selective
membrane suffices.
On the basis of the experience gained
in this trial, an old, no longer functional,
measurement method was replaced by
a 9187 sc chlorine dioxide sensor. The
option of connecting two probes to the
SC 100 controller will allow a second
instrument to be installed without
difficulty at this site if necessary.
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