Measuring Dissolved Ozone
Company develops new method of measurement for bottled water plants
Shake a bottle of beer, uncap it, and the dissolved gases
spray out. Sounds like a promising start for measuring the concentrations of dissolved
gases in water. That's essentially the breakthrough. The new method efficiently
mechanizes this gas-water partitioning principle long known to chemists as
"Henry's law." Mechanizing gas release from water using Henry's law
is not new. The approach is. The result is a new dissolved ozone monitor that
overcomes many nagging problems with existing technologies.
Current Methods for Measuring Dissolved Ozone Concentrations
By way of review, the common current methods for measuring
dissolved ozone include
reduction potential (ORP), also known as Redox. ORP reacts to any oxidizing
influences in the water such as chlorine. ORP is simple, rugged and
inexpensive, but it is not ozone-specific, and its range for ozone is rarely
much above 1 ppm. ORP in many cases is the right measurement where total
oxidizing ("germ killing") power is of interest such as some swimming
pool and drinking water systems that combine chlorine and ozone.
cell (polarographic). This is a chemical reactive cell protected by a special
membrane. The cell can be in a sample stream or sometimes in the main stream.
It has a wide dynamic range, usually into the 10s of ppm and reads to high
precision. Electrochemical cells and instruments are fairly expensive, and abuse
to the cell membrane by particulates, salts, mishandling, etc., often requires
frequent costly maintenance.
test (i.e., Hach colorimeter). No online version. These are packaged reagent
ampules of indigo trisulfonate that is bleached by ozone. The results are read
by a colorimeter, usually digital. When carefully done, the procedure is quite
accurate, but it easily can be misperformed, and there is a constant waste
product of broken glass ampules.
It was time to devise a better way to measure dissolved
ozone concentration. After much trial and error a satisfactory design was
achieved. (Experimental apparatus is shown in Figure 1.) The new instrument
strips ozone from a .15 L/min (3 gal/hr.) sidestream using a proprietary
process. To do this, the stripper section requires 14 psi (1 bar) water
pressure. There is a 1⁄4-inch NPT female port or a hose barb connection
to the instrument. A heated metal oxide semiconductor sensor is used by the
sensing instrument itself. The DOM-1 instrumentation package includes a water
strainer, pressure regulator and hardware to mount everything to the plant
wall. Easy calibration to a laboratory reference is created. Replacement parts
are low cost and easily replaceable by plant personnel.
An early trial of the engineering prototype against tests of
the same water by a Hach Model 850 Colorimeter are shown in Figure 2. The
linearity is good, and the r2 correlation is .9968.
The first generation of instrumentation is for the 0-2 ppm
dissolved ozone range. It should prove to be well-suited for monitoring and
control applications in bottled water plants, swimming pools and various
industrial processes such as food washing.
Ozone Beta Tests
In the early summer of this year, instruments were sent out
to selected customers for testing ("beta sites"). Results have been
better than hoped for. Preliminary top interest is for bottled water plants
where operators want to be sure there is enough ozone in the water for
effective sterilization and yet not so much as to cause a bromate ion problem.
Other applications of current interest are small water companies, jug water
stores and food and beverage plant process water treatment.
Consistency and ease of operation have been key points
reported. The beta testers have found that analysis and experimentation is
required to find the best point in the plant flow stream to connect the
instrument. Also, the connection line from the plant pipe to the
instrumentation should be short in order to minimize ozone half-life loss.
Also Works Well for Dissolved VOCs
One of the beta site testers checked the operation of the
gas stripper for VOCs such as benzene dissolved in water. This is important
because the feed water or process water for many systems must be free of
organics, petrochemical derivatives, solvents, etc. Most of these are collectively known as VOCs (volatile
organic compounds). The stripper's output fed a standard flame ionization
detector detecting benzene. The results were sensitivity below 10 ppb and very
high accuracy. Applications could include checking source water to drinking
water plants, recirculating water in cooling towers, recycled manufacturing
process water and ground water remediation.
The instrument is as accurate as the gas sensor, which is
coupled to the stripper. The stripper itself is very precise. The only variable
that affects calibration is temperature as you can verify by the much greater
amount of gas released from shaken warm beer vs. that released from cold beer.
(This is all predicted by Henry's law.) As long as the instrument is operated
indoors where the temperature is stable, this is not a problem. For outdoor use
and advanced systems, microprocessor-based sensing probes are being developed
that will automatically adjust the calibration for temperature changes.
The first version of the technology for ozone was designed
to be easily understood by plant mechanics and plumbers. Fittings are standard
NPT, and there are such features as strainers for stray particles, which are a
problem for other kinds of sensors. The filters are quickly removed, rinsed off
The first model to be introduced to the market will have an
LCD digital readout, set-point controlled relay for controlling alarms and
generators and a 4-20 mA loop output to plant data systems. Next generation
designs are planned to be microprocessor-based with output in factory data LAN
The new instrument is lower in cost than electrochemical and
ultraviolet-based systems. It is essentially ozone-specific compared to the
lower cost ORP instruments that are sensitive to all oxidants in the water. Due
to the probe not being immersed in the water, the instrument virtually is
maintenance free. However, as is the case for all new technology developments,
unforeseen problems can arise.