©2004
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Chlorine dioxide was first
produced from the reaction of potassium chlorate and hydrochloric acid by Davy
in 1811. Not until the industrial-scale preparation of sodium chlorite, from
which chlorine dioxide may more readily be generated however, did its widespread
use occur
The most common traditional
methods of generating chlorine dioxide involve mixing sodium hypochlorite (NaClO),
acid (HCl2) and sodium chlorite (NaClO2) or chlorine gas
(Cl2) and sodium chlorite.
NaOCl + HCl + 2NaClO2
2ClO2 + 2NaCl + NaOH
Cl2 + 2NaClO2
2ClO2 + 2NaCl
New technology has dramatically
improved the onsite generation of chlorine dioxide. This new technology
generates chlorine dioxide by electro catalytic and electrochemical techniques.
These generators typically use only one precursor, sodium chlorite. The
elimination of both chlorine and acid in the process has resulted in a much
safer and simpler generation process. Some of these new generators produce
chlorine dioxide directly in an aqueous solution with a concentration below 1000
mg/L. This low concentration in an aqueous solution dramatically enhances the
safety of the process.
Chlorine Dioxide has the
chemical formula ClO2 and is a yellow to brown coloured gas at room
temperature and pressure. It is a highly reactive oxidant and for all practical
areas of water disinfection, it must be generated on site using proprietary
reaction and dosing equipment
By comparison: At room
temperature, chlorine is a greenish-yellow poisonous gas. When added to water,
however, chlorine combines with water to form hypochlorous acid that then ionises
to form hypochlorite ion - 'bleach'
In general, chlorine dioxide
has been found to produce fewer organic by-products with naturally occurring
dissolved organic material. Chlorine dioxide is an explosive gas, but is stable
in water in the absence of light and elevated temperatures. ClO2 is capable of
oxidizing iron and manganese, removing colour, and lowering THM
(Trihalomethanes) formation potential. It also oxidizes many organic and sulphurous
compounds that cause off-tastes and odours.
The
physiological mode of inactivation of bacteria by chlorine dioxide has been
attributed to a disruption of protein synthesis.
In the case of viruses, chlorine dioxide preferentially inactivates the
outer protein layers, rather than nucleic acids.
It is well known that ClO2
does not react with ammonia; however, this is only one of many chemicals not
affected by “The Selective Oxidiser.”
ClO2 does not react
with:
acids, alkanes, alkynes,
alcohols, aldehydes, aliphatic amines, ammonia, azole, carbohydrates, ethers,
fats, glycol, ketones, methanol, polysaccharides, saccharides, unsaturated fatty
acids and unsubstituted aromatics, among others.
Organic contaminants, such as
those mentioned above, are regularly found in cooling and process water systems.
The cooling system contaminants could be ammonia in a semiconductor plant,
glycol in a process heat exchanger, oil in a textile air washer or a steel mill,
food from a cooker or methanol from a chemical plant. Because ClO2 does
not react with these contaminants, its demand is based on the microbiological
loading in the water only. This demand impact is a major advantage in the areas
of cost and performance.
As regards to the cost of ClO2,
it is certainly more expensive on a pound for pound basis, than chlorine gas (as
much as 1OX), or hypochlorite (4X). It is only when the consumption of the
chlorine by the systems demand is factored in, can the economics favour the ClO2
– cases have been witnessed in heavily contaminated systems where a 1:25
replacement by ClO2 for chlorine, resulted in satisfactory treatment
program where one did not exist prior. (John Murphy, Microcide Consultants).
A few
notes on some systems that have switched to Chlorine dioxide:
Washing
of cut lettuce for hamburgers for fast food chain in NZ. Processing done by
Essentia Foods.
Quality
requirements:
Salmonella zero
Listeria zero
E.Coli zero.
Must pass sensory evaluation
test criteria (no chlorine taste).
Appearance of lettuce must be
good.
TPC must be within guidelines
at day 10.
TPC = Total Plate Count
(microbiological surface contamination)
Description
of old chlorine disinfection system.
Chlorine
at 100-200ppm. Dosed using sodium hypochlorite 12.5%
Terrible chlorine smell in
factory with workers complaining of eye and skin irritations.
Impossible to control chlorine
residual and required manual chemical addition every 15 minutes.
pH control not possible as
always creeping high.
Required to dump a lot of water
to maintain chlorine residual which was high cost for chilling and extra ice.
E.Coli was not always zero.
Always concerned about Listeria
as Listeria not affected by chlorine at low temperatures.
TPC at day zero was
inconsistent usually 1 x 105, 3 x 105 and occasional 1 x
106 counts
Description
of new ClO2 system installed in 1996
Chlorine
dioxide at 1.0ppm in 2 stage wash. First wash stage is 8 deg.C and
second wash stage is 2 deg.C.
Dosing is done automatically
using Bellozon generator and automatic residual control.
No chemical smell in the
factory at all.
Operators do a check on the
dosing equipment every hour or so but don’t add any chemical manually.
pH is automatically controlled
to 7.5.
Very little dumping of water
and only chilled water is used. Chemical running cost is very low around $1.5k
per year.
E.Coli is always zero.
No concerns about Listeria as
ClO2 will easily kill Listeria at low temperatures.
TPC at day zero is consistent
and always less than 7 x 104
Tomato
washing : 3
plants in Australia. Girgarre Country Foods, SPC Foods, Unilever
Tomatoes are brought to the
processing factory by truck and then transported by flume to the tomato paste
production area. Chlorine Dioxide is used to destroy moulds on the tomatoes and
in the flume tank.
Tomatoes
are dumped from the truck onto a conveyor.
Coarse rinse with town water
sprays to remove dirt and stems, leaves etc.
Tomatoes fall into flume tank
(20 m3). The flume water is pumped to the sorting conveyor and back in a closed
circuit with the tomatoes. Operators remove unacceptable product.
Make-up condensate water is
continually added (5 m3/hr) from the tomato paste process.
Chlorine Dioxide is dosed into
the flume water to maintain concentration of 0.2 - 0.4 ppm of ClO2.
pH of the flume water goes to 4.0 and this is not corrected as it is acceptable.
Method of concentration
control
Bellozon
CDKa3A (420 g/hr) dosing directly into flume. By-pass water is the condensate
flow.
If flume water is used as
by-pass then no static mixer or VA flow meter/check valve as these block up.
Control to 650mV using D1CAW0R
redox controller and proportional control with industrial redox probe.
This system is only used in wet
weather and occasionally during dry weather. Mould is a bad problem when there
is a lot of rain during harvest.
Number of installations
3
plants in Australia. Girgarre Country Foods, SPC Foods, Unilever.
Previous Treatment
Used
sodium hypochlorite and due to the high concentration of organic material in the
flume water, had difficulty maintaining any free chlorine residual. This meant
that moulds were not controlled and surfaces were fouled. In addition, operators
would occasionally stop work due to chlorine smell in the sorting area.
Advantages/benefits
Low
concentration of chlorine dioxide is very effective in destruction of moulds on
the tomatoes. These moulds would negatively affect the past production process
if present.
Low concentration of chlorine
dioxide is very effective in destruction of moulds in the flume water. If
untreated, the moulds attach to surfaces of tanks and flumes and look like
“meat”. Eventually, they foul screens and smell.
Chlorine dioxide effective at
pH 4.0
No smell for operators
Very low running costs
No chlorinated organic
by-products.
Disadvantages
When
the redox probe fails, chlorine dioxide overdoses and then smell is produced.
This can be avoided by using two redox systems in parallel.
Possible build up of chlorite
in the flume tank can affect the skin of operators when they handle the tomatoes
i.e. hands, arms and face. If they wear gloves this can help.
copyright
2003 Dioxide Pacific Pt
Vegetable
washing.
Vegetables of all kind are
washed, cut and packed (e.g. in plastic bags).
Customers are supermarkets and
fast food producers.
Previous
treatment
Usually
Chlorine is used for microbiological control with concentrations varying between
100 - 200 ppm.
Problems
with previous treatment
Chlorine
created a smell problem during processing in the processing hall with operators
complaining of eye and skin irritation.
pH very often above 7.5 where
microbiological treatment is often not effective with chlorine.
Case
Study
Parripak
Food, in England - changed to Chlorine Dioxide
Batch
washing
Water
change every 6-8 hrs. typical dosage: 6 ppm
Spraying, typical
concentrations
Onion rings 6 ppm
Carrots 1 ppm
Benefits of chlorine dioxide
Shelf life increased by factor
3
copyright 2003 Dioxide Pacific Pt