Chlorine dioxide - ClO2
1. What is stabilized chlorine dioxide
2. How it works
3. How effective is it
4. How is it implemented
5. Where is it implemented
A) Drinking water
B) Washing vegetables
C) Biofilm removal and control
D) Prevention and control of legionella
E) Cooling towers
F) Scrubbers
1. What is stabilized chlorine dioxide - ClO2?
Like ozone and chlorine, chlorine dioxide is an oxidizing biocide rather than a metabolic toxin. This means that chlorine dioxide kills microorganisms by interrupting the transport of nutrients to the cell wall rather than interrupting a metabolic process.
The stabilized chlorine dioxide is buffered ClO2 in aqueous solution. Adding an acid to the required concentration activates the disinfectant.
2. How does it work?
Of the oxidizing biocides, chlorine dioxide is the most widely used oxidant. Both ozone and chlorine are much more reactive than chlorine dioxide and will be consumed by most organic compounds. However, chlorine dioxide only reacts with reduced sulfur compounds, secondary and tertiary amines and some other extremely reduced and reactive organics. This allows for much lower doses of chlorine dioxide to achieve a more stable stock than chlorine or ozone. Properly produced chlorine dioxide (not all chlorine dioxide products are the same) can be used effectively at a much higher organic charge than ozone or chlorine due to its selectivity.
3. How effective is it?
The effectiveness of chlorine dioxide is at least as high as chlorine, albeit at lower concentrations. But there are more and more significant advantages.
1. The bactericidal yield is relatively unaffected by pH values between 4 and 10.
2. Chlorine dioxide is clearly superior to chlorine in the destruction of seeds, bacteria, viruses and other pathogens on an equal residual basis.
3. The required contact time for ClO2 is shorter.
4. Chlorine dioxide has better solubility.
5. No corrosion associated with high concentrations of chlorine. Reduces long-term maintenance costs.
6. Chlorine dioxide does not react with NH3 or NH4 +.
7. It destroys THM precursors and increases coagulation.
8. ClO2 destroys phenols and has no distinct odor.
9. It is better at removing iron and magnesium compounds from chlorine, especially complex molecules.
4. How is it implemented?
Chlorine dioxide can be produced on site by special certified equipment.
Stabilized Chlorine Dioxide (SCD) is produced whenever it is needed. It can be dosed in an existing or new procedure where disinfection is required. This makes it easy to use, safe and a versatile disinfectant.
5. Where does it apply?
A) Drinking water disinfection
Chlorine dioxide - ClO2 has been used for years in the disinfection of drinking water (USA since 1944). The need arose when it was discovered that chlorine and similar products formed certain DPD (residual byproducts) such as THM (trihalomethane).
Since then, many aquaculture companies based in the UK and US have started using ClO2. However there are more reasons for using chlorine dioxide:
1. The bactericidal yield is relatively unaffected by pH values between 4 and 10.
2. Chlorine dioxide is clearly superior to chlorine in the destruction of seeds, bacteria, viruses and other pathogenic organisms on an equal residual basis.
3. The required contact time for ClO2 is shorter.
4. Chlorine dioxide has better solubility.
5. No corrosion associated with high concentrations of chlorine. Reduces long-term maintenance costs.
6. Chlorine dioxide does not react with NH3 or NH4 +.
7. It destroys THM precursors and increases coagulation.
8. ClO2 destroys phenols and has no distinct odor.
9. It is better at removing iron and magnesium compounds from chlorine, especially complex molecules.
B) Washing vegetables
Chlorine dioxide - ClO2 is an excellent product for washing vegetables. The ability to kill spores, viruses and fungi at low concentrations is essential.
ClO2 is a proven product that can be used to solve several food-related problems. It does not affect the taste, smell or appearance. It is safe to use and complies with food regulations. Below are some examples where chlorine dioxide has been applied.
1. Apples: control of E. coli and Listeria
2. Potatoes: protection against 'late-seed' and 'silver dandruff'
3. Lettuce, celery and onions: compared to sodium hypochlorite, vitamin C content increased and potassium content decreased
4. Citrus fruits: protection against rot ("green mold" and "sour rot") have proven successful at various pH values, low concentrations and limited contact time.
C) Biofilm removal and control
A biofilm is a layer of microorganisms contained in a matrix (slime layer), which forms on surfaces in contact with water. Incorporation of pathogens in biofilms can protect the pathogens from concentrations of biocides that would otherwise kill or inhibit those organisms freely suspended in water.
Biofilms provide a safe haven for organisms like Listeria, E. coli and legionella where they can reproduce to levels where contamination of products passing through that water becomes inevitable.
It has been proven beyond doubt that chlorine dioxide removes biofilm from water systems and prevents it from forming when dosed at a continuous low level. Hypochlorite on the other hand has been proven to have little effect on biofilms.
D) Prevention and control of Legionella
In the prevention and control of legionnaires disease causing microbes, chlorine dioxide has taken an eminent roll. The specific characteristics of the disinfectant make sure ClO2 gets the job done where others fail.
Biofilm in the piping can protect legionella from most of the disinfectants.
Chlorine dioxide however removes the biofilm and kills the bacteria, spores and viruses.
E) Cooling towers treatment
Cleaning and disinfecting cooling towers is essential for several reasons. Most of which are well known. Clean pipes mean higher heat exchange efficiency, pump lifetime improvement and lower maintenance costs.
Most people however, are unfamiliar with the fact that cooling towers pose a possible health risk. The high temperature condition is ideal for the growth of several pathogen organisms (like legionella).
The usage of chlorine dioxide comes with several advantages:
a) It is a very powerful disinfectant and biocide
b) It prevents and removes biofilm
c) Unlike chlorine, Chlorine dioxide is effective at pH between 4 and 10. No dumping and filling with fresh water required
d) The corrosive effects of chlorine dioxide are minimal compared to the corrosive effects of plain tap water
e) The bactericidal efficiency is relatively unaffected by pH values between 4 and 10. Acidisation, therefore is not required.
f) Chlorine dioxide can be used as a spray. All parts therefore, can easily be reached.
g) And last but not least: less environmental impact.
f) Scrubbers
Scrubbers are similar in design to cooling towers. The primary difference between the two is that scrubbers are pressurized systems, while cooling towers are vacuum systems. Scrubber's re-circulate water and spray it across the top of the system, counter-currently to the airflow. The function of re-circulating water is to absorb odour-causing species from the air.
Chlorine dioxide added to the re-circulated water reacts rapidly with odour-causing species that have been absorbed in the water, as well as those species that remain in the air. Usually, a very low chlorine dioxide residual, around 0.2-ppm, is sufficient to ensure odour control.