ClO2
ClO2
MINIMIZE CHEMISTRY AND OPTIMIZE DISINFECTION
One alternative to bleach is chlorine dioxide (ClO2). Chlorine dioxide was first used to replace chlorine as a pulp bleaching aid, because of the formation of toxic chlorinated by-products from chlorine use (equation 3a). Chlorine dioxide does not chlorinate organic molecules, but instead oxidizes them (equation 3b).
HOCl + Organic Molecule > Chlorinated Organic Molecule (3a)
ClO2 + Organic Molecule > Oxidized Organic Molecule (3b)
As an example of this important chemical difference, Table 1 provides the biocidal efficacy and trihalomethane (THM) formation of HOCl and ClO2 in a side-by-side pear dump tank study.
There are two primary points to learn from this study. First, ClO2 at 1/10th the dose of chlorine gives better performance for minimization of penicillium and yeast populations in the dump tank water. Three part per million of ClO2 outperformed 30 ppm of NaOCl. This is consistent with numerous studies in drinking water and cooling water industries, showing that ClO2 is less consumed than chlorine by the organic debris in the water.
Secondly, the THM level in the chlorinated dump tank was 2549 ppb, of which 2540 ppb was in the form of chloroform (a hazardous chlorinated organic by-product). The THM level in the ClO2-treated dump tank was 154 ppb. It is likely that the only 154 ppb THM level was a result of residual chlorine in the water used to fill and maintain the level in the dump tank.
Table 1. Chlorine dioxide is less affected by organics than chlorine and does not form chlorinated organics.
Some of the biggest concerns with ClO2 relate to its gaseous nature in water. However, when properly understood, this attribute can provide the industry with numerous benefits. As a dissolved gas in water, ClO2 does not take part in the acid-base chemistry that dictates the effectiveness of chlorine. Instead, ClO2 will provide the same degree of antimicrobial efficacy
regardless of the pH. This allows ClO2 to be used in very low pH dump tanks to de-scale apples, under conditions where chlorine cannot be used due to “gassing off” of toxic Cl2 gas.
The biggest challenge with the use of ClO2 is that it cannot be purchased in a drum like bleach—it must be generated on-site. Most traditional ClO2 generation systems produce a mixture of ClO2 and sodium chlorite, the starting material that is not completely converted to ClO2 by the generator. Using this method, both ClO2 and the salt precursor are added to the water system.
However, new generation systems are coming into the market that produce a pure, salt-free form of ClO2, allowing for maximum disinfection, minimal chemical and salt addition to the water, and minimal formation of potentially hazardous by-products.
SAFETY ISSUES RELATED TO THE USE OF CHLORINE AND CHLORINE DIOXIDE
There have been numerous instances of misapplication of chlorine and chlorine dioxide in the tree fruit industry that have resulted in “gassing off” of the oxidants, where a cloud of noxious gas rapidly fills an entire packing house room. Most of the issues with “gassing off” pertain to water temperature, pH, and oxidant dosage.
The most common occurrence of “gassing-off” with chlorine or bleach is when the pH of a dump tank water system is rapidly reduced with acid in order to use the tank for de-scaling hardness from the surface of apples. Recall from equation 2 that when the acidity is increased, the chemical equilibrium shifts to the formation of HOCl. Equation 4 shows another reaction that occurs at low pH—the release of chlorine gas, Cl2, from solution.
HOCl + HCl > Cl2 (gas) + H2O (4)
As the pH drops, Cl2 gas can form, escape from solution, and result in the release of strong noxious chlorine odors into the packing house. In addition, a majority of the chlorine added to a dirty dump tank is transformed into chloramine (equation 5) through reactions with natural amine-containing organic molecules in the water.
HOCl + Amine > Chloramine + H2O (5)
Chloramines are much more volatile than chlorine, and are readily “gassed off” at reduced pH.
Being a dissolved gas in water, chlorine dioxide is much more volatile than chlorine or chloramines. In addition, gas-phase ClO2 is comparably hazardous and noxious as chlorine. For this reason, the primary mechanisms for ClO2 to “gas off” must be well understood in order to maintain a safe and effective ClO2 application.
First, ClO2 should never be applied to a heated water system, such as an apple dump tank that is being maintained at >80 ºF. As a dissolved gas in water, ClO2 is rapidly released from solution with increasing temperature. For effective microbial control, any biocide must be maintained at a sufficient residual. At dump tank water temperatures >80 ºF, ClO2 cannot be maintained at a sufficient residual in the water system. Attempts to do so will result in “gassing off” of the majority of the ClO2 and a release of noxious gas into the packing house. Chlorine dioxide residuals of 0.5 to 1.0 ppm can be safely maintained at 70 ºF, while ClO2 residuals of 3 to 5 ppm can be safely maintained in 50 ºF water.
Secondly, chlorine dioxide will “gas off” from a water system if the concentration in the water gets too high. A situation that can cause a high ClO2 concentration is when a neutral pH dump tank is rapidly acidified for the purpose of de-scaling apples. The mechanism of “gassing off” is different than that discussed previously for chlorine. As was stated before, ClO2 is stable in water
regardless of the pH. However, ClO2 can be regenerated within the acidic dump tank water through catalytic reactions (equations 6a and 6b).
ClO2 + Microorganism > Dead Microorganism + ClO2- (6a)
ClO2- + H+ (acidity) > ClO2 (6b)
When ClO2 reacts in water the majority of it is converted into the chlorite anion (ClO2-), equation 6a. Over time the water system builds up a chlorite residual. Upon acidification of the dump tank, the residual chlorite reacts with acid to regenerate ClO2 within the dump tank (equation 6b). If the chlorite residual was high and if the pH was rapidly dropped from 7 to 2, then the reaction in equation 6b will increase the ClO2 concentration that may result in a “gassing off” situation. Because the rate of reaction 6b is slow, ClO2 may continue to “gas off” for hours or days, depending on the chlorite residual in the water.
In order to avoid this, the pH of a ClO2-treated dump tank should be decreased gradually in accordance with on-line ORP monitoring. As acid is added, more ClO2 is regenerated in the water, which will increase the on-line ORP value. After a few minutes the ClO2 concentration will decrease again, due to reaction 6a, and more acid can be added to bring the ORP reading back up. Small intermittent additions of acid in unison with the resulting ORP readings will allow for a safe transition of the dump tank from neutral to acidic. Once the desired acidic pH is reached, ClO2 can be used without concerns of “gassing off” despite the low pH condition. In fact, a reduced feed rate of ClO2 can be used under low pH conditions, due to the catalytic nature of reactions 6a and 6b.
Finally, unlike chlorine, ClO2 is excellent at removing biofilms, the microorganism populations living on the walls of all water systems. Typically, >99% of all microorganisms live on wetted surfaces and not in the bulk water. Chlorine is consumed by indiscriminant chemical reactions on the surfaces of biofilms, with most of those reactions having nothing to do with killing microorganisms. Most reactions of chlorine are with the polysaccharide protective coating that covers and protects biofilms. This is why it takes such a high dose of chlorine to provide effective microbial control, and why such high concentrations of chlorinated organics are formed during chlorination of “dirty water” systems. Because chlorine cannot effectively penetrate biofilms, a chlorine residual can be maintained in a “dirty water” system despite the presence of a healthy biofilm hiding beneath its slimy polysaccharide surface.
On the other hand, ClO2 is much more selective in the chemical reactions it undertakes, giving it the ability to penetrate biofilms in search of certain chemical functional groups. In a very simplistic view, Figure 2 shows how ClO2 can penetrate a biofilm to attack microorganisms with targeted reactions on sulfide-containing amino acids or protein disulfide linkages. These targeted reactions allow for effective biofilm destruction at very low ClO2 doses. Since ClO2 is so effective at reacting with and removing biofilms, it may require a long time and a high dosage of ClO2 to completely remove an existing biofilm and to achieve a ClO2 residual. Therefore,
another mechanism of how ClO2 may “gas-off” from a dump tank water system involves setting a ClO2 dosing rate that for the first few hours of a day does not provide a residual, despite a constant ClO2 feed. Once the biofilm has been significantly removed, suddenly the ClO2 concentration will rapidly increase in accordance with the feed rate. Through the use of on-line ORP monitoring, this “gassing off” mechanism will be negated. ORP monitoring will detect the rapid increase in ClO2 concentration and turn off the metering pump before an unsafe ClO2 concentration can build.
ClO2
MINIMIZE CHEMISTRY AND OPTIMIZE DISINFECTION
One alternative to bleach is chlorine dioxide (ClO2). Chlorine dioxide was first used to replace chlorine as a pulp bleaching aid, because of the formation of toxic chlorinated by-products from chlorine use (equation 3a). Chlorine dioxide does not chlorinate organic molecules, but instead oxidizes them (equation 3b).
HOCl + Organic Molecule > Chlorinated Organic Molecule (3a)
ClO2 + Organic Molecule > Oxidized Organic Molecule (3b)
As an example of this important chemical difference, Table 1 provides the biocidal efficacy and trihalomethane (THM) formation of HOCl and ClO2 in a side-by-side pear dump tank study.
There are two primary points to learn from this study. First, ClO2 at 1/10th the dose of chlorine gives better performance for minimization of penicillium and yeast populations in the dump tank water. Three part per million of ClO2 outperformed 30 ppm of NaOCl. This is consistent with numerous studies in drinking water and cooling water industries, showing that ClO2 is less consumed than chlorine by the organic debris in the water.
Secondly, the THM level in the chlorinated dump tank was 2549 ppb, of which 2540 ppb was in the form of chloroform (a hazardous chlorinated organic by-product). The THM level in the ClO2-treated dump tank was 154 ppb. It is likely that the only 154 ppb THM level was a result of residual chlorine in the water used to fill and maintain the level in the dump tank.
Table 1. Chlorine dioxide is less affected by organics than chlorine and does not form chlorinated organics.
regardless of the pH. This allows ClO2 to be used in very low pH dump tanks to de-scale apples, under conditions where chlorine cannot be used due to “gassing off” of toxic Cl2 gas.
The biggest challenge with the use of ClO2 is that it cannot be purchased in a drum like bleach—it must be generated on-site. Most traditional ClO2 generation systems produce a mixture of ClO2 and sodium chlorite, the starting material that is not completely converted to ClO2 by the generator. Using this method, both ClO2 and the salt precursor are added to the water system.
However, new generation systems are coming into the market that produce a pure, salt-free form of ClO2, allowing for maximum disinfection, minimal chemical and salt addition to the water, and minimal formation of potentially hazardous by-products.
SAFETY ISSUES RELATED TO THE USE OF CHLORINE AND CHLORINE DIOXIDE
There have been numerous instances of misapplication of chlorine and chlorine dioxide in the tree fruit industry that have resulted in “gassing off” of the oxidants, where a cloud of noxious gas rapidly fills an entire packing house room. Most of the issues with “gassing off” pertain to water temperature, pH, and oxidant dosage.
The most common occurrence of “gassing-off” with chlorine or bleach is when the pH of a dump tank water system is rapidly reduced with acid in order to use the tank for de-scaling hardness from the surface of apples. Recall from equation 2 that when the acidity is increased, the chemical equilibrium shifts to the formation of HOCl. Equation 4 shows another reaction that occurs at low pH—the release of chlorine gas, Cl2, from solution.
HOCl + HCl > Cl2 (gas) + H2O (4)
As the pH drops, Cl2 gas can form, escape from solution, and result in the release of strong noxious chlorine odors into the packing house. In addition, a majority of the chlorine added to a dirty dump tank is transformed into chloramine (equation 5) through reactions with natural amine-containing organic molecules in the water.
HOCl + Amine > Chloramine + H2O (5)
Chloramines are much more volatile than chlorine, and are readily “gassed off” at reduced pH.
Being a dissolved gas in water, chlorine dioxide is much more volatile than chlorine or chloramines. In addition, gas-phase ClO2 is comparably hazardous and noxious as chlorine. For this reason, the primary mechanisms for ClO2 to “gas off” must be well understood in order to maintain a safe and effective ClO2 application.
First, ClO2 should never be applied to a heated water system, such as an apple dump tank that is being maintained at >80 ºF. As a dissolved gas in water, ClO2 is rapidly released from solution with increasing temperature. For effective microbial control, any biocide must be maintained at a sufficient residual. At dump tank water temperatures >80 ºF, ClO2 cannot be maintained at a sufficient residual in the water system. Attempts to do so will result in “gassing off” of the majority of the ClO2 and a release of noxious gas into the packing house. Chlorine dioxide residuals of 0.5 to 1.0 ppm can be safely maintained at 70 ºF, while ClO2 residuals of 3 to 5 ppm can be safely maintained in 50 ºF water.
Secondly, chlorine dioxide will “gas off” from a water system if the concentration in the water gets too high. A situation that can cause a high ClO2 concentration is when a neutral pH dump tank is rapidly acidified for the purpose of de-scaling apples. The mechanism of “gassing off” is different than that discussed previously for chlorine. As was stated before, ClO2 is stable in water
regardless of the pH. However, ClO2 can be regenerated within the acidic dump tank water through catalytic reactions (equations 6a and 6b).
ClO2 + Microorganism > Dead Microorganism + ClO2- (6a)
ClO2- + H+ (acidity) > ClO2 (6b)
When ClO2 reacts in water the majority of it is converted into the chlorite anion (ClO2-), equation 6a. Over time the water system builds up a chlorite residual. Upon acidification of the dump tank, the residual chlorite reacts with acid to regenerate ClO2 within the dump tank (equation 6b). If the chlorite residual was high and if the pH was rapidly dropped from 7 to 2, then the reaction in equation 6b will increase the ClO2 concentration that may result in a “gassing off” situation. Because the rate of reaction 6b is slow, ClO2 may continue to “gas off” for hours or days, depending on the chlorite residual in the water.
In order to avoid this, the pH of a ClO2-treated dump tank should be decreased gradually in accordance with on-line ORP monitoring. As acid is added, more ClO2 is regenerated in the water, which will increase the on-line ORP value. After a few minutes the ClO2 concentration will decrease again, due to reaction 6a, and more acid can be added to bring the ORP reading back up. Small intermittent additions of acid in unison with the resulting ORP readings will allow for a safe transition of the dump tank from neutral to acidic. Once the desired acidic pH is reached, ClO2 can be used without concerns of “gassing off” despite the low pH condition. In fact, a reduced feed rate of ClO2 can be used under low pH conditions, due to the catalytic nature of reactions 6a and 6b.
Finally, unlike chlorine, ClO2 is excellent at removing biofilms, the microorganism populations living on the walls of all water systems. Typically, >99% of all microorganisms live on wetted surfaces and not in the bulk water. Chlorine is consumed by indiscriminant chemical reactions on the surfaces of biofilms, with most of those reactions having nothing to do with killing microorganisms. Most reactions of chlorine are with the polysaccharide protective coating that covers and protects biofilms. This is why it takes such a high dose of chlorine to provide effective microbial control, and why such high concentrations of chlorinated organics are formed during chlorination of “dirty water” systems. Because chlorine cannot effectively penetrate biofilms, a chlorine residual can be maintained in a “dirty water” system despite the presence of a healthy biofilm hiding beneath its slimy polysaccharide surface.
On the other hand, ClO2 is much more selective in the chemical reactions it undertakes, giving it the ability to penetrate biofilms in search of certain chemical functional groups. In a very simplistic view, Figure 2 shows how ClO2 can penetrate a biofilm to attack microorganisms with targeted reactions on sulfide-containing amino acids or protein disulfide linkages. These targeted reactions allow for effective biofilm destruction at very low ClO2 doses. Since ClO2 is so effective at reacting with and removing biofilms, it may require a long time and a high dosage of ClO2 to completely remove an existing biofilm and to achieve a ClO2 residual. Therefore,
another mechanism of how ClO2 may “gas-off” from a dump tank water system involves setting a ClO2 dosing rate that for the first few hours of a day does not provide a residual, despite a constant ClO2 feed. Once the biofilm has been significantly removed, suddenly the ClO2 concentration will rapidly increase in accordance with the feed rate. Through the use of on-line ORP monitoring, this “gassing off” mechanism will be negated. ORP monitoring will detect the rapid increase in ClO2 concentration and turn off the metering pump before an unsafe ClO2 concentration can build.
