Facilities Increase Profits Through Use of Chlorine Dioxide
Dr. Greg Simpson has been a big asset to PureLine for several years. He has made us better. He has made us smarter. We are honored to have Greg as a member of our team. Greg wrote this article several years ago with William S. Pickrell, Mark Germer and Jennifer Miller, Ph.D.. We hope you find this information interesting. As always, please remember PureLine for all your Chlorine Dioxide needs. PureLine is your ClO2 GO2 company. If you would like a full re-print of this article (Complete with references) please contact PureLine.
Abstract: Proper cooling is critical to the safe and profitable operation of industrial plants. Critical exchangers are frequently elevated or lie at or near the end of the cooling water circuit, i.e., the cooling water flow is reduced. These exchangers require good heat transfer.
Water-side heat transfer reduction results primarily from two factors, the formation of insulating scale and fouling deposits. Insulating scale is formed by the regular deposition in a predetermined crystal lattice structure of a marginally soluble salt that has approached its solubility limit. This solubility limit is a function of several factors, among which are concentration of salt forming species, pH, and temperature. Scale can be very difficult to remove and is in general very dense. The control of insulating scale is accomplished by maintaining the pH in a relatively non-scaling range, or incorporating the newest technology in scale control chemicals, or both.
A fouling deposit is formed when salts deposit in an amorphous way, that is, there is no definite crystal lattice structure formed. The origin of a fouling deposit often can be traced back to the formation of bacterial biofilm and is perhaps a more serious but less obvious problem.
Chlorine has been the primary biocide used for control of bacteria and its associated biofilm for industrial cooling systems. It performs well and is usually economical. Historically, unless there were economic or performance reasons, alternatives to chlorine were not considered.
The use of gaseous chlorine for industrial microbiological control is declining for a number of reasons. Various alternatives have been explored, including bleach, bleach with bromide, solid bromine release materials, non-oxidizing biocides, ozone, and chlorine dioxide, among others.
Chlorine dioxide offers some unique advantages, due to its reduced dosage requirements, effectiveness over a wide pH range, and speed of disinfection. These performance characteristics make it ideally suited for microbiological control in many industrial cooling systems, particularly those that are contaminated with organics or ammonia. One of its best performance characteristics is the ability to disperse, dissolve, and remove bacterial biofilm.
This paper discusses the use of chlorine dioxide for control of bacteria and the associated biofilm in industrial cooling systems. This paper describes the problems associated with biofilm and compares the effectiveness of chlorine vs. chlorine dioxide in its control and removal. Several case histories are presented which illustrate these exceptional characteristics for ‘stressed’ cooling systems, e.g., those systems that are contaminated with a high loading of organics, a high level of amine or ammonia, or a high pH. The economic benefits are described.
Problems associated with bacterial growth: Bacterial growth causes several problems in industrial plants. These problems can be split into two general categories: those that directly impact profitability and those that indirectly impact the profitability of a plant.
Problems Which Have Direct Impact: Problems which impact plant profitability directly can be broken into two types; those with immediate impact and those with long term impact.
Problems that have immediate impact includes such things as the growth of bacterial biofilm. Biofilm causes restrictions in heat transfer,1 which can result in production losses. (Cooling water exchangers normally have a built in excess capacity that allows for some fouling.) This problem is one that has a direct and immediate impact, and can become apparent once the restriction in heat transfer is removed.
Relatively sudden changes, i.e., changes over a day or so, can become apparent by observing such things as production rates, process temperatures out of critical exchangers, and cooling water basin temperatures. The dollar value associated with this problem changes with time, as it can be a function of many things, including the value of the process stream being cooled, whether the process stream is used internally by another unit, and the relative costs of replacing the process stream being treated. In general, it is easier to determine the economic impact of this type of problem than that of a problem such as corrosion.
Corrosion, a problem which directly impacts economics over a long term, reduces the life of operating equipment. Corrosion can be caused directly by bacterial action, and by action of bacterial enzymes, the effects of which can survive cell death. Because biofilm tends to be patchy and non-uniform over the surface, the differences in oxygen concentration at the metal surface result in corrosion.4 It is difficult to assign a dollar value to this type of problem, as a prediction of equipment life under more favorable circumstances is required.
Problems Which Have Indirect Impact: A problem on the horizon for large industrial facilities is the liability which may result from the potential health risks to workers of a poorly disinfected cooling system. Legionella, commonly found in many cooling towers,5 has been identified as the cause of Legionnaires’ Disease.6 Legionnaires’ Disease occurs primarily in people with compromised immune system, such as organ- transplant patients, people with AIDS, and cancer patients receiving chemotherapy. The mode of transmission of Legionnaires’ Disease is thought to be by inhalation of water vapor that is contaminated by these bacteria. This includes poorly disinfected cooling water. 7
Legionella are by nature sessile bacteria. In the bulk water they are relatively easy to disinfect. They thrive in biofilm and in certain protozoa.8 Thus, Legionella can proliferate in poorly disinfected cooling water. The microbiological control agent chosen for such towers must be able to penetrate the biofilm, remove the biofilm, and prevent the formation of biofilm. Since protozoa can be thought of as microscopic cows, which ‘graze’ on the biofilm, controlling the biofilm will limit the food source to higher life forms, and they will not proliferate under such circumstances.
About 2.4 million cases of pneumonia (Ref 9) occur annually in the United States. Of these some 10,000 to 100,00010 are actual cases of Legionnaires’ disease. However, according to the Center for Disease Control in Atlanta only 1,000 to 3,000 are reported. (Ref 11) The reason they are not reported is the lack of a rapid, suitable, reliable test for infection.( Ref 12)
There is a relatively high mortality rate for those who contract Legionnaires’ disease, even if the disease is identified and suitable antibacterial medication is provided. (Ref 13,14) Even healthy individuals with immune systems which are not compromised may have some lung damage as a result of inhaling water vapor contaminated with Legionella. This is because Legionella are commonly found in clumps. If one of these clumps is inhaled, and the clump settles in the lung, a white blood cell may attempt to engulf this invader. If the clump is too big to engulf, the white blood cell backs off and produces enzymes to deactivate the clump. The clump is eventually inactivated, but the surrounding lung tissue is also destroyed. (Ref 14) Thus even healthy individuals can have lung damage from being around poorly disinfected cooling towers.
Numerous law suits have been filed over Legionnaires’ disease. (Ref 15) One such action was recently brought by victims of this disease involving a small HVAC cooling tower located on top of a building. The building owners and the water treatment service company have been cited as defendants.16 Although this specific case involved a very small cooling system by industrial standards, it is only a matter of time before such actions move to the heavy industrial arena.
In many heavy industrial plants, process leaks can result in microbiological growth that is difficult to control at best. In many of these cases, the best than can be done normally with conventional microbiological control methodologies is to maintain status quo, battling bacterial growth in the bulk water, with the biofilm remaining healthy and supporting a complex community of bacteria.
The decision to repair or replace a leaking bundle is a balancing act, where production losses that would be incurred from a shutdown to repair the bundle must be weighed against other factors. These other factors have traditionally included the potential of recovering the costs associated with shutdown through an increase in production resulting from repair or replacment of the bundle, increases in costs of chemical treatment to maintain microbiological control during a process leak if the bundle is allowed to operate in a leaking mode, and safety factors associated with a large hydrocarbon leak. With the increasing knowledge of adverse health effects of a Legionella-contaminated cooling system, the potential health risks to those exposed to airborne water vapor from such a tower must now be considered in these ‘other factors’.
For water treatment service companies, the decision as to which microbiological control methodologies are recommended to an end user must be dictated by a consideration as to which chemistry offers the best available technology for Legionella control. Only in this way can the water treatment service company be protected.
II. Problems with Chlorine Usage: Chlorine is a relatively strong oxidant. It is reactive with many organics. (Ref 17) In the pulp and paper industry, there is a trend away from use of chlorine as the primary bleaching agent, due to the formation of chlorinated organics, in particular chlorinated dioxins and furans. (Ref 18) In some cooling systems the use of chlorine is restricted due to the reaction of chlorine with organics in the makeup water supply and the formation of chlorinated organics which results. (Ref 19) In addition, chlorine and/or bromine is objectionable in some systems due to the formation of trihalomethanes, THMs. (Ref 20)
Another problem associated with the use of chlorine is corrosion. (Ref 21) Systems which have leaks can be especially troublesome. Small hydrocarbon leaks increase the demand for chlorine, directly and indirectly, through the increase of bacterial activity. The increase in chlorine which is required also results in an increase in corrosion. (Ref 22) In systems with makeup of low alkalinity, increases in chlorine feed can have drastic impact on the pH, due to the formation of strong mineral acid during the hydrolysis reaction of molecular chlorine.
Chlorine, while acceptable as a disinfectant at neutral pHs, becomes less effective with increasing pH. (Ref 23) In addition, in some cases chlorine has been shown to be ineffective at controlling biofilm. (Ref 24)
In one study, biofilm was grown and an oxidation-reduction potential microprobe was inserted into the biofilm. (Ref 25) The ORP was measured as the microprobe was inserted slowly into the biofilm. The results clearly indicate that chlorine does not penetrate very far into the biofilm. This work suggests that a weaker, less reactive oxidant will be better at biofilm penetration than one which is more reactive. For example, chloramines have been cited as being somewhat better than chlorine for biofilm removal, under some conditions. (Ref 26)
III. Chlorine Dioxide for Microbiological Control: Chlorine dioxide, a gas at ambient temperatures, is very soluble in water. ClO2 has an odor similar to that of chlorine, and the aqueous solubility of ClO2 is significantly greater than that of chlorine. For stability reasons, the gas is rarely used directly as a disinfectant. Instead, it is produced and used as a dilute aqueous solution.
ClO2 is made by oxidizing chlorite ion, ClO2-, or reducing chlorate ion, ClO3-. For small applications, ClO2 is made almost exclusively by oxidation of chlorite ion, normally involving chlorine, either gaseous molecular chlorine (Equation 1) or oxidation of chlorite with hypochlorous acid (Equation 2).
Cl2 + 2 ClO2- > 2 ClO2 + 2Cl- Equation 1
NaOCl + 2ClO2- + 2HCl > 2ClO2 + 3NaCl + H2O Equation 2
The chemistry of generation,27 equipment for generation,28 and safety aspects surrounding producing aqueous ClO2 (Ref 29) are described in detail elsewhere.
Chlorine dioxide has achieved fairly widespread usage in the pulp and paper industry for pulp bleaching because it imparts a higher degree of brightness to the paper without degrading the fibers. (Ref 30) It has also seen increasing usage as an alkaline whitewater biocide for control of biofilm. (Ref 31) One reason it is used extensively in the pulp and paper industry is the very positive environmental benefits.32 Paper mills which have a high percentage of replacement of chlorine by chlorine dioxide for pulp bleaching have seen a dramatic decrease in the amount of chlorinated organics formed during the bleaching process. (Ref 33) In addition, the type of chlorinated organics is different from that produced when chlorine is used. (Ref 34)
ClO2 reacts with far fewer organics than chlorine does, (Ref 35) and it does not directly chlorinate organics. (Ref 36) Some of the reasons for the superior performance of ClO2 is illustrated in Table 1. Table 1 shows the relationship between oxidation potential and oxidation capacity of commonly used oxidants. Oxidation potential can be thought of as a measure of how strongly an oxidant reacts with an oxidizable material. (This is not strictly rigorous because kinetics are not included.) However, the trend appears to be generally true.
Table 1: Characteristics of Common Oxidants
Oxidant
Formula
Oxidation Reduction Potential
Oxidation Capacity
Ozone
O3
2.07
2e-
Hydrogen Peroxide
H2O2
1.78
2e-
Chlorine Dioxide (v)
ClO2 (v)
1.56
Hypochlorous Acid
HOCl
1.49
2e-
Hypobromous Acid
HOBr
1.33
2e-
Chlorine Dioxide (aq)
ClO2 (aq)
0.95
5e-
For example, ozone is the most powerful oxidant used. ClO2 gas is also a strong oxidant, having an oxidation potential of 1.56V. However, in aqueous solution, it is significantly weaker as an oxidant, having an oxidation potential of only 0.95V. This has led to considerable confusion in the literature, as some have reported that ClO2 is a ‘stronger’ oxidant than chlorine, which has an oxidation potential of 1.49V.
It is possible that those reporting this have confused oxidation potential with oxidation capacity. Oxidation potential can be thought of as a measure of aggressive an oxidant is in its reaction chemistry. Oxidation capacity, on the other hand, has to do with how many electrons can be accepted by an oxidant in a reaction. The oxidation capacity of ClO2 is reported to be about 2.5 times that of other commonly used oxidants, because it has the capacity to accept up to 5 electrons in a reaction, unlike other commonly used oxidants, which generally accept up to two. For this reason, ClO2 is said to have 250% of the oxidation capacity of free chlorine (263% when molecular weight differences are considered).
ClO2 has been shown to be significantly less corrosive than chlorine in industrial cooling systems. (Ref 37)
One of the most significant benefits of use of ClO2 for microbiological control is the impact it has on microbial biofilm. It is not only a very effective, highly selective disinfectant, but also, due to its high selectivity, is very effective as a biofilm control/removal agent. When fed at about 1 – 2 ppm based on recirculation rate, it reacts rapidly with the biological material in the bulk water. If ClO2 is fed directly to pump suction and there is a significant demand in the bulk water, a ClO2 residual will not be observed in the pump discharge until the demand in the bulk water has been satisfied. Once this has happened, ClO2 in the bulk water will move through the system with something approaching plug flow, reacting with, dispersing, and removing the vast majority of biofilm in the system. Once the surfaces are clean the residual in the return will approach 50 – 80% of that in the pump discharge.
The following case histories serve as examples of systems to which ClO2 has been applied. In each case, the application of ClO2 has been accompanied by a significant, measurable change in the system.
IV. Case Histories: In some cases the application of ClO2 to problem cooling towers can have dramatic, immediate results, depending upon the strategy of treatment. In the following case histories, certain instances of cooling systems where the removal and control of biofilm had immediate impact on plant economics are described. Where possible, a dollar value is assigned to the impact. Sometimes the dollar value is related to an increase in throughput. In other examples it may be due to a reduction in costs of biological control.
Case History 1 – Gulf Coast Petrochemical Plant
Plant Overview: A processing unit in this petrochemical plant utilized a cross- flow cooling tower with conventional wood slat fill for process cooling. The recirculation rate was 45,500 gpm. Chlorine gas was used at an average rate of 600 lb./day, with feed of supplemental bromide. A chronic leak of process contaminants into the cooling water caused rapid microbiological growth.
Conditions Pre-ClO2: The following sections describe the conditions the plant experienced prior to use of ClO2.
Corrosion Before ClO2: The high chlorine / bromide feed rate coupled with a fairly massive growth of healthy biofilm throughout the system resulted in excessive corrosion rates of around 10 mpy on carbon steel with a pitting index of routinely > 20 and
frequently reaching 50-60, as measured by a continuously recording corrosion rate meter. This corrosion rate was considered by plant personnel to be better than when chlorine was used alone.
The healthy biofilm present throughout the system created an optimum environment for the growth of sulfate-reducing bacteria, SRB. These bacteria are notorious for causing corrosion directly. Their presence resulted in a substantial decrease in equipment life. SRB were measured routinely in the bulk water. During the recent turnaround, one of the process stream condensers was replaced after a life of 15 years.
Cooling Tower Efficiency before ClO2: As a result of rapid microbiological growth, there was a healthy algal-biofilm mat on the distribution deck. This mat plugged many of the distribution nozzles and as a result caused the water to short-circuit through the tower. Inefficient cooling resulted.
The drift eliminator was also heavily fouled with this algae/biofilm. As a result, airflow was restricted through the tower, causing poor heat transfer and inefficient cooling.
Biofouling of Critical Exchangers before ClO2: Due to the process leak, the performance of the two main process condensers was degrading. Decreased cooling had been observed and frequent backwashing of exchangers was required to remove malodorous SRB biofilm and recover some cooling capacity.
Foaming before ClO2: The rapid growth of microbiological organisms and the feed of chlorine/bromide resulted in troublesome dark foam on the tower deck. The foam had become a chronic problem. Pieces of foam of various sizes would come loose and drift to the ground immediately surrounding the cooling tower, unless there was wind. In the case of wind, the foam would land on workers, workers cars, and any other structure surrounding the tower. As the foam that landed on cars dried, a tenacious, brown deposit that was very difficult to clean remained. Frequent washing of automobiles was required.
Offensive Odors before ClO2: Over time, this foam, containing pieces of detached biofilm which included dead and viable organisms, accumulated adjacent to the cooling tower and began to decay. As this dead biomass decayed, an offensive odor was produced.
Analytical before ClO2: Another consequence of the high levels of organics present in the tower and the presence of aerobic and anaerobic biofilm was that despite the high feedrate of chlorine, no free residual chlorine could be measured. Only total chlorine could be measured.
ClO2 Trial: These conditions had existed so long that plant personnel had learned 8
to live with this problem. It was perceived as the ‘normal operating condition.’ Still, the noxious odors, the annoyance of foam getting on people and cars, the corrosion rate, the reduced heat transfer, and the inability to achieve a free chlorine residual, created an interest in exploring alternatives to chlorine / bromide. A two-month demonstration with ClO2 was proposed.
ClO2 Demonstration Treatment Regimen: Feed of chlorine dioxide was implemented in May of 1998. The treatment regimen included three phases, cleanup, optimization, and maintenance.
During the cleanup portion of the treatment, ClO2 was fed to pump suction at 1 – 2 ppm, based on recirculation rate. During the initial cleanup phase, ClO2 is normally fed either for 8-hour periods, or continuously, depending upon the expectations of plant personnel and conditions. For this plant, an early decision was made to apply ClO2 continuously to expedite cleanup. The cleanup phase lasted 72 hours. During this phase, critical exchangers were backwashed. (Aged biofilm that has crosslinked and become like rubber cannot be dissolved by any oxidant. However, but the anaerobic layer of biofilm underneath this rubber-like material can be dispersed, and the biofilm dislodged. Once this anaerobic layer of biofilm holding the rubbery biofilm to the surface has been removed, backwashing easily removes this material from exchangers. In some cases, backwashing or other sudden changes in flow velocities and/or direction is required to remove the aged biofilm.
Once the cleanup phase was accomplished, the system was treated intermittently for several hours on a daily basis. The process leak provided a continuous food source for bacteria, resulting in a very rapid rebound of bacteria and its associated bilofilm, when ClO2 was used alone. For this reason, a low level of chlorine was fed continuously with an intermittent feed of ClO2 for control of biofilm, odors and foaming.
Currently this plant is using ClO2 for 5 hours/day on a Monday, Wednesday, and Friday schedule, with a small amount on Saturday. Chlorine is fed continuously to maintain a 0.1 – 0.2 ppm free residual.
Conditions with ClO2: Plant personnel have observed a number of benefits resulting from the use of ClO2. These benefits are described below:
Corrosion with ClO2: General corrosion rates, which had been excessively high before application of ClO2, began to trend downward, with the corrosion rate currently < 5 mpy. This reduction was attributed to control of MIC. The pitting index, which had been very high, was reduced to levels more consistent with a well treated cooling system, < 1.
Cooling Tower Efficiency with ClO2: At the start of the proposed demonstration period, the tower basin temperature was > 100 F.
Within 48 hours after the cleanup phase had begun, the basin water temperature had dropped to around 90 F. The ambient temperature was in the mid to high 90s, and humidity was high. Cleaning of exchangers was not all that was accomplished. Otherwise, the temperature of the basin would have increased, as the water going through the exchangers picked up more heat. But the reverse occurred. Several factors were responsible for this decrease in temperature.
The algal mat on the distribution deck blocked many of the distribution nozzles, resulting in a short-circuiting of the water and inefficient air/water contact over the cooling tower. Thus proper cooling was not accomplished. Cleanup of this algal mat allowed the water to be distributed uniformly over the entire deck, allowing all of the nozzles to work properly. Good air/water contact occurred, and an improvement in cooling efficiency was observed.
Thebuildupofbiofilmonthedrifteliminatorrestrictedairflow,andtheairshort- circuited through the tower, preventing good air/water contact. Once this biofilm was removed, the air was drawn through the tower as designed, thus improving efficiency.
After cleanup, plant personnel noticed a slight increase in amps of the fan motors, providing confirmation of improved cooling. This increase was due to the improved operating efficiency of the tower. The wet air resulting from better evaporation was heavier and more difficult to pull than dry air. A small increase in amps of the fan motors would be predicted.
Biofouling of Critical Exchangers with ClO2: Within the cleanup period, the process outlet temperature of one process condenser decreased by about 2 F, and the other showed a decrease of about 4 F. Though not very large, such a decrease in process outlet temperature indicated that more efficient cooling was taking place. Thus, with the condensers operating more efficiently, greater process throughput was achieved.
Foaming with ClO2: When bacteria lyse, or burst open, surface-active chemicals are released into the water. For this reason, a well-disinfected cooling tower normally has a small amount of white foam on the basin. When large amounts of bacteria, in the bulk water, and in the biofilm, are disinfected, a large amount of dark, dirty foam can be produced.
The foaming problem has been brought under control. ClO2 has cleaned the system so that disinfection of large concentrations of bacteria, in the bulk water and in the bacterial biofilm, was not producing the brown, stable massive foam that it once was. Instead, only a small amount of white foam on the basin was being produced.
Offensive Odors with ClO2: The offensive odor due to decaying vegetative matter from the drifting foam has been eliminated in two ways; the ClO2 oxidized the odor-causing species that were produced by decaying biofilm, and the biofilm has been removed. In addition, pools of SRB laden water surrounding the tower have cleared up and have not returned.
Analytical with ClO2: With a much cleaner system, chlorine at around 275 lb/day was sufficient to achieve a free chlorine residual. The measured total chlorine residual became only slightly higher than that of the free chlorine residual.
Economics: Before ClO2, the costs of microbiological treatment were $126,000 annually, and microbiological control was not achieved. Costs of the ClO2 – based program were slightly less than this, with much superior results. Unfortunately, it is difficult to put a dollar value on the increase in production that was achieved, because the product is used internally by another unit. A summary of the results for this case history is shown in Figure 1.
Case History 2 – Midwestern Power Plant
Plant Overview: This Midwestern once-through power utility has one 2400 psig coal-fired boiler which provides steam to a turbine that operates at a capacity of 200 megawatts. The plant can sell all of the power it generates; thus it is critical to keep exchangers free of any heat-transfer limiting biofilm. Cooling water flow is approximately 170,000 gpm.
Conditions Pre-ClO2: The conditions described in the following section are those the plant experienced prior to use of ClO2.
Microbiological Control before ClO2: Microbiological control in this cooling system has been a problem, historically. Another plant, just upstream of this utility, processes food. The wastewater from this plant is discharged into the river. Although the wastewater is treated, it is contaminated with a high loading of microbiological nutrient, which causes extremely rapid bacterial growth.
When chlorine gas was used as the primary microbiological control agent, continuous feed was required. Because chlorine was fed continuously, dechlorination was also required. The feedrate of chlorine was sufficiently large that feed of liquid chlorine was required to get sufficient chlorine into the system to achieve control. Usually 10 one-ton cylinders were kept on site.
When chlorine was applied to surface waters that were contaminated with organics from decaying vegetation, chlorinated organics formed. Concerns over the formation of chlorinated organics resulted in the state requiring a reduction to a 4- hour per day total treatment.
Biofouling of Main Condenser before ClO2: Normal operating backpressure on the turbine was about 2 mm Hg. If the backpressure increased to 6-8 mm, the plant was not able to produce power at the desired level and was derated.
ClO2 Trial: The extremely rapid formation of bacterial biofilm, coupled with the required reduction in chlorine feed, resulted in problems. After the restriction in chlorine feed was imposed, the backpressure increased to levels that were near the limit and the battle to maintain a backpressure which did not require derating was constant and difficult. Alternatives were tried including a move to supplemental bromide that did not help. Several other alternatives were tried but were eliminated. Chlorine dioxide was proposed and an evaluation scheduled.
ClO2 Demonstration Treatment Regimen: Feed of chlorine dioxide was implemented in early 1996. In this case, since the plant was a once-through utility, with minimal residence time, no ‘cleanup’ phase was used. Instead, the treatment regimen involved continuous optimization. ClO2 was fed to the pump suction at slightly over 0.5 ppm (well below the demand in the bulk water) intermittently, for various duration and frequencies of treatments. These various duration and frequencies were evaluated during program optimization, and the optimum treatment in summer months was to feed ClO2 for 4 – 30 minute periods or for 3 – 40 minute periods. Both seemed to work as well. The dosage was reduced somewhat during the colder winter months.
Biofouling of Main Condenser with ClO2: At the optimized level of treatment, microbiological control has been achieved very well, as subsequent inspections have revealed. No plant derating has been required, except for short periods when the generator was out of service for maintenance.
Economics: Total costs of chlorine and dechlorination chemicals were around $350,000 annually. This was the cost for chemicals alone, and did not include any maintenance on chlorinators, dechlorination feed equipment, or other ancillary costs. At $130,000, costs for ClO2 were slightly over 1/3 of the cost of previous treatment.
Ancillary Benefits: As a consequence of this type of treatment regimen, no periodic treatment for zebra mussels has been required, unlike other plants along the river. The summary of this case history is shown in Figure 2.
Case History 3 – Western Ammonia Plant
Plant Overview: The cooling system in this western ammonia plant has a recirculation rate of 39,000 gpm. The tower is cross flow design with conventional slat fill. The cooling tower services the ammonia plant, the urea plant, the CO2 plant, the ammonium nitrate plant, two nitric acid plants, and the refrigeration unit. Microbiological control in this plant had been unsatisfactory since plant startup several decades ago.
Conditions Pre-ClO2: The conditions described in the following section are those experienced by the plant prior to use of ClO2.
Historical Microbiological Control before ClO2: Microbiological control in this plant was extremely difficult. Even with excessive chlorine feedrates performance was poor. The high chlorine feedrate did not control microbiological growth very well at all, and the high feedrates of chlorine depressed pH and increased corrosion.
Chlorine with bromide ion was tried. Conventional non-oxidizing biocides were tried. Neither chlorine alone, nor chlorine coupled with bromide, nor any of the alternatives were effective at controlling microbiological growth to what the plant considered was an acceptable level.
The high levels of ammonia in the tower would frequently overpower the chlorine, and the nitrifying bacteria would grow explosively, resulting in the formation of nitric acid and a severe pH depression.
Cooling Tower Efficiency: This system was similar to that of Case History 1, where the rapid microbiological growth and algae on the distribution deck caused pluggage of many of the distribution nozzles. The water short-circuited through the tower and inefficient cooling resulted.
Airflow was restricted through the tower because the drift eliminator was also heavily fouled with this algae/biofilm. Poor heat transfer and inefficient cooling resulted.
Biofouling of Critical Exchangers: Many exchangers were biofouled to varying degrees. This resulted in heat transfer reduction that in turn caused production losses. These conditions had existed since the plant had been built. Plant personnel had learned to live with this problem although they knew better control was possible.
ClO2 Trial: The problems with microbiological control in this plant resulted in evaluations of almost all alternatives. Chlorine dioxide was presented as an alternative, and a short demonstration was proposed.
ClO2 Demonstration Treatment Regimen: Feed of chlorine dioxide was implemented in 1995. In this system, ClO2 was applied at 1.5 ppm to the existing chlorine header, allowing the ClO2 to ‘sweep the basin.’ In addition, no formal ‘cleanup’ phase was employed. ClO2 was applied for a duration that would be considered normal for such a tower. The demand for ClO2 was so high that the feed duration had to be was increased. Over a period of weeks, the ClO2 feed duration was adjusted upward until a residual in the return could be measured.
Cooling Tower Efficiency: Like that of Case History 1, the temperature changes were somewhat different than expected. With a tower operating in a consistent manner, if the biofilm were removed from an exchanger, one would expect the cooling water temperature out to increase, and with a constant level of cooling, the delta T to increase. However, the delta T decreased. The plant records ‘delta-T’, the difference between the cooling water basin temperature and the temperature out of the main ammonia condenser. The delta T decreased from about 35 °F to 6-9 °F in somewhat over 10 weeks.
Case History 1 and this one illustrate a very important point. One cannot separate the improvements in heat transfer of exchangers from an increase in performance of a cooling tower. Caution must be used when looking at changes in one piece of equipment in a plant. The system as a whole must be considered.
Economics: At one point, the costs for microbiological control were in the neighborhood of $800,000/annually. Although costs have been reduced to less than half of the costs of alternatives, and the results with alternatives was very poor, they are still high when compared to similar systems treated with ClO2. Work is on-going to determine the reason for the extremely rapid recovery of bacterial growth in this system.
In addition to the reduction in costs for microbiological control, with much superior results, the control and removal of heat transfer limiting biofilm in the system resulted in an increase in production rates. The dollar value associated with this production increase has been estimated by plant personnel to be around $70,000/month. The important points from this case history are illustrated in Figure 3.
V. Discussion: The question as to why ClO2 has such excellent biofilm removal capabilities arises frequently. There appear to be two primary reasons: biofilm penetration and effect of chlorite ion.
In a previous section, work was cited which showed the relative inability of chlorine to penetrate a biofilm. Weaker oxidants such as chloramine could more easily penetrate the biofilm and kill bacteria. However, despite the better penetration of chloramine than chlorine, unlike chlorine, chloramine does not remove the biofilm. ClO2, being non-ionic and being much less reactive with most organics than chlorine, would also be able to penetrate a biofilm effectively.
A contributing factor has to do with the reaction chemistry of ClO2. Although ClO2 has the capacity of accepting up to 5 electrons in an oxidation-reduction reaction, under normal circumstances it accepts only one electron and forms chlorite ion, ClO2-. The role of chlorite ion in controlling or removing biofilm has been described in more detail in another report.38 Although the chlorite ion is not very effective as a disinfectant, in general the chlorite ion plays a significant role in biofilm removal.
One group showed that adding a chlorite solution to a system laden with biofilm resulted in removal of the biofilm once a bacterial nutrient was added to the water.39 The nutrient caused a rapid increase in metabolism of bacteria present in the biofilm, producing acids. These acids caused an activation of the chlorite solution to form ClO2, and the ClO2 formed removed the biofilm.
In other reports, (Ref 40) similar observations as to the slow bacterial recovery of systems treated with ClO2 have been described. Thus, the chlorite ion formed when ClO2 reacts is available to be reactivated to ClO2 when it encounters a part of the biofilm that is metabolically active.
Of all the alternatives to gaseous chlorine, chlorine dioxide is one of the least used although it is arguably the most effective for biofilm control under most conditions. For systems that do not have severe leaks or other conditions that would preclude use of chlorine, chlorine might be the best choice for economic reasons.
That is, why use a sledge hammer(ClO2) for microbiological and biofilm control when a tack hammer(Cl2) would suffice? After all, if there are no problems, why change from chlorine? The analogy breaks down at this point because in every case, the sledge hammer(now Cl2), produces secondary negative effects, i.e., increased corrosion, pH depression, production of TOX or THMs, potential for release of large amounts of gas, than would a tack hammer(now ClO2). That is, with ClO2 there is sufficient power for disinfection and biofilm control without the downside associated with using excessive amounts of chlorine. Thus, ClO2 acts as a sledge hammer towards biofilm, but only as a tack hammer in its secondary negative effects. This is illustrated in Figure 4.
If the use costs for chlorine dioxide were equivalent to that of chlorine, chlorine dioxide would undoubtedly have long since replaced gaseous chlorine as the mainstay of microbiological control agents for industrial cooling systems. It performs better under almost all circumstances and the downside is much less than with use of chlorine.
However, the use costs of ClO2 can be 2-3 times that of Cl2. Because the reaction chemistry of ClO2 is so different than that of Cl2, it is not possible to say that in all circumstances, ClO2 will be X times the cost of Cl2. In most cases, ClO2 will be a bit more expensive than Cl2. The three case histories presented here illustrate that, depending upon circumstances, use costs of ClO2 can be much less expensive than that of Cl2, and this does not include the substantial increases in production that were attributed to improved biofilm control. How much more expensive is something that will be determined by conditions in the specific cooling system. For systems treated with ClO2, much of the first years’ annual cost will be incurred during the ‘cleanup’ phase.
During field trials that employ the cleanup phase, the most dramatic changes to the system occur in the first few days (Case History 1), although continued improvement continues to be observed with time (Case History 3). The dramatic changes are easily observable to those familiar with the system.
VI. Summary and Conclusions: These case histories have revealed the following:
♦ Use of ClO2 can show dramatic, immediate results in systems that are contaminated by hydrocarbons
♦ Use of ClO2 can show dramatic, immediate results in systems where process throughput is limited by poor heat transfer caused by biofouling
♦ Use of ClO2 can show dramatic results in systems where cooling tower efficiency,e.g., air and water flow, is restricted by biofilm
♦ A ‘cleanup’ phase can significantly shorten the time required to observe http://www.pureline.com/contact-pureline/improvements in performance of various pieces of equipmentIn addition, with the health effect concerns for Legionella, proper use of ClO2 may provide additional benefits, as described below:
♦ ClO2 is more effective than chlorine on Legionella in laboratory studies41-43
♦ ClO2 is more effective than chlorine on Legionella in field work44
♦ ClO2 is very effective at control of biofilm21,31,37,38,45—47
In summary, if there are no pressures to eliminate chlorine from environmental, safety, or performance reasons, then chlorine should be the microbiological control agent of choice. However, if microbiological control is difficult because of an incursion of bacteria laden makeup water, and there is a source of bacterial nutrient, then chlorine dioxide offers some unique advantages.
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