Sodium hypochlorite and chlorine gas have been used for microbiological control of cooling water systems for more than 40 years. Over the years, the performance, applications considerations, and limitations of chlorine-based chemistries have presented opportunities for alternative chemistries. Advancements in microbial-control technologies have brought different chemistries — bromine-based chemicals, peracids, peroxide, ozonation — and nonchemical technologies for treating industrial cooling water systems.
Despite the chemistry advancements, application and process conditions continue to present challenges. With growing environmental and regulatory restrictions and an emphasis on safety, the need for new and alternate chemistries is ever-existent.
The microbiocidal properties of chlorine dioxide have been known for a long time, but it has seldom been considered as an alternative oxidizing biocide for industrial cooling water systems. Unlike conventional chlorine- and bromine-based chemistries, chlorine dioxide cannot be shipped at high concentrations and has to be produced on-site with one, two or three precursors.
In the past, the main uses of chlorine dioxide were municipal water treatment or bleaching applications in pulp mills. Yet, numerous studies have documented the advantages of chlorine dioxide in many challenging water conditions. For example, chlorine dioxide remains undissociated; therefore, it is more effective under high pH conditions. By contrast, chlorine and bromine undergo molecular dissociation under high pH water conditions.
The biocidal species of chlorine (hypochlorous acid) and bromine (hypobromous acid) undergo pH-dependent dissociation to hypochlorite and hypobromite ions, respectively. This negatively affects biocidal efficacy for chlorine-based chemistries at pH levels greater than seven (pH>7) and for bromine-based chemistries at pH levels greater than eight (pH>8).
Figure 1 demonstrates the dissociations of hypochlorous acid, hypobromous acid and chlorine dioxide versus pH. Various studies also have demonstrated that chlorine dioxide can outperform chlorine- and bromine-based chemistries at controlling slime-forming bacteria such as Pseudomonas.
All the advantages of chlorine dioxide for microbial control in challenging water conditions, however, have not led to an uptick in the use of chlorine dioxide for industrial cooling water treatment.
In recent years, the regulatory requirements for a new technology have challenged water treatment companies’ ability to develop and bring new technologies to the market. At the same time, recent environmental, regulatory and water scarcity concerns and trends — combined with a higher consideration for safety and reliability — have invoked an interest in using chlorine dioxide for cooling water treatment. This has resulted in the development of new chlorine dioxide generation processes that utilize reliable on-site generator designs and application conditions.
Long-established oxidizer technologies are subject to:
- Homeland security regulations
- Strong safety demands for oxidizer transportation, including storage and handling
- Strict discharge regulations for disinfection byproducts, which limit products such as adsorbable/total organic halogens (AOX/TOX) or trihalomethanes (THM).
- The increased use of raw or recycled water.
The limitations of conventional oxidizer technologies have prompted increased consideration for the use of chlorine dioxide in cooling water treatment.
Storage and Handling Regulations for Chlorine Gas
Chlorine gas is subject to considerable regulatory burden and expense to comply with regulations such as OSHA’s process safety management (PSM) program and EPA’s risk management plan (RMP). Using chlorine gas may require the installation of scrubbers or containment systems, which can be an expensive proposition.
Under OSHA Standard 29CFR 1910.119, Process Safety Management, the threshold quantity of chlorine is 1,500 lb. So, having even a single-ton cylinder on site obligates the user to comply fully with the requirements. As a consequence, the time required and associated costs mandated by the regulations — for preparing the documentation, conducting the safety reviews, training employees, purchasing the protective equipment and monitors, and maintaining the program — can be considerable.
The risk management program in Section 112(r) of EPA’s Clean Air Act requires those affected to go above and beyond the PSM requirements. For chlorine, the threshold quantity is 2,500 lb, and those affected must comply with the RMP rule. The requirements include the preparation of emergency-response programs, public notifications and more record keeping.
Chlorine dioxide avoids all of the PSM and RMP requirements and associated costs if less than 1,000 lb of chlorine dioxide is stored. Because chlorine dioxide requires on-site generation, only small quantities are produced for cooling water applications. This helps keeps quantities significantly below the 1,000 lb threshold.
Discharge Limits for Disinfection Byproducts
Disinfection byproducts are compounds produced as a consequence of the application of a microbial control agent. AOX, adsorbable organic halides, and THM, or trihalomethanes, are two notable terms used to identify major types of disinfection byproducts.
AOX and THM values are global parameters that are monitored as a measure of pollutants being introduced into the environment. These parameters cover a large group of substances that range from simple volatile substances such as trichloromethane (chloroform) to complex organic molecules such as dioxins/furans, which have a number of toxic properties.
In the past, the undesired disinfection byproducts (DBPs) created as a result of chlorination caused, in many circumstances, environmental challenges and compromises. For instance, the pulp-and-paper industry once used large quantities of chlorine for bleaching and microbial control. Due to environmental and regulatory reasons, the use of chlorine and chlorine-derived oxidants in pulp bleaching has fallen significantly in most of the world.
In the early ’90s, bleaching technology shifted from using elemental chlorine to chlorine dioxide. This helped to substantially reduce the formation and subsequent discharge of AOX and THM.
The stringent AOX and THM regulations established in Europe some time ago are now being adopted within the U.S. market. National Pollutant Discharge Elimination Systems (NPDES) permits are increasingly stringent for AOX and THM discharge limits, causing operational challenges and adoption of practices that enable compliance. In many cases, operations are first trying to limit the feed of chlorine. This approach reduces AOX and THM but compromises microbiological control, which can cause heat exchanger fouling, reduced performance and potential public-health risks.
By contrast, chlorine dioxide is a selective oxidant in that it does not react with all organics. This results in a significant reduction in AOX and THM. Also, reduced consumption by undesirable compounds leaves more chlorine dioxide available for microbiological control — even at low dosage rates. This makes chlorine dioxide a favorable and preferred alternate to conventional oxidizing technologies for microbial control in industrial cooling water systems.
Use of Raw and Recycled Water in Industrial Facilities
Increasing water scarcity in the United States has forced many industrial operations to use more raw water or recycled water. This poses challenges because raw and recycled water can contain high organic loads and higher levels of bacterial contamination. The use of conventional oxidants such as chlorine prompts reactivity with organics contaminants and reduces the available actives for microbial control. As a result, significantly higher levels of chlorine need to be dosed in order to maintain adequate microbiological control. A higher chlorine dose means higher chloride contribution, which can cause a higher corrosion potential. It also can result in an increase in the potential for AOX/THM formation and compromise discharge limitations, resulting in violations.
Industrial processes using water sources such as raw and recycled waters need to actively monitor the water quality to adjust the chlorine feed level and monitor the consequential impact of that feed level on discharge limits and corrosion.
By contrast, chlorine dioxide does not react with most organics; therefore, should water quality conditions change, its effect on water treatment would be less. In most cases, the low residual levels of 0.1 to 0.3 ppm can be maintained relatively easily with changes in water quality. As a result, chemical inventory, corrosion levels and discharge limits can be managed easily. The same conditions apply to operations using recycled water or industries such as ammonia plants or refineries dealing with process leaks that have a significant impact on the efficacy of chlorine.
2 Case Studies: Chlorine Dioxide Use in Industrial Cooling Systems
One study was conducted at a Texas power plant in 2014. The zero-liquid discharge (ZLD) power plant was using bleach as a biocide where raw water following clarification served as makeup water in the cooling water system. In addition to the bleach, the plant used sodium permanganate to oxidize and remove manganese coming in with the raw water. Both oxidizers were replaced with chlorine dioxide, which successfully oxidized and removed the manganese in the clarifier and improved algae control (figure 2).
Reduction in Disinfection Byproducts. To understand the impact of implementing an oxidizing biocontrol program change (switching from sodium hypochlorite to chlorine dioxide) on the concentrations of THM and AOX/TOX in the cooling water blowdown, the concentrations of THM and AOX/TOX in the cooling water were monitored according to EPA standard methods. Two weeks after initiating the treatment with chlorine dioxide, the THM concentrations in the cooling water — monitored as bromodichloromethane, bromoform, chloroform and dibromochloromethane — were reduced significantly from 3.2, 0.7, 16.3 and 2 ppb, respectively, to below the detection limit of 0.5 ppb (table 1).
Similar to the THM results, during the chlorine dioxide treatment, the AOX and TOX levels were reduced by 47.4 percent and 50.0 percent, respectively (figure 3). This would be a significant improvement and an advantage for operations that need to meet NPDES discharge regulations for hazardous compounds.
Figure 4 shows the reduction in chloride and sulfate levels after chlorine was replaced by chlorine dioxide as the oxidizing biocide program for cooling water treatment. The reduced total dissolved solids (TDS) loading going to the ZLD process (crystallizers/evaporators) has an added benefit for the operation.
Another important consideration with oxidizing microbial-control programs is their impact on corrosion rates. Figure 5 shows the significant reduction in copper corrosion rates observed during the chlorine dioxide trial compared to the period during which sodium hypochlorite was dosed under similar free chlorine/chlorine dioxide residuals before and after the trial. The results were achieved with the same cooling water cycles, pH control and corrosion-inhibitor product levels.
Another study was conducted in a large U.S. ammonia plant in 2015. The cooling water was challenged by ammonia contamination in their cooling water system at levels that ranged from 15 to 50 ppm. Also, bacteria growth was fueled by high phosphate levels in the gray water used in the tower makeup. These combined system stresses made the chlorine gas treatment inefficient and, at times, ineffective.
The plant agreed to trial chlorine dioxide as a replacement for chlorine gas for 90 days in one of their cooling water systems while monitoring microbial control, corrosion rates and cost of the program. The plant selected a two-precursor chlorine dioxide program, which is based on sodium chlorate and sulfuric acid. For large-scale production of chlorine dioxide, sodium chlorate programs typically are more cost effective than chlorine dioxide programs that use sodium chlorite as a precursor.
The chlorine dioxide summer 2015 trial data was compared with the chlorine gas summer 2014 data. In both summers, the tower was operated under similar operational conditions such as water makeup quality, pH, cycles and using the same corrosion-inhibitor program.
Table 2 shows the positive results obtained during the trial using the two-precursor system (sodium chlorate/hydrogen peroxide and sulfuric acid). Sessile bacteria counts, corrosion rates and cost of program were reduced significantly during the trial phase
In conclusion, recent studies have confirmed that chlorine dioxide is an effective alternative oxidizing program to sodium hypochlorite and chlorine gas for treatment of industrial cooling water systems. Industrial plants using raw or recycled water, experiencing process leaks, facing AOX/THM discharge violations, not meeting their microbial control goals or operating cooling towers at pHs higher than 7.5 should consider the use of chlorine dioxide.
ZLD plants also can benefit from lower chloride and TDS levels. While selecting the best suitable chlorine dioxide program, operators should review reliability data and safety features of the on-site generator. Generators should be backed by a thorough preventive maintenance plan and use an automation program (including email alert systems) to minimize equipment breakdowns and ensure an emergency repair response to avoid interruptions of plant operation.
New security guidelines, stricter environmental regulations and increased water scarcity will force many plants to find replacement options for chlorine gas and sodium hypochlorite. Chlorine dioxide is an effective alternative to conventional oxidizer programs.