Chlorine is purchased commercially as gaseous chlorine, liquid sodium hypochlorite or calcium hypochlorite (solid). Chlorine gas, still the most common form of disinfectant, has been recognized as a potential terrorist target and safety problem and is heavily regulated. As a result, many facilities have switched or are considering switching to safer disinfection alternatives, and federal actions are encouraging this conversion.
The most common choice for those converting away from chlorine gas is sodium hypochlorite, which is available delivered in bulk at a free available chlorine (FAC) concentration ranging from 10 to 15 percent. It is also possible to generate hypochlorite on-site at a low concentration (less than 1 percent FAC) by passing a brine solution through an electrolytic cell. Hypochlorite as a bulk chemical is referred to by various descriptors, including delivered hypochlorite, commercial hypochlorite, concentrated hypochlorite and bulk bleach. On-site generation of a dilute hypochlorite solution (less than 1 percent) is abbreviated as OSG.
The decision process when evaluating alternatives to chlorine gas focuses on safety, maintenance requirements and costs. Delivered hypochlorite is often perceived as safe, easy and cheap. However, a proper evaluation shows that on-site generation is not only safer than bulk bleach, but also offers significant maintenance advantages and presents a return on investment within one to three years.
This article outlines the benefits and drawbacks of delivered sodium hypochlorite versus OSG. It will help consultants, engineers, city planners and utility personnel evaluate the potential costs and benefits of each disinfection alternative prior to conversion. This type of analysis will also provide the water utility with a valuable tool to secure conversion funding at the federal or state level.
Relevant safety topics for delivered hypochlorite include the potential formation of cancerous by-products, the concentration of the hypochlorite solution, shipping and transfer of hazardous materials, community safety concerns, and storage and operational safety. Relevant safety topics for OSG include hydrogen and electrical safety. Comparison of the two alternatives shows that OSG is a much safer alternative than delivered hypochlorite.
The production of commercial strength hypochlorite is a multistep process, starting with electrolysis of a brine solution to form chlorine that is similar to the process utilized by OSG. This chlorine is then reacted with sodium hydroxide to create sodium hypochlorite. Often, seawater is used as the brine source for production of hypochlorite because of its availability and low cost. However, seawater is high in bromide concentration, which ultimately converts to bromate during the electrolytic process. Thus, some commercial bleaches may exceed the EPA drinking water limits for bromate, which has been classified as a cancer-causing agent. In contrast, the salt used during the electrolytic process of OSG can be carefully controlled. The food grade and solar salts more commonly used with OSG systems for potable water treatment have bromide levels that are far below the levels of concern for bromate formation.
Delivered hypochlorite and OSG will form total trihalomethanes (TTHMs) in water with organic precursors. However, evidence shows that OSG reduces the formation of TTHMs, most likely due to the ability to reduce the chlorine added to the water supply with OSG while maintaining an adequate disinfectant residual.[i] [ii]
Commercial hypochlorite is highly concentrated and high in pH. Typical chlorine concentrations range from 10 to 15 percent, or 100,000 to 150,000 milligrams/liter (mg/L), and pH values of 12 to 13 are common. In contrast, OSG is generated at a concentration ranging from 0.25 to 0.8 percent, or 2,500 to 8,000 mg/L, far below the 1 percent safety threshold of the Hazardous Communications Standard. In fact, no regulatory requirements are associated with hypochlorite generated on-site at low chlorine concentrations. In addition, the pH value of OSG is around 9, only two orders of magnitude away from a neutral pH, rather than 5 to 6 orders of magnitude as with commercial bleach.
Shipping and transfer of hazardous chemicals
Ordering a chemical in bulk necessitates the transfer and storage of large quantities of hazardous material, increasing the likelihood of an accident. The Chlorine Institute Inc., a nonprofit trade association of companies created to support the chlor alkali industry and public safety, recommends developing procedures for handling and diluting sodium hypochlorite and provides guidelines on its website.[iii] In contrast, OSG produces a hypochlorite solution at a concentration that is at least 15 times more dilute than delivered hypochlorite. Typically, OSG systems specify less than 24 hours’ worth of storage, reducing the likelihood of a large spill.
Accidents with delivered hypochlorite have occurred when a tank truck of acid or acidic chemical was unloaded into a sodium hypochlorite solution storage tank, or, conversely, when a trailer of sodium hypochlorite solution was unloaded into a tank containing acidic chemicals. The result of this scenario is a massive chlorine gas plume. Chlorine gas is produced from hypochlorite when the pH is reduced, which occurs with the introduction of acid. The following examples demonstrate that concentrated hypochlorite is still a hazardous chemical, despite its perception as a safer alternative.
- According to George Clifford White in his Handbook of Chlorination and Alternative Disinfectants, one of the largest chlorine gas leaks that ever occurred at a water treatment plant was due to the use of sodium hypochlorite, not chlorine gas. The accident occurred at a large treatment plant in the East, when a tanker truck load of ferric chloride (pH = 4.0) was dumped into the hypo tank by mistake. This lowered the hypo pH from about 12 to 5 almost instantly, releasing approximately 5455 kilograms (12,000 pounds) of hypochlorite as chlorine gas in one great mass.[iv]
- At a facility in the U.S., approximately $500,000 in damages occurred when sodium bisulfite, a dechlorination chemical, was accidentally delivered into a hypochlorite tank.[v]
- In 2003 in Ohio, an employee mistakenly mixed 5 gallons of sodium hypochlorite with 3.96 liters (15 gallons) of hydrofluorosilicic acid, causing a toxic fume (chlorine gas release). He was taken to the hospital for treatment while a hazardous materials (hazmat) crew secured the chemicals.[vi]
Some operators minimize the risk of operator error by requiring two separate keys for delivery. Other recommendations from the Chlorine Institute Inc. include extensive operator training, secured devices on tank loading lines with a controlled key given to specified operators and checklists requiring the confirmation of the chemical name by examination of the shipping papers and the placarding of the truck or tank car.[vii]
By contrast, salt, the feedstock for OSG equipment, is not classified as a hazardous chemical, and no industrial accidents involving salt have been reported to date. Salt has none of the safety concerns associated with the shipping and transfer of bulk hypochlorite.
Community members and prominent citizens are typically against the transport and storage of chlorine chemicals. Besides increasing the potential for accidents at the treatment plant, use of a bulk chemical presents an increased risk for the larger community.
For example, on May 26, 2006, in Austin, Texas, a hazmat team worked most of the afternoon to contain a liquid chlorine leak from a tanker truck that closed down U.S. Highway 290. The truck driver himself was taken to the hospital and treated for breathing problems. Residents and students had to be evacuated since the leak impacted an elementary school, middle school and high school. Local resident Herschel McFarland said, "You smell it. … You know it’s bleach. It’s just a big smoke fog … and you can see it coming. We both have breathing problems, and [the chlorine gas produced after the leak] affected us. So really it’s probably best that we don’t even go back.”[viii]
Storage and operational safety
Chlorine gas can be formed not only with the addition of acids, but also by introducing water to commercial strength hypochlorite. In Pomona, California, in 1997, a tank cracked and caused a massive leak of sodium hypochlorite. Plant personnel attempted to wash down the leak and inadvertently caused a chlorine gas cloud, which overcame two auto mechanics next door.[ix]
Sodium hypochlorite has been involved in explosions and exothermic reactions during dilution. In 1999 in Alamogordo, New Mexico, a chemical transfer of bleach injured five workers in an explosion and killed one man who died from burn injuries.[x] Exposure of delivered hypochlorite to a heat source can provoke an explosion, and spontaneous ignition can occur when materials such as rags are soaked with commercial strength sodium hypochlorite.
Enclosed areas may also be cause for concern since chlorine gas generated during the natural degradation of sodium hypochlorite or as a result of leaks in the delivery system can overwhelm people in places with inadequate ventilation. "Enclosed areas such as a pump house or a filter plant tunnel are a real hazard, requiring the same safety gear as that required for Cl2 gas."
Many states now require secondary containment for sodium hypochlorite users as an added measure of protection.
Ironically, commercial strength hypochlorite has a worse safety record than chlorine gas. Studies reflect an increasing rate of sodium hypochlorite safety incidents over several years.
A study performed in Australia compiled safety incident data on chlorine gas and sodium hypochlorite from 1996 to 1998 via the Emergency Response System. More than 90 percent of delivered sodium hypochlorite accidents involved customer storage or the customer process error, rather than manufacturer error. Conversely, only 25 percent of the incidents associated with chlorine gas were a result of customer errors. The authors attribute this trend to a lack of operator training compounded by the false perception that delivered sodium hypochlorite is safe.[xii] Because regulatory guidelines are less stringent for liquid sodium hypo than chlorine gas,[xiii] standard risk management practices are often inadequate to protect the operator from the potential mishandling of hypochlorite.[xiv]
The main safety concern associated with the electrolytic process of OSG is the production of small amounts of hydrogen gas. Hydrogen gas (H2) can be explosive under certain conditions. The lower explosive limit (LEL) of hydrogen is 4.1 percent by volume in air and the upper explosive limit is 74.2 percent by volume in air. This means that any concentration of hydrogen in air less than 4.1 percent will not explode (too "lean" in fuel) and that air containing greater than 74.2 percent hydrogen will not be explosive (too "rich" in fuel). Therefore, at concentrations of 4.1 to 74.2 percent, hydrogen is explosive in an oxygen environment. H2 is the lightest gas with a vapor density of 0.069 (relative to that of air taken to be 1.0) and the smallest in molecular size, causing it to seek the highest point in a room or container in a normal room atmosphere. Because this potential fuel source is produced in any electrolytic process, proper venting of hydrogen is mandatory for the safe operation of OSG equipment.
In a proper installation, hydrogen is diluted to nondetectable levels and vented to the atmosphere. Consultants should investigate the hydrogen venting design of OSG manufacturers to verify that the design has been properly tested and verified. Acceptable designs include liquid barrier mechanisms and fan-driven dilution air systems. The liquid barrier system is elegantly designed to provide a simple solution. As long as the velocity of the oxidant stream in the drop tube is lower than the rate of bubble rise in the oxidant tube, all of the hydrogen gas will be trapped and vented out of the system through the hydrogen vent piping that discharges external to the building. Sizes for the liquid barrier system and the dilution air vent systems are available in a variety of ductwork pipe diameters, and the system size is matched to the H2 generation rates for the selected system. When designed properly, the systems will bring the H2 concentration to less than 25 percent of the lower explosive limit. Manufacturers can provide complete engineering parameters.
Basic electrical safety must be followed when working with on-site generators. This is true of all electrical equipment. On-site generators are designed to minimize electrical hazards. The voltage varies depending upon the size and manufacturer of on-site equipment. For example, most small OSG systems are far below the upper limit of 40 volts direct current (VDC). This means that operators can touch the cell, work on the system and perform basic maintenance without the risk of electrical shock. Larger systems may have voltages in excess of 40 VDC, but they are typically designed with an electrical interlock that shuts down the cell when the cell enclosure is opened. All high voltage components are isolated and rarely require maintenance.
The effectiveness of adequate hydrogen venting systems and electrical safety is reflected in the strong safety record of the OSG industry, with more than 6,000 installed units in the U.S. from many manufacturers. Delivered hypochlorite cannot claim the same safety record, with the potential for an accidental chlorine release or explosion occurring during transport to the site, transfer to storage tanks, storage, degradation, and delivery to the distribution system. Delivered sodium hypochlorite is a concentrated chemical with a hazardous classification, a higher potential for formation of cancerous byproducts than OSG, and a worse safety record than chlorine gas, the substance it is replacing.
Operation and maintenance
Operation and maintenance issues relevant for delivered hypochlorite include pH, solution stability and chlorate formation, scaling and leakage, and gasification. Operation and maintenance topics relevant for OSG include cell cleaning, salt quality and remote monitoring of system operations. Comparison of the two alternatives reflects much less maintenance required with OSG than with delivered hypochlorite.
During production, an excess of caustic (base) is added to stabilize the hypochlorite solution, resulting in pH values 12 to 13 for most commercial hypochlorite. This high pH later requires addition of acids to the water being treated in order to neutralize the effect of the caustic.
Since on-site generated hypochlorite has a pH around 9, it is 1,000 to 10,000 times less caustic than concentrated hypochlorite. Plant operators can either eliminate the addition of acid or at least greatly reduce it when using OSG.
Solution stability and chlorate formation
Even with the addition of caustic, hypochlorite starts chemically degrading shortly after its production. Degradation of concentrated hypochlorite results in the formation of oxygen gas (O2) and chlorate (ClO3-), reducing the disinfection capability of the solution as show in the equations below.xii
3OCl– —- ClO3– + 2Cl–
OCl– + OCl– —- O2 (gas) + 2Cl–
The more concentrated the hypochlorite, the faster it degrades. The process is further accelerated by the presence of trace transition metals such as nickel and copper. It has also been shown that increasing temperature increases the rate of decomposition.[xv] For example, an unventilated hypochlorite storage shed will accelerate the rate of decomposition.
Degradation of the hypochlorite solution adversely affects plant operations since more hypochlorite has to be injected into the line to achieve the same disinfection power. Many sites choose to dilute the delivered hypochlorite to stabilize the solution and enhance accuracy in hypochlorite feed metering systems or to minimize chlorate (ClO3-) formation. The dilution process must be carefully planned and implemented to avoid an uncontrolled exothermic reaction.[xvi] Using pure chemicals to make the hypochlorite and nonmetallic-lined hoses to transfer the hypochlorite is critical for maintaining product purity and safety.[xvii]
Gordon et al. studied the rates of liquid bleach decomposition and chlorate (ClO3–) formation.xii A 15 percent full-strength hypochlorite was diluted by 2/3 and 1/3 and monitored at two different temperatures. The higher the concentration and the hotter the temperature, the more pronounced the chlorine degradation was.
Hypochlorite degradation after 28 days based on concentrations and temperature
|Commercial Hypo Initial FAC (%)||Temperature||Measured FAC (%)||Decomposition (%)||Measured ClO3– (g/L)|
Table 1. These degradations are shown after 28 days. Courtesy of Winfred Kpodo and Carollo Engineers.
While chlorate formation is not currently regulated in the U.S., studies indicate that it is harmful to the environment. As a result, or perhaps in anticipation of the introduction of a regulatory limit, many operators strive to reduce chlorate production. If a utility uses a single tank to store liquid bleach, a residual chlorate concentration is probably building in the tank. Thus, hypochlorite storage tanks should be periodically flushed and cleaned.
In contrast, on-site generated hypochlorite is produced on demand at a low concentration and is typically used within 24 hours, so degradation is a nonissue. The OSG systems of one manufacturer produce only about 33 parts per billion (ppb) of chlorate for every 1 parts per million (ppm) dose, a level far below even the proposed regulatory limits of concern.
Scaling and leakage
Scaling commonly forms when carbon dioxide and calcium are present in the system and the pH rises above 9.0. The high pH of commercial hypochlorite is sufficient to cause the formation of calcium carbonate scale, which can plug pumps, backpressure devices and piping, especially at injection points and solution diffusers. If the facility chooses to dilute the hypochlorite solution, it must use softened water to prevent the further addition of calcium to a high pH environment, exacerbating the buildup of calcium scale. The high pH of the hypochlorite solution also gradually dissolves the silica in standard PVC glue.
In contrast, hypochlorite generated on-site has a pH at or below 9, so calcium carbonate scale, clogged equipment and dissolved PVC glue are not concerns.
The continuous formation of oxygen and chlorine gas because of delivered hypochlorite degradation causes many operational problems. At room temperature, Unified Sewerage Agency in Hillsboro, Oregon, found hypochlorite will generate oxygen at a rate of 1 percent per day. This means that 100 gallons (378 L) of hypochlorite will release approximately 1 gallon (3.7 L) of oxygen daily. The presence of nickel ion, which appears to catalyze decomposition to produce O2 gas, only worsens the situation.
Gas accumulates in high spots and interferes with delivery, especially when the systems are run intermittently. Operators must usually eliminate the raised portions of pipe or replace the lines with smaller diameter piping. Gas in the hypochlorite delivery system can also cause pump, valve, tank and plumbing failures. Certain types of pumps lose prime or overheat, making it difficult for operators to find pumps that suit their needs for accuracy, capacity and dependability. Valves are also susceptible to failure. In two documented cases, hypochlorite left trapped between two closed valves caused the ball valves to explode. Several other types of valves have cracked.[xviii] Finding piping systems resistant to hypochlorite leaks is also difficult because hypochlorite exploits the weaknesses of the piping system. Hypochlorite leaks through most of the mechanical fittings, and glued plastic joints can fail within months.
When system delivery is compromised by gas accumulation, a steady drop in residual occurs. Once the air has passed, the chlorine residual can rise sharply. These inconsistencies in hypochlorite delivery make standardization of disinfection and later processes, such as dechlorination, difficult to predict and time-consuming to monitor.
Employee exposure must also be considered. Staff clothing is often bleached or filled with holes due to normal chlorine off-gassing. If an accident occurs, worker consequences may be more severe.
[i] Hamm, Beth, 2002, "DBP Reduction Using Mixed Oxidants Generated On Site", AWWA Journal, November 2002, Volume 94, Number 11: 49-53.
[ii] Hunt, Jonathan P., et al. HSMM. "Onsite Generation of Sodium Hypochlorite – a Key to Reducing Trihalomethane Concentrations?" WEFTEC 2005.
[iii] "The Chlorine Institute, Inc. exists to support the chlor-alkali industry and serve the public by fostering continuous improvements to safety." Accessed from the Chlorine Institute website, Aug. 5, 2006.
[iv] White, G. C., "Handbook of Chlorination and Alternative Disinfectants" 4th edition. 2004. John Wiley & Sons, Inc. New York p. 113.
[v] Baur, Rob "Solutions for a Solution" Operations Forum, pp. 18-22, 1999.
[vi] "Ohio Officials Evacuate Water Plant after Toxic Mistake", Morning Journal News. Oct. 24, 2003.
[vii] The Chlorine Institute, Inc. "Sodium Hypochlorite can release chlorine gas; Never mix sodium hypochlorite with acids or acidic chemicals" released 11/16/04 available at www.cl2.com or http://www.chlorineinstitute.org/SodiumHypochlorite/ArticleList.cfm?CFID=10702&CFTOKEN=1cc925c5c805e145-F5FC44BF-9027-AC8F-FD3D7DDBC11CFD85
[viii] "290 East Open Again after Chlorine Leak", KXAN.com. May 26, 2006.
[ix] "Chlorine is both helper, hazard", Daily Bulletin.com. May 27, 1997.
[x] "Chemical Explosion Kills One", Alamogordo Daily News. August 14, 1999.
[xi] Gordon, G. et al., "Predicting Liquid Bleach Decomposition" Vol. 84, Issue 4, pp.142-149, 1997.
[xii] Travaglia, Teresa. Orica. "Chlorine Gas vs. Sodium Hypochlorite." 67th Annual Water Industry Engineers and Operators Conference, Sports and Leisure Centre – Wodonga, Sept. 1-2, 2004.
[xiii] U.S. Department of Transportation Emergency Response Guidebook (ERG). 2004.
[xiv] Regal Chlorinators. "The Case for Responsible Legislation for Regulating Chlorine." http://www.regalchlorinators.com/html/pub._592-4.html. Accessed August, 3, 2006.
[xv] Adam, L. et al "Hypochlorous Acid Decomposition in the ph 5-8 Region. Inorg. Chem.. 31:3534 (1992)
[xvi] The Chlorine Institute, Inc. "Dilution of Sodium Hypochlorite solutions (NaOCl)" released 11/16/04 available at www.cl2.com or http://www.chlorineinstitute.org/SodiumHypochlorite/ArticleList.cfm?CFID=10702&CFTOKEN=1cc925c5c805e145-F5FC44BF-9027-AC8F-FD3D7DDBC11CFD85
[xvii] Chlorine Hose Safety Advisory.pdf, Downloaded Aug. 21, 2006. http://www.csb.gov/index.cfm?folder=news_releases&page=news&NEWS_ID=29
[xviii] White, G. C., "Handbook of Chlorination and Alternative Disinfectants" 4th edition. 2004. John Wiley & Sons, Inc. New York p. 114.
Khalil Kairouz is an associate with Carollo Engineers, a consulting firm in Costa Mesa, California, dedicated to water and wastewater treatment engineering. Albert Rau is a contributing author and the president of UDECM Inc., an engineering firm in Ontario, California. Winfred Kpodo is an engineering consultant in Italy.