Chlorine Gas
Chlorine gas is still a commonly used disinfectant. It’s highly effective and comparatively low-cost. 

Depending on the amounts stored and used, a 
Risk Management Program (2500 pounds or more on site) and/or Process Safety Management Program (1500 pounds or more on site) may need to be in place. The reportable quantity for a spill is 10 pounds. Appendix F of EPA's Risk Management rule includes some useful hazard review checklists, what-if questions and HAZOP procedures for chemicals commonly used at wastewater treatment facilities.

Chlorine Institute is another great educational resource for safety and security information as well as technical data. The website includes a bookstore with pamphlets and manuals, including the Water and Wastewater Operators Chlorine Handbook. This is a must-have manual for anyone working with gas chlorine. You can download a pdf version for free or order a hardcopy for $70. Wall charts are also available with information on handling chlorine cylinders and ton containers. A good selection of videos (DVD) is also available, including Chlorine Safety for Water and Wastewater Operators ​ ($80).

guideline for chlorine is also a good reference. Also, be sure to read the Safety Data Sheet before handling chlorine gas. (This one is from Praxair, be sure to get a Safety Data Sheet from your supplier). Here are some key items to know about chlorine gas safety. Much of this information is right out of the Chlorine Institute’s handbook mentioned above. Also, review OSHA's Chemical Database report on chlorine.

Physical Properties
Color – greenish yellow
Odor – pungent, similar to laundry bleach. Detectable by smell at concentrations of 0.2 to 0.4 ppm.
Specific gravity – Approximately 2.5. This means gas chlorine is 2.5 times heavier than air, and will sink to the lowest level in a building or area.
Boiling point is -29.15 degrees F. Liquid chlorine that escapes from a cylinder or ton container will immediately convert to gas.
One volume of liquid chlorine converts to 460 volumes of gas.
Liquid chlorine that vaporizes on skin can reduce the temperature enough to cause frostbite.
Chlorine is not explosive or flammable, but can react violently with many substances.
Chlorine reacts with water to form hypochlorous and hydrochloric acids, with hypochlorous acid being the main disinfectant.

How Chlorine Exposure Affects Humans

Chlorine is a severe irritant. As noted above, when mixed with water (including moisture in mucous membranes, eyes and skin), it forms an acid. The primary route of exposure is through the eyes and respiratory system. Exposure to chlorine has effects ranging from irritation to death, depending on the concentration and time of exposure.

The OSHA ceiling level (the maximum limit of any worker exposure) is 1.0 ppm.

A level of 10 ppm is considered Immediately Dangerous to Life and Health under the National Institute for Public Safety and Health (NIOSH).

At low levels for a short time, chlorine can cause eye irritation, coughing, sneezing and throat irritation. At higher levels, labored breathing and vomiting may occur. Death can result from suffocation.

Dangerous Chlorine Reactions

Chlorine has the potential for violent or explosive reactions with certain substances. It is very important to separate chlorine from the following:

Ammonia and ammonia compounds

Hydrocarbons – oils, greases, solvents, even in small amounts.


Most plant operators store 150 pound cylinders or ton containers. 

Large plants sometimes use rail cars. The Chemical Safety Board (CSB) has published a
Safety Bulletin on the dangers of a major chlorine release during unloading. This was instigated after the CSB investigated two accidents, including a 2002 accident that resulted in the release of 48,000 pounds of chlorine.

Check out the 
CSB's investigation and Safety Bulletin.

Always secure cylinders and ton containers to protect them from falling, rolling or being dropped.

Both cylinders and ton containers have fusible metal plugs that will melt when the temperature gets between 158 and 165 degrees F to relieve pressure. These pressure relief valves keep the containers from rupturing during a fire.

Chlorine may be stored indoors or outdoors, though shading from sunlight is recommended for outdoor storage. Storage areas should be away from HVAC intakes, as chlorine gas could be distributed throughout a building in case of a leak.

Separate the chlorine storage area from incompatible materials, especially ammonia, sulfur dioxide, and hydrocarbons like fuels and oils.

The chlorine storage area should have a well-maintained chlorine gas detector installed, complete with alarm and call-out capability if a leak occurs when the plant is unmanned.

Unloading Chlorine Cylinders and Ton Containers

All employees receiving chlorine cylinders and containers must be properly trained.

Always use proper equipment to unload cylinders and ton containers. Chain cylinders to a hand truck, or move with a forklift if already secured in a storage rack. 

Make sure the protective valve housing is on securely

Never lift a chlorine cylinder by its protective valve housing!

Use a properly rated hoist or forklift to relocate ton containers. When using a hoist, remember that the total weight of the ton container is nearly 2 tons. A one-ton hoist is not sufficient for lifting a ton container.

The hoist and cables must be in good operating condition. Have a professional inspect the hoist each year and repair or replace it when necessary.

Never stand under a hoisted container. Stand to either side.

Once the containers or cylinders are unloaded, secure them properly at the site. Always store cylinders in an upright position. Store ton containers with the two valves lined up vertically.

Chlorine Leak Detection

Check for chlorine leaks by using a plastic squeeze bottle with a solution of ammonium hydroxide in the bottom. Squeeze to allow only the ammonia vapor (NEVER the liquid) to detect the presence of chlorine. If a leak exists, it will form a white cloud.

In addition, chlorine leak detection instrumentation is recommended, and required in some states. Leak detectors are typically connected to an alarm system with call-out capability for unmanned plants. Leak detectors must be properly maintained, calibrated and tested.

Chlorine Leak Response

NEVER respond to a chlorine leak unless you have been properly trained and have the necessary safety equipment—including a self-contained breathing apparatus and protective suit.

If your plant has its own HAZMAT Team, or provides the operators with the proper training and certification, follow your standard operating procedures.

Otherwise, call 9-1-1 or whatever agency is listed in your plant’s Emergency Response Plan.

To speed response and recovery, each treatment plant should have the appropriate Chlorine Institute Emergency Kit at the site:

Kit A: for 100 or 150-pound cylinders
Kit B: for ton containers
Kit C: for tank cars and tank trucks

Cylinder containment vessels: for 100 or 150-pound cylinders

Whether a chlorine leak is handled in-house or not, your Emergency Response Plan should detail the exact procedure. Most importantly, practice, practice and practice the procedure. Finding the Emergency Response Plan and reading it is not a good option in the middle of a chlorine gas leak.

Connecting and Disconnecting Chlorine Cylinders and Containers

Changing cylinders or containers is one of the most likely opportunities for exposure to chlorine. It is extremely important to make sure all operators are thoroughly trained before attempting the task.

100 and 150-Pound Cylinders:

Make sure the cylinder is upright and properly secured. The yoke and adapter connects the cylinder valve outlet to the feed system. A gasket must be used on the valve face and MUST be replaced with every new connection. Failure to replace the gasket will often lead to a chlorine leak. Do NOT reuse the old gasket.

Ton Containers:

Ton containers should be secured in a horizontal position in a cradle, with the two valves aligned vertically. The top valve will feed gas, the bottom will feed liquid. A yoke and adapter connects the cylinder valve to the outlet feed system. As with the cylinders, a gasket must be used on the valve face and MUST be replaced with every new connection.

If the vacuum regulator connects directly to the cylinder or container, follow the manufacturer’s instructions for connection. Otherwise, follow the directions in Section 6 of the Chlorine Institute’s Water and Wastewater Operators Chlorine Handbook.

Make sure the appropriate personal protective equipment is available and worn during the procedure:

Self contained breathing apparatus with full face mask
Hard hat
Safety shoes
Long sleeved shirts
Clothing and gloves should be free of oil or grease.


The events of 9/11 made treatment plant security all the more important.

Control access to chlorine cylinders and containers with gates, locked buildings and other barriers. Provide bullet-proof shields for containers that are not indoors.

If funding allows, use electronic gates and doors with access badges that record the comings and goings at the plant. If not, ensure all visitors sign in and show a photo identification.

Include security requirements in your specifications when taking bids for chlorine or other hazardous chemicals. These requirements may include training for the company’s staff, a written security plan, a list of delivery drivers with record checks completed, and more.

Learn More

Most chlorine vendors provide training for operators and other plant staff. Study the literature from the Chlorine Institute. Keep your Emergency Response Plan updated. And practice what to do in an emergency.

Center for Disease Control-Emergency Response Information 

National Transportation Safety Board Report on Chlorine Leak after Train Collision in Macdona, TX 2004
Chemical Safety Board Report on Chlorine Leak at Festus, MO 2002
Video-Youngstown, OH chlorine leak at WWTP 5/4/15

Rogersville, TN chlorine leak at WWTP 2011
Chlorine Rescue
Reaction of Chlorine Gas with Sodium
Graniteville Train Disaster 10 years later

Chlorine Leak at Scottsville, IN 2016

Chlorine Safety

Chlorine is a common disinfectant in the water industry. Although many treatment plants have switched to ultraviolet or other disinfection methods, the majority of plants use some form of chlorine for disinfection—chlorine gas, sodium hypochlorite, calcium hypochlorite, or chlorine dioxide.

The use of chlorine has saved millions of lives by preventing the spread of waterborne diseases. But it’s critical to understand the dangers when working with chlorine at your plant.

This section will focus on gas chlorine.

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