Treatment Plant Safety
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Sulfur Dioxide Safety

Sulfur Dioxide

Operators commonly use sulfur dioxide to dechlorinate wastewater effluent after disinfection with chlorine, sodium hypochlorite or calcium hypochlorite.

The hazards and safety precautions are quite similar to those of chlorine gas.

Sulfur dioxide forms sulfurous acid when it contacts mucous membranes. Exposure can damage the eyes and lead to blindness, and inhalation can cause severe respiratory damage or even death.

Depending on the amounts stored and used, a Risk Management Program (5,000 pounds or more on site) and/or Process Safety Management Program (1,000 pounds or more on site) may need to be in place. The reportable quantity for a spill is 1 pound.

A level of 100 parts per million in air is immediately dangerous to life and health.

Dangerous Sulfur Dioxide Reactions

Sulfur Dioxide has the potential for violent or explosive reactions with certain substances. It is very important to separate sulfur dioxide from the following:

·         Powdered metals

·         Strong alkalis (sodium hydroxide, fluorine)


Most plant operators store cylinders or ton containers.

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.

Sulfur dioxide 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 sulfur dioxide storage area from incompatible materials.

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

Unloading Sulfur Dioxide Cylinders and Ton Containers

All employees receiving sulfur dioxide 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 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. 

Sulfur Dioxide Leak Detection

Check for sulfur dioxide 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 sulfur dioxide. If a leak exists, it will form a white cloud.

In addition, sulfur dioxide 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.

Sulfur Dioxide Leak Response

NEVER respond to a sulfur dioxide 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 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 sulfur dioxide gas leak.

Connecting and Disconnecting Sulfur Dioxide 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 sulfur dioxide 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.

o   Self contained breathing apparatus with full face mask

o   Gloves

o   Hard hat

o   Safety shoes

o   Long sleeved shirts

o   Pants

o   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 sulfur dioxide 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  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

Some chemical vendors provide training for operators and other plant staff. Keep your Emergency Response Plan updated. And practice what to do in an emergency.

The Center for Disease Control has a number of good informational documents about sulfur dioxide, including the Occupational Health Guideline for Sulfur Dioxide, recommended reading.

Material Safety Data Sheet

Be sure to review the MSDS for sulfur dioxide before working with this chemical.

Sulfur Dioxide Accident Reports

Gas leak at Richmond's WWTP 

Penalty for Risk Management Program Deficienies 

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