Chlorine Gas Continues to Be a Safe Disinfection Method
for Water and Wastewater Treatment Plants Worldwide

Chlorine (Cl2), one of the most common liquefied gases used in the disinfection of water and wastewater, has played a vital role in the reduction of disease worldwide for more than 100 years. With the development of effective safety products such as scrubbers and vent exhaust gas arrestors; authorization of environmental health and safety regulations; and strict enforcement of applicable codes, gas chlorination continues to be an effective treatment method proven to be safe, reliable and very economically priced to meet varying application needs.

In response to concerns over the safe storage and transportation of gaseous chlorine, water utilities are working to secure their reservoirs, treatment plants and distribution systems. Municipalities have further reduced risk because of local, state and federal code restrictions against exhausting chlorine fumes outside a building. The NFPA1/Uniform Fire Code (UFC) and Building Officials and Code Administrators (BOCA) codes, for example, have been adopted across many utilities as a way to address leak containment and mitigation associated with the storage and use of liquefied gases. These codes are used as the design basis by producers, transporters, designers and users of liquefied hazardous gases.

Understanding code documentation fosters compliance with today’s environmental health and safety regulations
Some municipal water and wastewater treatment plant operators and managers may believe they face a dilemma when it comes to complying with today's environmental health and safety regulations: How to comply with these regulations – including USEPA’s Risk Management Program and Article 80 of the UFC – without switching from chlorine gas to a potentially more costly disinfection agent that has the potential to increase operator work load? Unfortunately, interpretation of the NFPA 1/UFC by suppliers of chlorine alternatives and of shut-off valve and containment devices can be misleading to operators and managers. Code paragraphs are often taken out of context and quoted without regard to clarifications that are included elsewhere in the documentation.

Here is an example of how information in the Code may be taken out of context: Any gas that has a lethal concentration (LC50) over 200 ppmv is considered a “Toxic Gas” and if less than 200 ppmv it is considered a “Highly Toxic Gas.”  Chlorine qualifies as a “Toxic Gas” since it has an LC50 of 293 ppmv. Containerized anhydrous chlorine under vapor pressure at the prevailing temperature is considered a “Toxic Liquefied Gas.” As such, since chlorine is not a highly toxic gas, special protection provisions are not required when stored under NFPA 1/UFC-specified threshold amounts. When the threshold amounts are exceeded, treatment is considered.  Based on this information from the Code, a reader might believe that chlorine scrubbers are no longer required in order to meet the regulations. But, reading deeper into the Code reveals that a “Toxic Gas” and “Highly Toxic Gas” are grouped when treatment systems are considered.

Gaseous chlorine is typically furnished in 150-pound, one-ton, multi-ton stationary tanks and railcar configurations and accompanying gas feed systems can be cylinder-, ton-, wall- or floor-mounted depending on the application. In the United States and many other countries, the “valved” transportable containers from which the chlorine is fed are equipped with special relief devices called “fusible plugs.” These plugs are designed to melt at approximately 160oF (71oC) to allow the contents of the container to discharge to the atmosphere, preventing rupture in case of exposure to high temperatures. It is important to note that the timely closure of the container valve will not prevent the plug from allowing chlorine to escape the container. Also, the implementation of a total containment device is not practical since a complete fusible plug discharge can be over in five minutes. 

Shut-off valves do not act on the fusible plugs. And, containment devices that encapsulate the entire 150-pound and 2,000-pound containers have relief valves designed to exhaust outside the containment device. For example, in the case of exposure to high temperatures, the fusible plug will open followed closely by the containment device relief valve opening into the room. 

Reading further, the Code specifies that containment vessels or systems shall be capable of fully containing or terminating a release. A shut-off valve or containment device cannot “fully contain or terminate a release” in a high temperature scenario. As a result, if the devices mentioned – such as shut-off valves or containment systems – cannot contain a fusible plug release, then all exhaust is to be directed to a treatment system that qualifies by the Code guideline as follows:

  • Treatment systems capable of diluting, adsorbing, absorbing, containing, neutralizing, burning or otherwise processing stored or used toxic and/or highly toxic gas.
  • Treatment systems capable of reducing the allowable discharge concentrations to one-half the immediately-dangerous-to-life-and-health threshold at the point of discharge.
  • Treatment systems sized to process the maximum worst-case release of the gas based on the maximum flow rate of release from the largest vessel.
  • Consideration of the single largest compressed gas vessel.

It is not to be mistaken: the UFC requires that a treatment system be available for “processing accidental release of gas” when using gas chlorination.

Scrubbers and vent exhaust gas arrestors are proven and often required safety treatment systems
Scrubbers have proven to be a highly cost-effective tool to mitigate accidental chlorine gas releases in municipal and industrial applications in accordance with NFPA 1/UFC requirements.

Wet scrubbers have traditionally been used by municipalities. However, dry scrubbers are increasingly being placed in municipal applications. While dry scrubber systems are typically more expensive than wet systems, lower operational and maintenance costs and their ease of use make them an excellent alternative to traditional wet scrubbers. Over a 20-year operational period, assuming complete media exhaustion, dry media replacement and disposal costs could be half of the cost of the caustic solution required for wet scrubbers.

The EST™ Dry Emergency Chlorine Scrubber from Severn Trent Services is designed to mitigate a worst-case release of hazardous gas through a fusible plug. This safe, user-friendly system offers a low cost of ownership. It does not require chemical maintenance or the use of chemical pumps. New and used dry media is non-hazardous and the system features only one moving part, the blower.

Severn Trent Services also offers the Capital Controls® VEGA™ vent exhaust gas arrestor, which is designed to treat occasional gas vent releases that normally accompany chlorine container changeovers. The VEGA prevents nuisance tripping of chlorine room air sensor alarms; helps compliance with local, state and federal code restrictions against exhausting chlorine fumes outside of a building; and keeps the building free from chlorine-related acid corrosion.

For more than a century, gas chlorination’s cost-effectiveness against most pathogens, its residual protection and its odor control have combined to make it an invaluable means of disinfecting world water supplies. A thorough understanding of, and conformance to, applicable federal, state and local codes and statutes will ensure this disinfection workhorse will continue to be an indispensable tool for municipalities for years to come.

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