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If you have spent much time lately looking around truck stops, car dealerships and diesel-fleet refuelling facilities, you may have noticed some storage tank and dispensing systems being installed.

The equipment looks familiar, but you may not know everything about the new substance that is being stored, metered and dispensed from these new systems. It’s called diesel exhaust fluid, or simply DEF, and it’s different than the substances most of us are used to.


In 2000, the U.S. EPA published Tier 2 emission standards for light-duty vehicles and trucks. It set common standards for all passenger cars, light trucks, and medium-duty passenger vehicles. The Tier 2 standards require new diesel vehicles, produced in model years 2009 and after, to meet an average oxide of nitrogen (NOx) emission level of 0.07 grams per mile. In 2001, the U.S. EPA published a rule setting requirements for new heavy-duty highway engines and vehicles produced after January 1, 2010. The diesel engine NOx standard for 2010 is 0.20 grams per brake horsepower-hour.

Manufacturers plan to meet these requirements by optimizing engine designs for low emissions and adding high-efficiency after-treatment. Diesel engine and vehicle manufacturers have considered several different types of NOx-reduction technologies in order to meet these requirements. One technology, selective catalytic reduction (SCR) using DEF, has been chosen by nearly all diesel-engine manufacturers because it can achieve as high as 90 percent NOx conversion efficiency. SCR technology adds DEF to the exhaust stream to promote these efficiencies

DEF Characteristics

DEF is a clear, colourless, non-toxic, non-flammable, non-combustible liquid. It is made up of 32.5 percent urea with the balance distilled or deionized water. Urea and water are completely miscible and do not separate in storage. For individuals associated with an underground and/or aboveground storage tank program, it is important to understand that DEF is NOT:

  • Motor fuel
  • Fuel additive
  • Flammable or combustible liquid
  • Volatile organic compound
  • Hydrocarbon liquid
  • Liquid that requires Stage I or Stage II vapour recovery
  • Substance that is currently regulated by the federal government.

DEF Is Not Regulated Under By EPA’s UST Regulations. The states have asked EPA’s Office of Underground Storage Tanks whether EPA regulates USTs containing DEF under the federal regulations. The primary issue was that DEF may contain a small amount of ammonia, which is a regulated substance under the U.S. UST laws.

According to a memorandum issued September 22, 2009, by Carolyn Hoskinson, director of EPA’s Office of Underground Storage Tanks, “since EPA expects that the presence of ammonia in a DEF UST will be minimal, it is EPA’s view that DEF USTs meet the de minimis exclusion and thus are not regulated as hazardous substance USTs under the federal regulation.”

The director’s memorandum also contains an important caveat. The memo notes that “some states may choose to be more stringent than federal regulations and require DEF USTs to fully comply with state UST regulations.”

While at this writing I have not encountered any state or local regulator choosing the “more stringent’ approach at regulating USTs containing DEF, it can happen, so PEI is cautioning contractors and facility owners to check first with state and local authorities having jurisdiction regarding any special requirements.
At this writing, it is still uncertain how Canadian federal and provincial governments will regulate DEF - if at all.

PEI’s Recommended Practices for the Storage and Dispensing of Diesel Exhaust Fluid (DEF) PEI RP1100-10

The Petroleum Equipment Institute (PEI) has published a recommended practice for the installation and operation of DEF storage and dispensing equipment that will preserve its quality and prevent releases into the environment.
The recommended practices apply to the storage, handling, and dispensing of DEF at motor-fuel-dispensing facilities and repair and maintenance garages. It is limited to storage containers that use a pump and/or meter to dispense DEF.

The single-copy price for RP1100-10 is $44 for PEI members; $95 for nonmembers. Member pricing is extended to all regulatory officials. For more information about this special pricing for regulators, contact Keith Wilson at PEI: 902-494-9696 or

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