NAME : Stephane A Page, Donald B Cherry, C Clair Claiborne
COUNTRY : Switzerland, USA
REGISTRATION NUMBER : DLGw24813
GROUP REF. : A2
PREF. SUBJECT : 2
QUESTION N° : 2
It is now commonly accepted that biodegradability is considered to be a highly desirable
property in dielectric insulating liquids used in transformers since major environmental
concerns over the toxic effects and collateral damages of uncontained transformer oil spills
have quite extensively been reported. The definition of biodegradation is the process by
which organic substances are broken down by the enzymes produced by living organisms.
Whenever any substance is placed in the environment, it begins to undergo degradation due to
the action of relevant substrate utilizing microorganisms. Every substance, irrespective of its
complexity and toxicity, is subject to this process. The term biodegradability is generally
associated with reduced environmental impact in the event of a spill and less toxicity to living
organisms. When used in the present context, it is referenced to a time frame. That is, for a
transformer oil to be referred to as biodegradable, a specific time limit must be used as a
reference.
There are two major types or classifications of biodegradability, primary and ultimate. Both
types are measured as a percentage change in relation to a period of time under standard
conditions. Primary biodegradation measures the loss of a product by alteration of the
chemical structure resulting in a change in the chemical nature of the product, not the partial
or total degradation or breakdown of the material into CO2 and water. CEC L-33-A-93 and
EPA OPPTS 835.3110 – developed by the Coordinating European Council (CEC) and by the
U.S. Environmental Protection Agency (EPA), respectively – are test methods for primary
biodegradation that determine the rate and extent of aerobic aquatic biodegradation under
laboratory conditions. Biodegradability tests based on the CEC method described above
indicate that mineral oils have poor biodegradability, i.e., typically between 15 and 35%, and
as low as 10%. While silicone oils are essentially non-biodegradable per the same method,
the latter shows that vegetable-based oils (or natural ester-based liquids) undergo typically
about 90 to 100% biodegradation in the specified period of time (21 or 28 days). Synthetic
ester-based liquid biodegradability is typically about 80% and can be as high as 90% per the
same CEC method.
Figure 1 – Biodegradation rates of different dielectric insulating liquids used in transformers
measured as per the CEC L-33-A-93 test method after 21 days [1]
Ultimate biodegradation, called mineralization, is the percentage of the substance that
undergoes complete degradation, i.e., how long it takes to achieve a specific percentage of
complete degradation. When this happens it is referred to as Ultimate or Complete
Biodegradation – which is the degradation achieved when a substance is totally utilized by
microorganisms resulting in the production of carbon dioxide, methane, water, mineral salts,
and new microbial cellular constituents. ISO Method 14593, ASTM D5864 and Method
OECD 301B – the Modified Sturm Test developed by the Organization for Economic
Cooperation and Development (OECD) – determine ultimate biodegradation through the
measurement of CO2 released. A material is classified as readily biodegradable when 60% or
more of the test material carbon is converted to CO2 in a 10 day window over a maximum
period of 28 days, as determined using these test methods. ISO Method 10708, OECD
Methods C, D, F, and Method EPA 560/6-82-003, number CG-2000 – the Shake Flask Test
adapted by the U.S. Environmental Protection Agency (EPA) – determine biodegradation
through the measurement of Biochemical Oxygen Demand (BOD). The pass criteria for these
tests is the same, i.e., at least 60% of the oil has to be degraded within a 10 day window over
a maximum 28-day period. Both natural and synthetic ester-based dielectric insulting liquids
are typically readily biodegradable.
Figure 2 – Biodegradation rates of different dielectric insulating liquids used in transformers
measured as per the OECD 301B test method [2]
The key point when specifying biodegradability is to ensure that a required percentage is set
in reference to a specific test method. Prior agreement to these parameters will serve to
prevent confusion or misinterpretation of the reported test results.
[1] R. Asano and S. A. Page, “Reduced environmental impact and improved safety and performance
of power transformers with natural ester dielectric insulating fluids”, Paper PR-13, 9th Petroleum and
Chemical Industry Conference Europe, Electrical and Instrumentation Applications, Prague, Czech
Republic, June 2012
[2] Cigré Brochure 436, Experiences in Service with New Insulating Liquids, October 2010