The recent natural gas explosion in San Bruno California (not far from EMLab P&K's San Bruno offices) has prompted many questions about the safety of our aging infrastructure. This event killed seven people, injured many more, and destroyed 37 homes. Another explosion of this kind occurred in the southwest killing 12 people. At least one possible cause of these explosions is microbial corrosion of the steel pipes that carry the gas.

It is hard to imagine that something as small as a bacterium can cause such devastation. However, there is power in numbers, and in the very powerful chemicals that bacteria can make from minute amounts of nutrients. Here is how it works.

First, where do the organism come from? As we know, bacteria are ubiquitous on earth, and there are at least some of them on virtually every surface that is not designed to specifically exclude them. You can imagine that these giant steel pipes are no exception. While attempts are made not to include large amounts of soil inside the pipes, they cannot be made sterile. Unfortunately, many of the bacteria that can cause corrosion produce endospores, which are highly resistant structures that can live for many years. These spores are common in soil, and are likely to be present throughout the many miles of gas pipeline throughout the world.

Why don't these explosions happen more often? The essential ingredient that is missing is water. The pipes are designed to exclude soil water, so that, as long as they remain intact, water is not present. How does water get into the pipes? In a well-designed, well-laid pipe, the water probably comes from the gas itself. It may also be that 50 years ago (which is when the San Bruno pipes were laid) the pipes were not as carefully constructed or laid as they are today.

The bacteria that cause metal corrosion are primarily anaerobic forms. When water is present in an environment with oxygen, a biofilm begins to form. The organisms in the upper layers of the biofilms use all of the oxygen so that an anaerobic niche is formed, allowing these corroding bacteria to grow. The most common bacteria that lead to corrosion of steel can use sulfur as an energy source, and even a small amount of sulfur present in the steel or on surfaces can be enough. The bacteria use the hydrogen in the water to reduce sulfates, sulfites, thiosulfates, and sulfur to hydrogen sulfide, which then reacts with metal to form metal sulfides.

Biocorrosion gradually weakens the walls of the pipes, and the high pressure gas (1000lbs/in²) eventually ruptures the pipe and is released at high speed through the failed metal. The rapidly escaping gas ignites, leading to a catastrophic explosion.

Of course, there are other causes for gas explosions, and even other microbial corrosion pathways that can be causative. For all of the microbial problems the solution is to assure that water does not enter and accumulate anywhere in the pipeline.

This article originally appeared in the November 2010 issue of Indoor Environment Connections. Reprinted by permission.

About Dr. Harriet Burge Dr. Harriet Burge is EMlab P&K's Director of Aerobiology and Chair of EMLab P&K's Scientific Advisory Board. Widely considered the leading expert in indoor air quality (IAQ), Dr. Burge pioneered the field more than 30 years ago. She has served as a member of three National Academy of Sciences committees for IAQ, including as Vice-Chair of the Committee on the Health Effects of Indoor Allergens. View Dr. Burge's Curriculum Vitae. Read "Ask Dr. Burge" articles

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