Fighting Fire with Soap: Alaska Experiments with Foam

While other people deal with frozen cars or flee to warmer climes, fire fighters at the Bureau of Land Management are evaluating last summer's experiments with fire suppression foams in Alaska.

Flying tankers drop thousands of gallons of chemical fire retardant on wildfires every summer. Last year some of those tankers dropped masses of soft, white bubbles. The newest weapon in the fire fighters' arsenal is foam, and it looks like a good one.

Superficially, it's a dish-pan chemistry. Foam is created by mixing water with a wetting agent, essentially a detergent that creates a mass of small bubbles when agitated. Dropped on a fire, the small, uniform bubbles insulate vegetation, keeping out heat and the oxygen needed for combustion. When the bubbles break, they release water. Soapy water has a lower surface tension than clean water, so it penetrates and soaks the vegetation more effectively.

Fire fighters have known about the benefits of foam for a long time, but until about 10 years ago it was thought to be too expensive to use on forest fires. Then the Texas state Forestry agency started experimenting with a by-product of the pulp industry called soap skim. They injected air into the soap skim with a compressor and got stiff foam, like shaving cream.

That prompted fire-fighting agencies around the country to experiment with other foaming agents. Dawn brand dishwashing soap seemed to work well. Researchers found that three gallons of concentrate could produce a thousand gallons of foam. This brought down cost dramatically. Now several chemical companies make foam products that require very low concentrations.

The foam used on fires is biodegradable and contains no phosphorus. In three to five days it breaks down into harmless materials such as carbon dioxide and water. Still, fire fighters try not to drop any retardant or suppressant into waterways or lakes.

Foam has many appealing features. It is cheaper than conventional retardants---about one cent buys a gallon of expanded foam, while retardant costs about 1.56 per mixed gallon. Foam is light weight, so it doesn't hurt people or damage buildings on which it falls. A foam drop leaves a highly visible brilliant white blanket: pilots can easily tell where their last airdrop left off.

Foams can be made dry and long lasting or wet and short lived. Dry foams stay in the tree tops, keeping a fire from traveling in the canopy. Wetter foams drip down into the Lower vegetation, soaking it and dousing ground fires. Besides being dropped from air tankers, foam can be sprayed from trucks or dropped from helicopter buckets equipped with air injection systems.

The BLM Alaska Fire Service and State of Alaska air tankers dropped more than 400,000 gallons of foam in 1988. With each drop, fire fighters collected data on water quality, vegetation type, temperature, wind speed, slope and aspect of land, fire behavior, and other variables.

They found that foam is effective on low- to moderate-intensity fires in Alaska, but several things can hamper its performance. As anyone who has washed dishes knows, cold or hard, mineral-laden water requires more concentrate to get good foam. Contamination from other types of retardant residue in the tanker or hoses can reduce foaming action. If the air tanker is flying too fast or too high when dropping the foam, or drops it in windy conditions, the foam can drift and miss the target.

Foam is a short-term suppressant and must be dropped close to the fire, The effective area, or footprint, covered by a foam drop is shorter and wider than that of conventional retardant. Since the object is to put a line of suppressant around the fire, more drops are needed to cover the same length of fireline.

Despite the problems, foam is cost effective. Chemical retardant costs 156 times more a gallon, so even after allowing for more drops, which increase aircraft cost, the Alaska Fire Service estimates that it costs about half as much to cover the same length of fireline with foam as with other retardants. Foam saved the Alaska Fire Service an estimated $353,000 in 1988.

Studies on the limitations and capabilities of foam will continue. Foam will not be right for all fires, but it could save a lot of money in the right applications. When summer returns, the planes will again fight fire with bubbles.