Environmental team turns to bioreactor to clean up contaminated sites

  • Published
  • By Merrie Schilter-Lowe
  • 60th Air Mobility Wing Environmental Public Affairs
TRAVIS AIR FORCE BASE, Calif. --  With "green remediation" and "greenhouse gas emissions" becoming common
terms, it stands to reason that the environmental restoration team at Travis
would look for groundwater cleanup technologies that are cheap, efficient
and require little upkeep.

One solution may be the bioreactor - a 20-foot-by-20-foot hole in the ground
about 25-feet deep. The hole is filled with inexpensive mulch to encourage
the growth of naturally occurring bacteria that break down solvents.

"It looks like an underground percolator that uses mulch and gravel instead
of coffee grounds," said Mark Smith, chief of the environmental restoration
branch. "Microbes attach themselves to the gravel particles and live off
the carbon in the mulch."

Travis has two bioreactors - one at site SS016 near the flightline and
another at site DP039, which once housed a battery and electric shop. While
some bioreactors are inside a stainless steel chamber, the units on base are
in ground.

The technical term is "in-situ bioreactor," meaning built in place
bioreactor. Unlike the old fashioned coffee percolator, which circulates
boiling water through coffee grounds, the bioreactor uses solar-powered
pumps to draw contaminated groundwater from a nearby extraction well.

The groundwater passes through an irrigation pipe that sprays the water over
the mulch. As the mulch decomposes, it releases carbon that supports the
growth of microorganisms.

The microbes feed by stripping chlorine ions from the contaminated molecules,
eventually transforming them into harmless compounds. The groundwater flows
back into the reservoir and the cycle is repeated.

Each time the groundwater circulates through the bioreactor, microbes
further break down the contaminants. The process is called "reductive
dechlorination" and it is the foundation for most biology-based solvent
cleanup strategies, Mr. Anderson said.

Groundwater is not part of the base's water supply, but some residents living
south of Travis rely on groundwater for daily use. Groundwater also flows
toward Union Creek and has the potential to migrate to the Suisun Marsh.
This is one reason the restoration team is searching for the most efficient
and effective cleanup technologies available.

"We're focused on protecting people's health and freeing the land from
environmental restrictions," said Mr. Anderson.

Most of the contaminated soil and groundwater sites on Travis contain
trichloroethylene, aka TCE. Until the Environmental Protection Agency banned
TCE as a possible carcinogen, it was widely used by industry to clean and
degrease everything from clothing to aircraft parts.

Some 23 groundwater sites on Travis contain TCE, but with TCE concentrations
as high as 180,000 parts per billion, site SS016 is the most challenging.
The EPA cleanup standard for TCE is five parts per billion, said Mr. Anderson.
"SS016 seemed the ideal place for a bioreactor based on the success of the
bioreactor at site DP039," said Mr. Anderson.

The Air Force Center for Engineering and the Environment provided funding
and technical support for the bioreactor at site DP039 two years ago. The
unit has worked so well that the restoration team had the newest bioreactor
installed during the summer.

Team members believe bioreactors have the potential to cut decades off the
original estimated cleanup time and save energy in the process. How quickly this will happen is not known.

"We'll collect and analyze groundwater samples quarterly over the next year
to get some indication of how well the bioreactor is performing," said Mr. Anderson.

The restoration team began cleaning up contaminated sites in the 1980s,
first by conventional methods such as pumping groundwater from the soil or
bedrock, treating it to remove contamination and then pumping the water back
into Union Creek.

The "pump and treat" method was very effective in the early stages.
However, as solvent levels dropped, more groundwater had to be extracted and
treated to remove the same amount of TCE, said Mr. Anderson. For example,
nearly a billion gallons of water was pumped to remove eight tons of
solvent. And the operating cost of the system was not cheap - about
$500,000 annually, said Mr. Anderson.

Since most of the contamination from site SS016 is under the parking ramp
and flightline, the team employed a cleanup method called thermal oxidation
at the site, said Lonnie Duke, environmental scientist on the restoration
team.

"Thermal oxidation uses a vacuum to draw the solvents out of the soil then
burns the contaminated vapors using natural gas," said Mr. Duke. But
thermal oxidation uses as much natural gas as 300 average-sized homes per
year and the process emits more than 200 tons of carbon dioxide annually.

"While the unit was getting rid of some contaminants, contaminant
concentrations in the groundwater remained high. We needed a more effective
cleanup strategy," said Mr. Duke.

In addition to being environmentally friendly and costing less to operate,
bioreactors are self-sustaining and require little maintenance. The mulch
may break down over time but it is easy for an environmental contractor to
add more mulch, said Mr. Duke.

Although the base has until 2012 to have remedies in place to clean up
contaminated sites, the Travis team hopes to be ahead of that schedule.