Removing methyl bromide from shipping operations
Researchers in the U.S. have patented a bioreactor that uses bacteria to eliminate ozone-layer-destroying methyl bromide emissions from shipping operations.
The deadline for the 1987 Montreal Protocol’s phaseout by developed countries of methyl bromide, a pesticide that depletes stratospheric ozone, came and went in 2005. Although some countries have achieved a 100% phaseout, others, including the U.S., received exemptions for certain applications for which they claimed that no effective or affordable replacement existed for the pesticide.
The focus of most of the actions taken so far has been on the protocol’s targeted application of methyl bromide as a soil fumigant to kill insects and pathogens in agricultural fields. But the protocol does not cover the pesticide’s uses for sterilizing structures and disinfecting goods before shipping. Such quarantine and pre-shipment applications account for about 25% of the more than 45,000 tons of methyl bromide consumed globally in 2000, according to the UN Environment Programme’s Methyl Bromide Technical Options Committee [2.1MB PDF].
“There’s no limit as far as what can be emitted” by these shipping and quarantining operations, says Laurence Miller, an oceanographer and chemist at the U.S. Geological Survey. Miller and his colleagues were therefore inspired to address this problem by developing a bioreactor to target these emissions, and they received a U.S. patent on their design this spring.
They first described the bioreactor in a paper published in ES&T. The technology harnesses methylotrophic bacteria, which oxidize methyl bromide to get energy. The system is designed to recapture the pesticide from the waste-gas stream created after fumigation. The proof-of-concept, 10-liter bioreactor successfully removed 100% of the methyl bromide from an air stream.
“Here we have an ideal system,” Miller says. The bacteria “are gobbling up and thriving on this thing that you want to get rid of.”
The global background levels of methyl bromide hover near 10 parts per trillion, much lower than the hundreds of parts per million pumped into shipping containers for fumigation, according to Miller. After about 36 hours, the operators open a vent, start a fan, and blow the gas out into the atmosphere. Miller’s system calls for this gas to be passed through the bioreactor instead. Just one pass is required, he says.
The bioreactor is a tower containing a liquid phosphate medium with cells of a bacterial strain called IMB-1, which Miller and his colleagues discovered in agricultural soils during past research. They realized that the bugs had an appetite for methyl bromide and therefore isolated the strain. The temperature is kept at 28 °C, and the waste stream is diluted with air to keep methyl bromide levels at less than 500 parts per million, because the cells die at higher temperatures and concentrations. As the gas bubbles up through the media, the bacteria oxidize methyl bromide, creating CO2, hydrobromic acid, and water. During the process, sodium hydroxide is added to the medium to neutralize the acid, and in the end, you get a “soup that has nutrients, sodium bromide, and live cells,” Miller says. “You can sterilize this with heat or bleach, and it can go into a sanitary waste stream.”
Miller’s calculations show that removing methyl bromide from the fumigation exhaust of a standard shipping container in half an hour would require a 10,000-liter bioreactor, which he envisions as a stainless-steel vat 12 feet (ft) tall and about 4 ft in diameter.
Kelly Goodwin, a U.S. National Oceanographic and Atmospheric Administration researcher who has studied the biogeochemistry of methyl bromide, points out that Miller’s system offers advantages over activated charcoal adsorption, a chemical process that has been shown to recapture methyl bromide emissions. The pesticide can be adsorbed on the charcoal and recovered, but the technique still creates about 535 ppm of emissions. Moreover, recycled methyl bromide does not have a market in the U.S., because it is not registered as a fumigant.
Despite these advantages, Miller says that commercial interest in building a pilot system has been scant because of the lack of regulations for methyl bromide’s shipping and quarantining uses.
That will change, according to an official of the European Commission’s (EC’s) Ozone Layer Protection Team. He explains that the parties to the Montreal Protocol focused on controlling emissions from soil fumigation, which they deemed a critical use of methyl bromide, for two reasons. It was the biggest application area, forming about 75% of the pesticide’s consumption, and was easier to tackle, because countries with shipping fumigation operations would need to agree on alternatives, which include heating, cooling, or irradiating the container. “But as critical-use phaseout is quite advanced, we should and will put more focus on [the shipping and quarantining] area and evaluate more possibilities to phase out [methyl bromide’s use],” he says, adding that he expects related rules to be promulgated in less than 5 years.
In the meantime, some members of the EU, such as the Czech Republic and Austria, have acted independently and completely replaced methyl bromide for shipping and quarantining uses, he adds.
Until similar actions are taken in the U.S., recapturing the emissions by using technologies such as the bioreactor “is a great step in the right direction,” says Susan Kegley, a senior scientist at Pesticide Action Network North America, especially because it is easier to clean up the waste from closed containers than from open fields. She points out the need for similar technologies for soil fumigation.
Recapture technologies, such as bioreactors and adsorbents, could be a good intermediary step, the EC official says, but they are not a final solution. “It is like if you break your hand and you only take drugs against pain but don’t put a plaster on your hand,” he says. “As a transitory method, this is well acceptable, but the key problem is use of methyl bromide. To pull out the problem at its root, we should force people to convert to alternatives.”
