2 February 2012
MICROBE MACHINE: Bioreactors containing aerobic microbes were tested a semiconductor fab with the aim of reducing volatile organic compounds in the plant’s waste stream.
2 February 2012—A Taiwanese research team says it has demonstrated an efficient biological treatment for water-soluble volatile organic compounds (VOCs) emitted from semiconductor and optoelectronic manufacturing processes. This development could offer a more environmentally benign and less costly alternative to today’s expensive methods.
According to the team leader, Ching-Ping Tseng, a professor of biological science and technology at National Chiao Tung University, based in Hsinchu, major pollutants in waste gases emitted from semiconductor manufacturing include water-soluble VOCs, such as acetone, isopropyl alcohol (IPA), and propylene glycol monomethyl ether acetate.
The research team selected aerobic (oxygen-using) microorganisms from local sludge and demonstrated that the microbes could decompose these airborne VOCs, reducing the concentrations of waste gases to levels meeting environmental regulations in Taiwan.
Experiments conducted in the team’s lab suggest that the method can remove about 90 percent of VOC pollutants. Environmental standards for the semiconductor and optoelectronic industries in Taiwan dictate that 85 percent of VOCs have to be removed.
Later, the team built two bioreactors at an undisclosed semiconductor manufacturing plant in Hsinchu Science Park, near Taipei. The collaborating plant produced low concentrations of airborne VOCs but at high flow rates. The twin bioreactors took turns treating the 5-cubic-meter-per-second waste stream, each operating 12 hours per day. Results after one year of observation showed a reduction of roughly 85 percent of VOC emissions. Tseng says such a bioreactor can remove 60 metric tons of water-soluble VOCs per year, turning it into a small amount of sludge that can easily be treated further.
According to Tseng, using such bioreactors for waste-gas treatment is cheaper than relying on activated carbon filters, now common in many semiconductor plants.
Compared with a plant equipped with four activated carbon filters, each handling a VOC waste gas stream of 2.5 m3/s, the maintenance cost per year for the two bioreactors is NT$13 million (US $439 000) lower. But the initial capital investment for the two bioreactors is NT$14.2 million (US $480 000) higher than the facility with four activated carbon filters. “From a long-term point of view, it’s worthwhile for semiconductor firms to switch to the biological method,” Tseng says.
Experts in environmental solutions for semiconductor and optoelectronic manufacturers say the biological method does indeed conserve more energy and is more cost-effective, but there is room for improvement. Compared with commonly used methods, the biological treatment facility needs more space, says Jackey Fu, technical vice president of Taiwan-based Desiccant Technology Co. And when the facility is shut down for repair or maintenance, Fu says, it takes much longer to restart the system, because aerobic microorganisms need time to grow.
The biological methods will have to compete with improvements to the existing technology. Fu says burning concentrated VOCs, one common method, demands more energy than the bioreactor scheme, but this method is getting more efficient. “For example, the adoption of a new type of incinerator, called a regenerative thermal oxidizer, helps firms halve the fuel consumption,” he adds.
According to Tseng, it takes a week to restart a bioreactor. “That’s why two or more bioreactors have to be built as backup systems, to reduce the risk of facility breakdown,” he says. He adds that microorganisms in a bioreactor can stay alive up to 10 days when deprived of VOC waste gases, so a shutdown of a few days shouldn’t be a problem.
Ching-Fong Chang, deputy minister of Taiwan’s National Science Council, says the three-year research project, which it sponsored, meets global trends in terms of energy conservation and environmental protection. “It offers high-tech firms an opportunity to stay environmentally friendly,” Chang says.
Whether those firms adopt the technology is an open question. Elizabeth Sun, a spokesperson for Taiwan Semiconductor Manufacturing Co. (TSMC), in Hsinchu, the world’s largest foundry, says it uses porous materials called zeolites to concentrate the VOCs and thermal oxidization to destroy them. Its average emission reduction rates exceed 95 percent. The company is considering new environmental solutions, but it has some concerns about the bioreactor technique.
Companies such as TSMC often try new chemicals and innovative manufacturing processes, creating unstable circumstances in which the microorganisms must grow, Sun says. Consequently, the biological system could fail.
In addition, the company believes that the space demand could be a challenge. Should TSMC adopt the biological method, it would probably need more than 10 times the area it currently devotes to waste-gas treatment.
“Currently, the biological method might not suitable for us, but it might possibly become one of our choices someday if it becomes more mature,” Sun says.
About the Author
Yu-Tzu Chiu is a Taipei-based reporter and frequent contributor to IEEE Spectrum. In October 2011, she reported on the Taiwanese government’s efforts to retune its patent system.