6 November 2008

Scrap iron goes green

Scrap iron can clean up industrial wastewater; just ask Wei-xian Zhang.

Zhang, a professor of civil and environmental engineering at Tongji University in Shanghai, just wrapped up a five-year research project in which he and his colleagues used 2 million pounds of iron to detoxify pollutants in industrial wastewater.

The project, which they conducted in Shanghai, was the largest in history to use iron in an environmental application. The iron, called zero valent iron (ZVI) because it is not oxidized, came in shavings or turnings from local metal-processing shops for less than 15 cents a pound.

The experiment did not start out with 2 million pounds. Rather, it began with small, experiments in the laboratory that used 90 pounds of iron to treat toxins in a solution. It graduated in 2005-06 to a pilot test using 2,000 pounds of iron to pretreat wastewater at a treatment plant in the Taopu Industrial District in Shanghai. Wastewater at the Taopu plant, which comes from small chemical materials and pharmaceutical companies, had previously undergone treatment with microorganisms alone. With ZVI augmenting the process, the remediation method improved.

Following the pilot test, the Shanghai city government approved a grant to construct a full-scale treatment reactor in the Taopu district capable of processing almost 16 million gallons of wastewater a day. This ZVI reactor connected to the biological treatment plant two years ago and has been in continuous use since. Shanghai’s Municipal Environmental Protection Bureau just certified the system.

The addition of ZVI to the traditional biological methods of wastewater treatment resulted in a significant improvement in pollutant levels, said Luming Ma, a professor of environmental engineering at Tongji and who also directs the National Engineering Research Center for Urban Pollution Control in the College of Environmental Science and Engineering. The removal of biological oxygen demand rose from 76% to 87%. Improvements also occurred with the removals of nitrogen (13% to 85%), phosphorus (44% to 64%), and colors and dyes (52% to 80%).

“Before this project, few people believed scrap iron could work in a wastewater treatment plant,” Ma said. “We have developed a copper-activated iron and used a systematic approach—from benchtop to pilot to full-scale tests—to show that ZVI-enhanced treatment can achieve dramatic improvements over biological processes used by themselves.”

While biological methods, including biofilms and aerobic organisms, are effective at treating municipal wastewater, said Zhang and Ma, who wrote a paper on the subject, they enjoy limited success in treating the less biodegradable and often toxic compounds in industrial wastewater, many of which are synthetic organic chemicals.

These chemicals attract to the surface of the iron, where they share electrons with the iron. They then become degraded and detoxified. ZVI, which undergoes oxidation during this exchange, has a useful lifetime of about two years in the treatment process.

The ZVI scrap iron is chemically similar to iron-based nanoparticles invented by Zhang that are now widely used in North America to clean decontaminated soil and groundwater. The nanoparticles contain 99.9% iron and about .1% palladium or other Noble elements.

Zhang said scrap iron’s low cost has potential to see use in developing countries where nanoparticles’ high cost (about $50 a pound) can be a barrier.

For more information, go to www.isa.org/environment.