An industrial engineer has developed a computer model to help recyclers more economically process raw materials from the millions of computers and other electronic trash landing in the waste stream every year.

   Recycling electronic hardware is becoming increasingly important because of the sheer quantity of potential e-junk. The number of personal computers that become obsolete every year is expected to rise steadily, from an estimated 41.9 million this year to 61.3 million by 2007, according to the National Safety Council's Environmental Health Center. A May 1999 report prepared by the center identified outdated laptop computers as the fastest-growing segment.

   Currently, recycling computers and television sets is not required in most places. But if new requirements are instituted in the future, recyclers will face a significant challenge trying to manage the surging flow of high-tech junk, said Julie Ann Stuart, an assistant professor of industrial engineering at Purdue University. Stuart came to Purdue in August but developed the computer model while she was a faculty member at The Ohio State University.

   Certain electronic components contain dangerous pollutants, including mercury, lead and cadmium, making it important to recycle discarded computers so they are not dumped in landfills. Some of the electronic products received by recyclers are still useful and can be repaired, while others are cannibalized for parts. The remaining junk is processed and, in some instances, reprocessed to extract and purify the metal, plastic and glass it contains.

   This reprocessing involves repeating various steps such as grinding, shredding and separating materials. However, at a certain point it becomes too expensive to continue purifying the bulk materials; each mechanical reduction and separation "run" costs money. Recyclers can end up losing money by spending more to reprocess some materials than is justified by current market values.

   Meanwhile, new junk is constantly arriving, making it important to efficiently manage the flow.

   Until now, there has been no analytical tool to help recyclers more economically reprocess the bulk materials. Stuart has created just such a tool. Her "discrete reprocessing model" can be used to determine the threshold at which it becomes unprofitable to continue purifying bulk materials.

   "Our model allows us to find the pricing threshold for reprocessing," Stuart said. "So, if the price of copper fluctuates, it helps us find how low the value could be to justify reprocessing."

   Stuart developed the model with Qin Lu, a doctoral student at Ohio State.

   "We are the first to look at the reprocessing decision," Stuart said, noting that recyclers have relied more on instinct than analytical tools. "There has been no formal way to decide questions like: When do you process something more than once? What level of purity do you want?"

   The model was detailed in a research paper that appeared in the October issue of IEEE Transactions on Electronics Packaging Manufacturing, a journal published by the Institute of Electrical and Electronics Engineers.

   Recycling electronic hardware is not commonly required by law, although Massachusetts has enacted the nation's first ban on dumping computer screens, television sets and other glass picture tubes at landfills. However, similar policies could become more common in the future, drastically increasing the flow of electronic products to recyclers and causing their inventories to rise sharply.

   Complicating matters is the fact that most recycling centers are small-scale operations and would be ill-equipped to handle surging demands on service.

   "A contributing factor to the closing of some recycling companies has been excessive material handling and inventory," Stuart said.