WALTER J. WEBER, Jr., is the Gordon M. Fair and Earnest Boyce Distinguished University Professor of Engineering at the University of Michigan. He is the author of over 300 peer--reviewed technical publications, has been a registered professional engineer since 1962, was named a Diplomate in the American Academy of Environmental Engineers in 1972, and elected to the National Academy of Engineering in 1985. Dr. Weber received his ScB in chemical engineering from Brown University, MSE in environmental engineering from Rutgers University, and AM in environmental chemistry and PhD in water resources engineering from Harvard University. FRANCIS A. DiGIANO is Professor of Water Resources Engineering in the Department of Environmental Sciences and Engineering (School of Public Health) at the University of North Carolina at Chapel Hill. Prior to this appointment in 1981, he served on the faculty at the University of Massachusetts at Amherst. Dr. DiGiano is a registered professional engineer, a Diplomate in the American Academy of Environmental Engineers, and the author of over 100 technical publications. He received his BS in civil engineering from the University of Massachusetts, MS in civil (sanitary) engineering from Tufts University, and PhD in sanitary and water resources engineering from the University of Michigan.