1. Department of Environmental Science and Engineering, Tsinghua University Beijing 100084, People's Republic of China
2. Department of Environmental Science and Engineering, Tsinghua University, Beijing 100084, People's Republic of China. Phone: 86-10-62773095; Fax: 86-10-62771472; E-mail: hanchang@mail.tsinghua.edu.cn
3. Department of Civil & Environmental Engineering, University of Alberta Edmonton, Canada T6G 2W2