1. M.S. Student, School of Environment, Tsinghua Univ., Beijing 100084, P.R. China.
2. Assistant Professor, Dept. of Civil and Environmental Engineering, Southern Methodist Univ., Dallas, TX 75275.
3. Associate Professor, Key Laboratory for Solid Waste Management and Environment Safety, Tsinghua Univ., Beijing 100084, P.R. China (corresponding author).