1. School of Environmental and Municipal Engineering, Qingdao University of Technology, Qingdao 266033, China
2. School of Information and Control Engineering, Qingdao University of Technology, Qingdao 266033, China
3. Faculty of Environmental Engineering, The University of Kitakyushu, Kitakyushu 808-0135, Japan
4. Department of Statistic, George Washington University, Washington, DC 20052, USA