**6. Acknowledgments**

The authors thank the National Natural Science Foundation of China (grants 51072170, 21021002), the National Basic Research Program of China (grant 2007CB935603) and the National High Technology Research and Development Program of China (grant 2009AA03Z327), and the Environment and Water Industry Programme Office (EWI) under the National Research Foundation of Singapore (grant MEWR651/06/160) for their financial supports.
