Nowadays, the level of urbanization is in rapid increasing, but the freshwater resources still have the problems of unbalanced distribution of regions and time. Besides, the freshwater resources are still in short supply. However, in fact, there is always a hidden resource in the city - the wastewater. The rapid population growth and increasing urbanization have increased the needs for wastewater treatment and recycling to ensure water resources adequacy, and then improve the environmental quality. In this day and age, there are two main ways to treat wastewater: centralized treatment and decentralized treatment. Both of these methods can reduce the pollution of wastewater to water resources and provide domestic water. In this article, we will introduce and compare the two approaches with real-world examples: The Shanghai Bailonggang Sewage Treatment Plant, located in Chaoyang Village, Heqing Town, Pudong New Area, and the decentralized sewage treatment system in Germany.