60 The Future of Water in African Cities
Adaptive Systems Work Best to Cope with Uncertainty Water management must take into account that the future is inherently uncertain. It is important to recognize that there is uncertainty about many future conditions including population distributions, economic growth, and climate change. A flexible strategy that can adapt over time to changing requirements is critical (Vairavamoorthy et al., 2008). The principles of IUWM outlined previously foster the use of flexible technologies and approaches to planning that can respond to the uncertainty about the future paths of expanding African cities. A more modular, decentralized approach to urban water management and a clustered growth approach to urban development are particularly important components in planning for an uncertain future (Bieker et al.,
Box 2.12
Flexible Design of Decentralized Wastewater Treatment and Reuse Systems: Xi’an, China In the newly developed housing area outside Xi’an in northwestern China, an expansive green space was considered necessary both to increase the real estate’s commercial value and to improve living conditions. Because the water demand for gardening is high, treated wastewater was used for watering to mitigate urban water shortages (Wang et al., 2008). In the city of Xi’an, there were two options for wastewater reuse: a supply of reclaimed water from treated wastewater through a centralized pipe system; and the use of decentralized wastewater treatment systems (DEWATS) for onsite reuse. DEWATS were cost efficient, and they could be implemented more quickly than the centralized system. The decentralized system could also be implemented incrementally, which provided the flexibility to trace the urban growth trajectory and to avoid up-front investment. A pilot project was implemented that included six residential buildings for 400 people. The wastewater management system includes a dual-piping sewer system for separate collection of blackwater and greywater. The greywater is treated by a process combining enhanced primary treatment with ozone-enhanced flotation that included chemical coagulation, biodegradation, particle pelletization, and separation in one unit. The treated greywater is used to replenish an artificial pond with a water surface area of 6,500 square meters (0.5 meters deep) that maintains a green belt that covers an area of 6,400 square meters. Source: Wang et al., 2008.