Figure 3.42 Schematic Diagram of a Water System

and collection networks, control valves, pumping stations, storage tanks, and disinfection facilities. The system serves distributed demand nodes.

The following sections highlight technological factors in designing water systems and the merits of the proper spatial distribution of demand nodes, which are governed by land use planning.

Water supply systems

The spatial distribution of demand centers: Cities should strive to limit urban sprawl to ensure that the demand for water is not overly dispersed and that the extent of distribution and collection systems is minimized. (The costs related to distribution networks typically account for 70 percent of the overall costs of a water supply system.) Denser land developments help minimize the costs of capital and operations. Energy consumption also typically declines because consumption is linearly proportionate to pipe length. In addition, dense developments promote environmental protection.

Distribution system spatial confi guration: It is important to achieve highly reliable water supply services. This often entails some redundancy in the network, such as loop networks that supply a demand zone through more than one main pipe. This may involve trading higher capital costs for improved reliability and reduced energy costs. Proper system reconfi guration leads to reduced total distribution costs (box 3.9). To avoid substantial increases in network costs, loop networking may be operated at lower pressures and through smaller pipes made of material that is less expensive than ductile iron.

Water treatment plant: The water treatment process may be simple. Biological treatment is preferred over chemical-intensive processes. Treatment plants must be close to water resources and, it is hoped, close to urban demand centers. To ensure the security of water supplies, cities must build the treatment plant with space to expand to meet growing demand and should consider building more than one plant associated with diff erent sources (if possible).

Groundwater and water wells: If the groundwater is viable, well fi elds should be developed and distributed near demand centers. This proximity leads to a simpler network and lower energy and capital costs. Distribution systems may often operate from minimal storage tanks because the aquifer represents a robust and feasible storage source. Moreover, many cities commonly use aquifers as natural storage receptacles for the surface water that infi ltrates from basins along river embankments and other areas (for example, Paris). Infi ltration is a natural treatment process that helps purify water at minimal cost.

Water pumps: Water pumps are the main energy consumers in water systems. The energy consumed is proportional to the set effi ciency of the pump motor. Normally, pumps are most effi cient at their designed operating points. However, owing to load changes, pumps are often operated outside peak effi ciency, and signifi cant energy is wasted. Variable speed pumps may be modifi ed to address this waste. Pump speed is adjusted to maximize effi ciency given any particular load. Pumps may also be noisy, but this may be mitigated by introducing acoustic insulators.

Storage Tanks: Depending on the mode of operation, storage tanks may be important com-

Figure 3.42 Schematic Diagram of a Water System

Source: Walski, Chase, and Savic (2001).