DEL GUAYAS

Outlining resource movement within the water and energy infrastructure networks that serve buildings presents distinct differences between the two systems (Table 2.5). Smart energy grids allow for bidirectional flows of energy, which subsequently permits on-site energy generation and offsets. Water pipelines only allow for unidirectional flow, and therefore water cannot be re-introduced into the existing system in order to offset consumption. The inclusion of a disposal step within the water network which is omitted in the energy sector highlights contrasts in the properties of each resource. Energy consists of many forms, beginning with natural sources which are transformed into convenient forms, such as electricity, in order to serve building end uses or functions. At the end use, the energy flow may again be transformed to meet the function, whether as motion, light, or heat. Conversely, water does not change forms, but mixes with impurities at different points in the network. Water generally serves a temporary purpose within the building that affects its quality, and the constant demand for high-

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quality water requires low-quality wastewaters to be discharged for treatment and eventual reuse.

Table 2.5: Comparison of the urban water and energy infrastructure networks.

Water Energy

Natural sources Surface water (lakes, rivers, streams) Groundwater (aquifer) Seawater (desalination) Fossil fuels Solar radiation Wind Biomass Geothermal gradients Gravity

Production methods Water treatment plant Desalination plant On-site groundwater wells

Power plant Hydropower facility Solar panels Wind turbines Distribution system Unidirectional pipe flows Bi-directional network

Building end use Consumption Electricity

Heat Light Motion Discharge methods Wastewater infrastructure

Wastewater treatment plant Stormwater infrastructure

When the boundary is drawn such that it includes the environment holding the water source, municipal water production, and subsequent treatment and discharge, building water use appears to be a minor factor. In this case, the environment becomes the resource stock that requires net-zero balance – water consumed must equal water generated in order to preserve the volume. Within this wide view of water use, water is removed from its natural source and conditioned at a centralized water treatment facility in order to meet quality standards. Treated water is then distributed to building structures through a pipeline network to meet customer demands. On-site water is consumed by a variety of end uses, and wastewater is generated simultaneously. Using a separate sewer infrastructure network, wastewater flows are directed to a centralized wastewater treatment facility where the water quality is improved before returning to the natural source stock. In cases where buildings are not served by a

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municipal sewer network, wastewater treatment and discharge occur on-site, such as through the use of septic systems.

The municipal water cycle agrees with the ecological protection mandated by some net- zero water requirements. Water consumed by buildings equals wastewater generated. The two streams are comparable due to shared quality standards that occur at the beginning and end of the cycle, making wastewater a renewable water source. However, this wide water cycle view includes challenges that limit efficiency and support net-zero approaches at the building scale. The transportation of water within the pressurized distribution system results in leakage losses; and therefore, not all of the water produced is delivered and consumed by the customer resulting in a system imbalance. In addition, the natural water source used for water production is often different from the location where treated wastewater is discharged. For example, pumping water from an underground aquifer for potable consumption and returning the used flow to a surface river disrupts the ecological cycle. In this case, the availability of the resource has changed and may no longer be fairly compared for net-zero balance. Pursuing net-zero by incorporating municipal facilities also affects infrastructure networks and security. Aging infrastructure networks incur stress due to changing population demands and require continuing maintenance. Vulnerable distribution systems and centralized treatment facilities reduce security of connected building sites; and facilities reliant on centralized processes are sensitive to service disruptions and variable pricing, whereas self-sufficient sites can better control resource flows and costs.

Minimizing the system boundary to the building site highlights potential flows that must be addressed in order to evaluate net-zero water. Distribution losses are limited, and water quantities are easier to verify within the smaller system boundary. Inflows include potable water, municipal reclaimed water, and precipitation, while wastewaters and runoff outflow from the boundary. Net-zero analysis requires water consumption to be compared to water generation, in which the demands within the building drive consumption that can be met by any

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municipal or alternative water source production. However, water generated on-site is constrained to wastewater and precipitation flows. Wastewater results from the building operation and is therefore an internally generated source that can be freely utilized. Precipitation occurs regardless of the building’s existence and initially contributes to an existing natural cycle. Utilizing captured precipitation in order to offset or eliminate municipal supplies supports building self-sufficiency, but may affect overall resource availability. Therefore, preservation of hydrologic flows is a necessary component for net-zero water analysis (Hoekstra, 2008).