The requirement for back-up power for distribution system facilities is driven by three Arizona Administrative Code (AAC) requirements as follows:
- R18-5-502.B - A potable water distribution system shall be designed to maintain and shall maintain a pressure of at least 20 pounds per square inch at ground level at all points in the distribution system under all conditions of flow.
- R18-5-503.A - The minimum storage capacity for a CWS [community water system] or a non-community water system that serves a residential population or a school shall be equal to the average daily demand during the peak month of the year. Storage capacity may be based on existing consumption and phased as the water system expands.
- R18-5-503.B - The minimum storage capacity for a multiple-well system for a CWS [community water system] or a non-community water system that serves a residential population or a school may be reduced by the amount of the total daily production capacity minus the production from the largest producing well.
Conformance with item 1 is usually implemented by the construction of one or more reservoirs to provide storage capacity for a distribution system service area. Properly designed reservoirs located at the high water elevation of a service area can provide hydraulic head to maintain a nominal system pressure of 20 psi. In some cases, isolated service areas may be fed by redundant supply sources instead of using reservoirs to maintain system pressure.
Conformance with items 2 and 3 is implemented by calculating the size of the reservoirs to provide sufficient capacity to meet the required capacity for peak and fire flow demands in combination with redundant supply sources feeding a distribution system service area such as surface water treatment plants, groundwater wells, booster pump stations and pressure reducing valve stations.
Emergency power may be required for groundwater wells, booster pump stations and pressure reducing valve stations acting as a source of supply to isolated service areas. For example, a service area with insufficient storage capacity fed by two other service areas via booster pump stations that are on the same electrical grid would not conform to the AAC requirements as loss of power would result in a loss of pressure in the distribution system.
An engineering analysis should be performed whenever a water distribution system is being modified or expanded to determine if emergency power is required for new or existing groundwater wells, reservoirs, booster pump stations and pressure reducing valve stations. The analysis should calculate the required minimum storage capacity for peak demand and fire flows and address the impact of system wide interruptions due to loss of power, major transmission pipeline ruptures and equipment failure.