Design Considerations for Automated Makeup Water Supply Systems
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Field Observations For some practices, field observations may be required to determine the design demand. For example, a washing station may require field observation of the time spent washing each vehicle.
Air-gap An air gap and other cross-connection requirements included in relevant building codes must be followed when combining potable and non-potable waters. An air gap physically separates two sections of pipe and is open to the atmosphere. The air gap must be located higher than the overflow drainage piping from the tank and the overflow drainage piping must remain free of blockage so that excess rainwater flows to the overflow system and does not back up and overflow at the air gap. Air gaps are not generally utilized or recommended in large outdoor storage tank applications due to freeze protection and loss of volume storage due to required air space.
Minimal make-up water storage Stormwater management objectives must not be compromised if a secondary or makeup water supply is used. The design must provide adequate storage for the next design storm. Make-up systems must place a minimal amount of volume in the storage at any one time. Utilizations of an air gap in a day tank is one option to address this issue.
Level Indicators Automatic make-up is typically triggered by a float switch, pressure transducer, or level indicator. The water elevation that triggers the make-up must be high enough to avoid running the pumps dry and must be lower than the passive draw-down orifice (if applicable). Float switches are preferred for critical operations including dry run protection and as fail safe cut off when transferring water to a day tank. Level indicators can be used for informational levels and non critical devices and are prone to failure in moist environments.
Wet wells An option for larger, underground storage tanks is to have a separate wet well at a lower elevation. This eliminates the storage of make-up water in the harvest storage tank.

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