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Can WaterGEMS or WaterCAD be used to perform a hydrosanitary design?
In a hydrosanitary design, pipes are sized based on reasonable peak demands. The usual way of doing this is the “Fixture unit method” where you total the fixtures downstream of any point and use “Hunter’s Curve” or some similar approach to determine peak flow.
This has been discussed in Advanced Water Distribution Systems Modeling and Management (pages 399-401). You can find even better references to it in many other places such as AWWA Manual M22 "Sizing Service Liens and Meters".
To design this system using WaterCAD or WaterGEMS, you need to first lay out the piping systems. Create a number of scenarios based on which specific toilets and sinks are running. Run the model and view pressures, flows and velocities.
It is best to use pressure dependent demands calculations in this type of system. WaterCAD/GEMS doesn’t use the fixture unit method, it calculates the actual flow and pressure.
Please be sure to account for the head loss through any backflow preventers and flow meters. The key input is the hydraulic grade you can expect from the supply system; use a conservative value.