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Answer

WaBack is a normally open non-return valve, which means opening it doesn’t require any pressure. It is advantageous in low-lying areas, where the pressure head is minimal. WaStop, on the other hand, is a normally closed non-return valve, which means it closes even in the event of a positive upstream pressure, providing the advantage of being really one-sided. The disadvantage is that opening it calls for a certain opening pressure.