Some systems use direct return , reverse return or serpentine loops that return the water to a storage area , providing continuous re-circulation .
Others are single - use systems that run from point of UPW production to point of use .
Some systems use direct return , reverse return or serpentine loops that return the water to a storage area , providing continuous re-circulation , while others are single - use systems that run from point of UPW production to point of use .