Since membranes are rated at a 98%+ rejection rate already, recirculating the high TDS waste water back through the membrane to attempt to gain an additional ~2%, will actually result in worse rejection rate. Meaning less performance and higher TDS output than with a single pass through. There's really no trade off, however as mentioned above there are some solid benefits to a dual membrane setup. Here are a couple of videos that outline how a dual membrane setup works and how pressure is a large factor in overall performance of the RODI unit. :)