All RO and RO/DI units on the market, with only a couple exceptions, are 4:1 waste ratio. Thats 4 gallons of brine or waste for every 1 gallon of permeate or treated water. This is not "efficiency" in the RO world, efficiency is considered the rejection rate or how much TDS the membrane removes, typically 90 to 98%.

Membranes must be flushed so they do not plug or foul and become useless. The best way to accomplish this is by that 4:1 waste ratio which continously flushes the membrane while it is in use. There are those who offer a "flush kit" which is basically a ball valve that bypasses the flow restrictor which you open to manually flush TDS away. So far I have not seen any data to back up these flush valves so I would say they have limited value if any at all. To be of any benefit they would have to be used each and every time before you shut the RO system off to flush any accumulated TDS away from the membranes surface. The trouble is we all automate our water making with float valves or float switches and solenioids so we never know when the RO is shutting off and would miss the flush. Once you have missed flushing it a few times the solids begin to solidify or cake on the membran and can never be flushed away so its best to stick with the 4:1 waste ratio.

There are a few vendors promoting low waste systems but this is something major manufacturers and vendors have tried for 25 years and dismissed since it does not work well long term. There is one exception to this rule but it is also a $800 system which is microprocessor controlled and uses DI water to flush the system not tap water so is proven to work well. Not cheap but worth it in the long run if you plan to keep the system for long and make much water.

In reality RO and RO/DI systems don't waste a significant amount of water. You probably flush 4 or 5 times that amount down the drain each day with the toilet and shower. I would pay more attention to how efficient, the rejection rate, the system is as this leads to cost savings in membrane and DI replacements and higher water quality.