RO/DI question

[hillegom]

No you can't shut the waste water off. If you do, all the impurities you are trying to get rid of, will attach themselves to your RO membrane and clog it. The waste water flushes these impurities away.
The membrane is the most expensive part of the system.
Some people contain the waste and use it for, clothes washing, toilet tank water, watering the garden. Do a search on this board and you will find a lot of uses in one of the threads.