Qoustion on Ro/DI Filter

I have a brs 4 stage value ro/di system which when I shut it off water comes through the black waste line and a lot of water gets out. I read you should keep water in there but I don't know how.

Any Suggestions on how to do this? Do I need a shut off vavle or something fancy? Also this happened to my filter, So how long can the canisters last without water, with outside temp being cold???:(