closed loop water turbine

I have in my thought cage and idea that I'm not sure is possible, but figured this was the group to chat about it. Would it be possible to recycle water that has run through a turbine back up and around again using a ram pump, essentially creating a closed loop water engine? Has anyone heard of such a thing? I've done some searching on the tubes, but haven't found anything yet.

Another half-baked idea is to somehow introduce grey water and filtration into the equation. It would cease to be a closed loop, but maybe better for different applications.