The velocity of the moving water, and it's in-compressible nature, are what provides the pressure/energy to make a ram pump work. The waste water has had all that energy removed (it's went to powering the pump to lift the water).
To get the waste water back into the stream, you have to add energy back into it with some other kind of pump. And of course that pump is going to have losses and waste energy, so all in all it's a losing proposition.
In simplified terms, the three laws of thermodynamics are:
#1 there is no free lunch. You can't get something for nothing.
#2 You can't even break even, no matter what you are going to lose some energy.
#3 You can't even get close to even; the colder it is, the more energy you lose.