The genius of lilies or duckweed is that there is no soil depletion. Duckweed doesn't even have real roots, just things that hang in the water and suck up nutrients. In fact, growing it helps solve other problems like sewage treatment and fertilizer runoff, where their nutrients are our waste.
I should have pointed out that yours was a good idea to solve this problem. I was wondering though if that method would be enough to meet all of the demand. Thats why I posted what I did.