For my setup, I simply use a length of corrugated plastic drain pipe that I bring out only when i am cleaning the S+G . This directs the drain water out to the lawn and away from the filter area. I also use this approach if I want to do a water change on the pond- simply use the waste port on the filter and let the water run out for as long as you want . You probably couldn't do this if you dumped your waste to a pit under the barrel. I might also be worried about the smell if all your waste is going in a small area . I would think the gravel would get dirty and clogged pretty quickly, since the waste water is pretty nasty.