Yeah, you aren't supposed to use flammable(especially highly flammable) solution in ultrasonic cleaners, something about how the process works. I get away with small amounts of denatured alcohol in solutions I use, but mine is only 35 watts. His is MUCH better than mine.
http://www.ultrasonicsdirect.com/sha...l-cd-2800.html
(same company, different site)
As far as disposal of the solution, I pour the solution out of my cleaner, into a pitcher, then into a large clear plastic jar. I let the solution sit(multiple tanks worth, since it's a small unit) and let gravity pull larger particles to the bottom. after gravity has the large particles, I pour the light stuff down the drain. I would try and filter it further, but to do that properly would be a tad expensive, and space consuming.