Originally Posted by Triguy
Like most, we don't fill the black tank very fast.
Wish they gave an option when ordering to change to 35gl FW, 35gl GW and 20gl BW.
I couple of camping trips ago we had filled up the grey tank by the last evening, and the black tank hadn't hit 1/3 yet. (And no- I didn't dump it on the ground, I took a Rubbermain bin, filled that, and then brought it into the trailer and dumped it down the toilet.)
So that got me thinking about water pressure and another solution. I cleaned out my black tank at home, and then filled my grey tank with water. I then made sure the drain cap was in place, and opened the grey valve to fill the cavity between the vavle and the drain cap. Then I opened my black valve, and sure enough- the pressure from the grey tank pushed water into the black tank. As soon as I heard the flow of water slowing down I shut the black valve first; and then the grey valve.
The black tank was now reading water in it, and the grey tank had dropped from full to 1/3. In my system that means about 15 gallons of grey made it into the black tank. Yes, there is still about a litre of water left near the drain cap, but once again a small container could be used to collect that to be dumped down the toilet.
So before anybody tries this with real waste, I would try it with clean water as I did. The other thing with the discussion of mods; I think it would be a lot easier to create a mod that allows the middle of the grey tank to drain into the top of the black. Once again the water pressure of a full grey should push enough water into the black to make a difference.