Dear Dave,
Why do RVs uniformly have separate gray and black waste tanks? After all, both tanks empty into the same sewer inlet. I can think of possible answers, but I’d just be guessing.
For example, using the gray water to back flush the black tank—but the mix of the two would put more water into the black tank, reducing buildup. And there would still be a tank flush line. And a simple back flow preventer (similar to the hollow ball used in wet/dry shop vacs) would block black waste from contaminating gray water traps. —Jim, 2022 Gulf Stream 189DD travel trailer
Dear Jim,
Thought this would be a good “comment starter” for our readers. Over the years I have had discussions with design engineers (I use that term loosely!) about the placement of holding tanks and the need for two gray water holding tanks, but never having just one tank for everything.
Sanitary issue
The first thought I had was that it might be a sanitary issue when the tanks get full and could have raw sewage coming into the shower or kitchen sink. Even with a back flow preventer, I wouldn’t want to take the risk.
I also think it would be very difficult to keep the odor from coming into the kitchen even with a “P” trap. The bouncing around on the road typically sloshes water out and the smell from the tank would come up.
Larger tank may require dumping more often
Another thought is that the gray water tank being mostly shower waste water fills faster and therefore is typically larger than the black. Therefore, having all waste going into one tank would be a very large tank that would need to be dumped more often. Plus, that would be a lot of weight for the size of tank that would be needed. Water weighs 8.3 lbs. per gallon. Since I had steak and mashed potatoes last night, my black water may weigh slightly more than that!
Some of the forums I researched had comments about being able to dump the gray water tank just about anywhere, which I disagree with. Most people that would dump their gray water tank in a rural ditch would most likely dump both.
Let’s see what others have to say.
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