Confused about gray/black water tanks...

[lyonkster]

I must have a total misunderstanding of how the gray and black water tanks are set up in trailers. We have a fifth wheel that is permanently boondocked. The toilet waste line has been rerouted to a remote (outside) composting toilet.

So from my (apparently incorrect) understanding of things, the only thing that should come out of the tanks is the gray water (from sinks) - nothing should come out of the black water tank. Yet after every visit to the trailer, when I drain the two tanks, more comes out of the black water line than out of the gray water tank. (just make it clear, what comes out of the black water tank is reasonably clean "gray" water, not toilet waste). How is this possible? The only thing I can think of is rain water coming into the black water tank through the vent, but A) the vent has a cap on it, and B) it hasn't rained here in years.

What am I missing?

[Steve Ownby]

Originally Posted by lyonkster

I must have a total misunderstanding of how the gray and black water tanks are set up in trailers. We have a fifth wheel that is permanently boondocked. The toilet waste line has been rerouted to a remote (outside) composting toilet.

So from my (apparently incorrect) understanding of things, the only thing that should come out of the tanks is the gray water (from sinks) - nothing should come out of the black water tank. Yet after every visit to the trailer, when I drain the two tanks, more comes out of the black water line than out of the gray water tank. (just make it clear, what comes out of the black water tank is reasonably clean "gray" water, not toilet waste). How is this possible? The only thing I can think of is rain water coming into the black water tank through the vent, but A) the vent has a cap on it, and B) it hasn't rained here in years.

What am I missing?

Many RV builders route one of the sinks into the black tank.

[lyonkster]

Originally Posted by Steve Ownby

Many RV builders route one of the sinks into the black tank.

Well that would certainly do it then! I was wondering if it was something tricky like that - is that intended to help flush out the black water tank?

[Steve Ownby]

Originally Posted by lyonkster

Well that would certainly do it then! I was wondering if it was something tricky like that - is that intended to help flush out the black water tank?

Well, it's true that more water in a black tank is generally a good thing, but I'm guessing that the main reason it that it simplifies their plumbing.

[lyonkster]

Thanks Steve! It all makes sense now!

[JohnRR]

My bathroom sink is hooked into the black tank as well I believe it's a fairly common occurrence in RVs.

[Tytlfamily]

The mid bathroom on my 34e Hurricane has toilet and sink drain into black tank also.

[nothermark]

And showers seem to be routed either way depending on the builder.

[MSHappyCampers]

I think Steve nailed it!

[Gary RVRoamer]

is that intended to help flush out the black water tank?

No, just an expediency for the convenience of the builders. The back tank is directly under the toilet (unless the toilet is a macerator type), so it's simple to run the adjacent sink drain into it as well. A bathroom sink doesn't usually contribute a lot of waste water, so it [hopefully] doesn't strain the capacity of the black tank too badly. A typically generates much more waste water and running it into the black tank is a poor design choice (which rarely stops an RV builder from doing anything that saves him time/money during assembly).

[lyonkster]

Thanks for all the replies folks, it makes perfect sense now. Now I'm kicking myself for not coming here first and asking the question - I spent way too much time trying to figure this out on my own, googling, and only reading that "gray water tank is for sink waste, black water tank is for toilet waste" - nowhere did it say that sink waste is commonly dumped into the black water tank. That little tidbit would have saved me lots of headaches .

[JohnBoyToo]

Do you let the black tank fill up before dumping ?
You said after every visit.... and wondered if it had enough volume to woosh it all out

THAT pyramid, once formed, may last as long as the ones in Egypt !

[lyonkster]

Originally Posted by JohnBoyToo

Do you let the black tank fill up before dumping ?
You said after every visit.... and wondered if it had enough volume to woosh it all out

THAT pyramid, once formed, may last as long as the ones in Egypt !

I do empty the tanks at every visit, but there is only sink waste in them - all the toilet/solid waste goes to the composting toilet .

[nothermark]

Not to put too fine a point on it but the bathroom sink is also a source for fecal contamination. That is why it is sent to the black tank.