Equalizing gray/black water tanks

[cooper31525]

I have a 2022 2108DS. I believe the black and grey tanks have the same capacity but the grey tank fills up way before the black. I have a portable tank that I use to empty the gray but I wondered if there is an easier way. In my research, I came across a method of adding a third valve at the outlet and allowing the black and gray tanks to equalize like in this video: https://www.gonewiththewynns.com/combine-rv-black-grey-tank/

Has anyone tried this. It’s just simple physics and makes sense but I’m wondering if there’s anything in the black tank that would malfunction with “stuff” running in the opposite direction. I’m remembering back to the first time I back flushed my black tank and found out that there was a faulty back flow preventer. There was water(probably not clean) that was going all over the place.

[NomadRider]

I think that would be a terrible idea.
The webpage that you reference indicates that they use a composting toilet and thus, were not using their black tank at all - it was an empty tank taking up space so they essentially increased their grey tank capacity.
Water will seek its own level so there will be free transfer of wastes between the two tanks. The biggest problem with this is that you have no means of flushing the blackish/gray tank.

[cooper31525]

Thanks for your reply. As long as the gray tank level is above the black tank level, the water should transfer from gray to black. As soon as level is achieved, you close the gray and black valves so no further transfer. No problem there. My question was if anyone knows of a valve or anything else in the black tank that would be damaged or become clogged by filling with pressure (elevation head differential between the gray and black tank).

[NomadRider]

No, there's nothing in there that'll become damaged - you're not talking about any significant amount of pressure. Even if one tank was 5' above the other you're only talking about 2PSI +/-. Residential waste systems are designed to handle up to 5 PSI (not that RV systems are as good as that but they are similar)

[cooper31525]

Originally Posted by NomadRider

No, there's nothing in there that'll become damaged - you're not talking about any significant amount of pressure. Even if one tank was 5' above the other you're only talking about 2PSI +/-. Residential waste systems are designed to handle up to 5 PSI (not that RV systems are as good as that but they are similar)

That’s good to know. Thank you.

[bigb]

I've heard about folks doing what you propose. Another idea is a pump to pump gray water into the black tank. When we travel our gray generally needs emptying daily but the black can go 5-6 days easy so I made a garden hose connection for the gray and adapters to fit 3" and 4" CG sewers so it's easy to drain every day, I only have to get the slinky out every 5 days or so.

I do have a pump that pumps the gray up over the axle to the bayonet fitting but it was a pita to use so I ended up making a separate drain for gray. I am considering diverting the pump hose to the black tank though in case we are ever in a situation with nowhere to legally empty the gray, it could get us by 1-2 days plus make the black drain better.

[Sazornek]

Somewhere, a long while back there was an article about setting up a system to use grey water to flush the toilets. It is an intriguing and useful way to transfer water from grey to black. It may not be a game changer but it is one way to better utilize the generally underutilized capacity of the black water tank. Concept works well if you want to install a water pump and some new pex. Gets messy if there is only one grey water tank as you do not want to pump kitchen scraps through the toilets. But if there is a second grey tank dedicated to a shower it is feasible. Also serves as a good option to conserve fresh water when boondocking.

https://wandrlymagazine.com/article/...-in-an-rv/amp/

Here is an article from a self built system in a greyhound bus conversion.

[Thindirtline]

I think there are many of us with the grey/black challenge. We put a rubber bin in our sink when washing dishes and dump it into the toilet when done. It fits into the left side of the sink. Our trailer is off-site or I would send a pic. They make collapsible bins that would work for this too.

[wahoonc]

I would be hesitant to try to use the first method posted to move gray water to the black tank, you are asking for a massive mess and contamination if something went wrong. I have used the method to equal out 2 gray water tanks on my previous rig. It worked fine.

The idea of using gray to flush the toilet is a good one and I have seen it done before.

Aaron

[SgtB]

I've read about numerous systems to take care of the gray tank. Personally, I'll leave mine alone. What I do not want is to have two black tanks. Switching back and forth could cause that. Of course that's my opinion.
Jim