Back flushing black with gray

[hoppers4]

I have been reading some of the other threads related to installing tank rinsers and Flush King (or lookalikes). I see where some folks use the gray water to back flush the black tank. I also noticed that some folks think this is a bad thing. Since this is something I recently started doing myself and thought it was a good idea, I'd be curious to know what the negative side of doing this would be. They're both just plastic tanks and they both have some pretty raunchy odors. I also don't think the gray tank would have any less bacteria than the black tank, just a different kind.
I don't ever dump my gray water on the ground so I don't see it as a problem if some of the black water happened to end up in my gray tank. Please let me know the down side to this practice.

Don

[emeryts]

just curious...how does one get their gray water in the black tank? If it can be done, it sounds like a great way to conserve water for rinsing black tanks.

[hoppers4]

Gray tank can be run into the black tank using either a Flush King or a 3" valve that can be coupled up to the output before the slinky. The intent is to hook a hose up to the Flush King, close the Flush King's output valve, open the black or gray tank valve and backfill either tank. This tank can then be re-emptied by opening the Flush King's output valve thus removing some of the effluent and TP than didn't get removed by the initial dump.
I was able to add Tornadoes to my previous RVs and didn't need this device but my current RV has inaccessible tanks. My only choice is the Flush King. In my opinion the Flush King uses a lot more water than the Tornado and takes longer.
During my first use it occurred to me to do a flush or two using the water from the gray tank. I emptied the black tank, close the Flush King's output valve and opened both the gray and black tank valves to allow the water from the full gray tank to run into the black tank. This would happen very quickly because they're both 3" pipes/valves. Then I closed the gray tank and opened the output of the Flush King to re-flush the black tank. Did this a couple of times and decided the black tank was good enough without wasting a lot of fresh water.
Anyway, in my opinion this is better than waiting for the black tank to backfill and using a lot of fresh water. As far as back flushing/cleaning the gray tank, that should still be done on occasion using the Flush King or adding water through a sink or shower.

[smlranger]

While I've not done it on my current rig, I have (when I had a TT) used a small 12V pump to pump water from the grey tank into the black tank. This was not to flush the black tank but to extend the period of time between visits to a dump station. Since most RV's have more black tank capacity, this seemed like a good solution.

With my current rig with an outside connection for a flush nozzle in the black tank, one could use a similar pump to pump grey water into the black tank via the flush system. This could be used to not only flush the black tank but to extend boondocking time.

[hoppers4]

Smlranger, the outputs of my gray and black tanks come to a common 3" single connection. I can put a valve on the single 3" output and open both gray and black valves to equalize the levels of the tanks.

[Flagelpater]

I am not an expert on this matter, but here is my reasoning on the subject.
They, maybe all, RV manufactures install both a Grey and Black tank when they equip the units with a sink and a toilet.
The tanks are, as far as I have seen, always two different sizes.
Both tanks have separate dumping valves.
The Owner's Manual tells you to open and dump the Black, close the valve and dump the Grey. I agree, they do Not tell you Do Not have both valves open at the same time. We all know the Owner's Manuals are lacking information.
The Flush King does not tell you to mix Grey and Black. It has you use water from somewhere else! Not sure where, but I hope it is not a drinking water source. The RV tank is most assuredly higher than the Flush King allowing for gravity to allow flow of the Black Liquid into the hose.
In a Stick House the code requires separation of the Toilet pipes from other drain pipes, not completely but by distance and traps. The house is also Not draining into a holding tank.
Why would the RV manufacturer go to all the expense to keep the two tanks apart if it were not a requirement? Why do they all have Black Tanks smaller than Grey? It would also be cheaper for them to use two tanks of the same size.
Like I said, I am not an expert on this, but I have had E. Coli infection from a trip to Mexico. I was told E. Coli is transmitted from human feces.
It would be nice if an Expert would chime in here.

[hoppers4]

Flag, I agree E. coli would be a bad thing! I'm just undecided as to whether having E.coli in the gray tank would be a bad thing.
One thought that did occur to me would be that if the gray tank was full and the shower was used, that might not be so good if the water backed up into the shower pan.

Don

[Flagelpater]

And, the kitchen sink is not very far away!

[PA Titan]

My tanks are the exact same size and mounted side by side therefore once the black is emptied and I close the Flush King and then open the grey, I would assume that I could run a good amount of rinse water from a full grey tank with no risk of anything backing into the grey tank. I never thought of doing this but I am intrigued with the idea. I like to rinse the black tank every trip and in my opinion this would work and if you empty the rinse water from the black and then immediately empty the grey, any small amount of "contaminated" water would just be flushed out. Just my two cents but I think I may try doing this. Thanks for input everyone!!

[hoppers4]

Titan, that's pretty much my way of thinking. Seems to me like a more efficient, conservative, and quicker method than using a lot of fresh water.

[PeteB]

I have used the third valve method for many years and it has been especially useful when dry camping as the gray tank always fills up before the black tank. By opening the gray valve and then the black valve the gray tank contents will self level into the black tank and allow more volume to the gray. The trick is to listen as you open the black tank valve and when you hear the transfer slow down, just close the valve.
Hope this helps.

Pete

[PA Titan]

Quote:

Originally Posted by PeteB I have used the third valve method for many years and it has been especially useful when dry camping as the gray tank always fills up before the black tank. By opening the gray valve and then the black valve the gray tank contents will self level into the black tank and allow more volume to the gray. The trick is to listen as you open the black tank valve and when you hear the transfer slow down, just close the valve. Hope this helps. Pete

Yes, that's the ticket!! Listen for the flow to slow and you will have close to 1/2 tank to rinse with. In my case black and grey are each 29 gal so there will be a good 10 - 12 gal which will give a good rinse with no fear of any cross contamination.

[Flagelpater]

I just read the instructions for Sani-Con. They DO open both valves! Dump the Black and then flush with the Gray. It looks like I am the only one concerned about E. Coli.

[hoppers4]

Some validation from Sani-con no less. Thanks flagelpater.

Sounds like the dry campers already have this method down pat. Thanks for the input Pete.

Don