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Old 05-29-2017, 11:30 AM   #1
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Backflush black tank from grey tank?

Was discussing the joys of tank dumping the other day with a fellow RVer.
He told me he back flushes his black tank from his grey tank. Huh?

Here's how he does it: Both tank dump pipes come together in a Y after their individual valves to a single dump pipe on his rig; he also uses a separate valve right at the main outlet where the cap goes. (Mine is set up this way too.)

He waits till both tanks are full, then dumps the black tank by opening both black and end valve at the sewer hose. When empty, he closes the black valve and then the end valve. He then opens the grey valve, then the black valve, thereby letting soapy grey water flow back up into the black tank. When he feels the grey tank is almost empty, he closes the grey valve, then the black valve, opens the end valve, and then opens the black valve again. He feels this helps keep the black tank somewhat cleaner with the soapy grey water in it to help clean it. Has anyone ever done this? Does it work? What are the consequences of this method? I have thoughts, but want to hear from those in the know.
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Old 05-29-2017, 11:41 AM   #2
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A good friend of mine has done this for over ten years and claims similar benefits and no problems.
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Old 05-29-2017, 11:44 AM   #3
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Just remember, there's no check-valve there...

So while the bulk will be grey tank INTO the black tank, there will be some cross contamination the other way; as well. How much of an issue that is, well; that's a personal thing, I suppose.
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Old 05-29-2017, 11:53 AM   #4
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Just remember, there's no check-valve there...

So while the bulk will be grey tank INTO the black tank, there will be some cross contamination the other way; as well. How much of an issue that is, well; that's a personal thing, I suppose.
That was one of my thoughts. What are the results of there is cross contamination?
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Old 05-29-2017, 11:54 AM   #5
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Originally Posted by OntarioFireFighter View Post
Just remember, there's no check-valve there...

So while the bulk will be grey tank INTO the black tank, there will be some cross contamination the other way; as well. How much of an issue that is, well; that's a personal thing, I suppose.
X2.....easier just to close the black tank after dumping, then filling the black tank with clean water using your tank flush system, dump it again. Repeat as many times as you like until the tank is clean. THEN close the black and dump the grey to flush the slinky
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Old 05-29-2017, 12:15 PM   #6
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X2.....easier just to close the black tank after dumping, then filling the black tank with clean water using your tank flush system, dump it again. Repeat as many times as you like until the tank is clean. THEN close the black and dump the grey to flush the slinky
He says he has enough grey water to flush out the slinky when he does it this way. Also, some CGs don't allow flushing? I always do the super flush job when I put the TT away; we have a wash area and dump station at my storage place.
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Old 05-29-2017, 12:50 PM   #7
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Personally, I have used this method for over 10 years in my previous coaches. But, they all had macerators on them and it was very easy. Since my present coach does not have a macerator on it, I am searching for a valve that can connect to the "Y" that can facilitate this method of dumping.

If done correctly, there should be no cross-contamination. By completely dumping the black tank first, there should be minimal water in the black tank. By opening the grey tank's valve, water will seek it's own level and it will rush out of the grey tank and into the black tank, with no back wash into the grey tank. Then, close both valves and then reopen the black tank's valve. Also, while I am doing this, I am also using the black tank's flush nozzle at the same time.

Repeat the process until the grey tank has just a little water left in there. Then flush the slinky with the remaining grey water.
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Old 05-29-2017, 12:52 PM   #8
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Nope. Ain't gonna do it. There's a reason those two tanks are separate.
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Old 05-29-2017, 01:01 PM   #9
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Intersting those last two posts are diametrically opposed. (did I spell that right?) Is it possible any bacteria that might find it's way from black to grey and 'come and get us?' I'm being serious.
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Old 05-29-2017, 01:05 PM   #10
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Backflush Black toi Gray

OMG, I can't believe that anyone would practice this procedure. Is it worth using your gray water with some amount of soap to clean (maybe) your black tank.? I have never saw a study that indicates that this would in fact help clean your black tank. The biggest downside would be that you might get black tank water into your gray tank. What are you saving by using the procedure, I don't understand? As they say each their own. Using the black tank flush on a regular basis seems so much better, and never have to worry about cross contamination.
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Old 05-29-2017, 01:17 PM   #11
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Contaminated water from black, will enter and remain in grey tak. No problem as long as there is water in the "P" traps. BUT, If stored, those "P" traps will evaporate, and contaminated air will enter the rig. Results in a very yucky smell....
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Old 05-29-2017, 01:20 PM   #12
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unless he has a strange setup, it can not be a "good" flush..... my tanks, and I suppose most others, are mounted at the same height and in most cases are the same size, ..... the flow from gray is not going to/can't EMPTY gray and FILL black, .... the flow can only leave gray into black until it reaches a medium level point in both tanks, and would never completely reach the top of the black tank, .....
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Old 05-29-2017, 01:25 PM   #13
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I like the idea of using the soap to help clean the black tank. I do not like the idea of possibly contaminating the grey tank that is connected to my sink and thereby the kitchen.

What I do is to put 15 gallons of clean water into my black tank after I am completely done flushing/rinsing. I add some Dawn Dish soap. I drive home, and then back to the next camping trip. The splashing of the soapy water in the black tank is constantly cleaning the tank while we are driving.

When first into the next campground I drain the black tank. Voila, a clean tank and sensors and NO cross contamination
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Old 05-29-2017, 01:44 PM   #14
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Like Maurice's plan but with laundry detergent and liquid water softner in a 2/3 full black tank to slosh between stops. Considering a wand as some say they are more effective than built-in washer.
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Old 05-29-2017, 01:45 PM   #15
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Then not only you have a black tank to clean out (easy task with using the Black Tank Flush) but you then have to somehow clean the gray tank out. I guess you could dump a ton of water and soap down the drain in the shower. Or you could put some sewer chemicals down the drain, which would sanitize the tank. Point being, you really don't know if you have black tank water in your gray tank. Why take a chance?
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Old 05-29-2017, 01:49 PM   #16
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Contaminated water from black, will enter and remain in grey tak. No problem as long as there is water in the "P" traps. BUT, If stored, those "P" traps will evaporate, and contaminated air will enter the rig. Results in a very yucky smell....
Ah. Good point. But then, grey water doesn't smell all that great either. Never thought about the dry trap, but then, I visit my camper once a month to check on it, and I DO add water to the sinks and shower for that reason.
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Old 05-29-2017, 02:11 PM   #17
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Originally Posted by paule253 View Post
OMG, I can't believe that anyone would practice this procedure. Is it worth using your gray water with some amount of soap to clean (maybe) your black tank.? I have never saw a study that indicates that this would in fact help clean your black tank. The biggest downside would be that you might get black tank water into your gray tank. What are you saving by using the procedure, I don't understand? As they say each their own. Using the black tank flush on a regular basis seems so much better, and never have to worry about cross contamination.
I didn't realize people drank out of their grey water tank.

That must be why you find it so disgusting, right?

Because otherwise, I can't think of any harm this would cause.
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Old 05-29-2017, 02:26 PM   #18
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Tank flush

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Originally Posted by MeepMeep View Post
X2.....easier just to close the black tank after dumping, then filling the black tank with clean water using your tank flush system, dump it again. Repeat as many times as you like until the tank is clean. THEN close the black and dump the grey to flush the slinky
We were told by dealer never use your black tank clean out with valve closed if your not paying attention and you over fill there are only a couple of places for that dirty water to go and it would not be pleasant.
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Old 05-29-2017, 02:29 PM   #19
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Most of the time I just read the form. But this one I could not resist. After being a farmer and dealing with water in spray tanks/ valves etc. there is good reasoning with what this guy is doing.
Honestly Guys; Any cross contamination is not going to be any worse if at all than what you get the moment you open your black tank valve every time you empty. that is when you will get the most contamination if any as the black tank water goes up against the gray valve. if the drain pipe is connected to the gray and black tank valve. As it is on most every camper I have ever seen. with the exception of the gray Galley tank.
With the force of the water flow from the gray tank into the Black it will have far more effect of washing the bottom of the black tank than most back wash systems. you will have 3" of flow, his way. as opposed to 1/2" of flow at 45 psi on back wash system.
Cross contamination doing this is a non issue..
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Old 05-29-2017, 02:43 PM   #20
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I don't even let the food on my plate touch.
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