Wednesday, February 8, 2023


Gray water tank can smell more than black. Here’s why and what to do about it

Who knew I could get so excited about flushing the gray and black water tanks? Okay, I know it is a bit odd, but as the designated tank flusher I am happy when the “gunk” comes out. While I am diligent about flushing out the black water with the built-in flush system, there was no built-in system for the gray water. When I complained to our RV tech about the smell when driving, he said that the gray water can smell worse than the black.


Mold in the gray water tank

I recently attended a rally seminar about “All things black and gray” and, boy, did I learn a lot! The presenter from Kleen Tank passed around glass bottles of “stuff” that had come out of holding tanks. He did advise, “Don’t open, don’t drop” while handing it off.

I recognized the black discs that had been coming out of the gray water holding tank and was disgusted when he said it was a massive amount of mold. He also had some discs as hard as metal that he passed around that had “mineralized” with the urine in the black tanks. He said that he finds the gray tank is usually worse than the black, particularly if people have built-in flush systems and had been keeping up with the black tank.

Flush rinser with gate valve and water meter

Best of all, he passed around this flush attachment and water meter. I knew the flush rinser existed, but our hose goes straight down through the bay and there was no way the flush attachment was going to fit. But wait! He showed how the gray and black dump pipe rotates outward and allows the flush rinser to attach! This is what he showed me.

Combined with a water meter, the flush rinser can easily clean out both gray and black water tanks.

The water meter measures how much water is going through a garden hose. It is usually used when watering a garden but it is an exceptional tool for measuring how much water in gallons or liters is going into RV tanks, particularly when flushing. (You can read more about this handy water meter here.)

Warning and Disclaimer:


It is not recommended because there can be disastrous results if one walks away from the RV or gets distracted while filling a closed gray water or closed black tank with water. Think black water eruption through the toilet into bathroom…

With that disclaimer noted, this was the method taught at the seminar to thoroughly flush out the black water and gray water tanks. This was also the method Kleen Tank used when flushing out our tanks. They had the additional step of a hose inserted into the tanks to pressure wash them along with flushing them out. That pressure wash is what a professional is for!

Flush steps

  1. IMPORTANT! Double check the gallon capacity of the holding tanks in the RV specs. If you are like me, it is a good idea to write them down!
  2. Attach water meter to a garden hose. Do NOT ever use a fresh water hose to flush! Use a backflow preventer to prevent contaminated water going into fresh water faucet. Kleen Tank adds the backflow preventer and water meter at the flush rinser. Others put the preventer and meter at the faucet.
  3. Attach the flush valve rinser to the sewer pipe opening and sewer hose.
  4. Pull gray water valve open for a few seconds and close to “lubricate” the sewer hose.
  5. Dump black water.
  6. When black tank is empty, follow directions on the flush valve rinser, close the rinser gate valve and fill the tank with water.
  7. Watching the water meter, fill UNDER the capacity of the tank. I went under by 20 gallons just to be safe.
  8. Turn off water and pull gate valve. If you can stand to watch, see how much more gunk was in there.
  9. Repeat with the gray water tank. If your tank was like ours, it was not a pretty sight!
  10. This flush rinser has a hose rinse setting too, but I found it rather ineffective and figured the gray water going through the sewer hose was rinse enough.
  11. Add at least two gallons of water to the black tank through the toilet and then treat as normal. A number of people are so happy with this type of flush they don’t add any chemicals, organic or otherwise, to the tanks.

And that’s what you call a royal flush! Happy flushing!



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Jim Johnson
13 days ago

Sadly, the spray gate valve trick is not universal. While we have separate black/grey tanks in the bathroom and each has its own gate valve, they drain through a ‘Y’ inside the belly cover. I have no idea of the actual plumbing, but not enough water pressure gets past the ‘Y’ to do much good.

The black tank has a built-in flush line (just floods the tank, no pressure wash). As mentioned in other comments a grey tank flush would be nice. My complaint is both the time and volume of flush water it takes to truly clean the black tank.

I have a ‘Y’ at the park spigot. One branch uses a water meter separated from a dedicated flush hose by a back flow preventer. As described, I partially fill and open the gate valve; repeat until clear.

I cannot use the road agitation trick as our RV is seasonally stationary. Periodically, I find myself doing a deep clean via wand through the toilet.

Larry Lee
6 months ago

The water meter and back flow preventer belong on the faucet, PERIOD.
Studies have shown that the contamination (if any) occurs within the first 6 inches of the hose nearest to the flush attachment. So that is no place for anything but the hose which is reserved for flushing ONLY. As Mike Sokol says, “OK, let’s be safe out there!”.

Steve Davis
1 year ago

Is there any reason to not use whatever treatment you use in your black tank in your grey as well? That’s what I do and it seems to work. Also, when I do disinfect my fresh water tank with bleach I flush a portion of it down my sink drains using the water pump, putting some bleach into the grey tank

tom wesrerfield
1 year ago

When our gray tank occasionally smells we put about 3 ounces of peroxide down the sink to kill bacteria, woks. Don’t use in black tank as it kills good bacteria,

Tyrone Scales
1 year ago

Hydrogen peroxide?

Chris Mead
1 year ago

I’ve never understood why manufacturers never installed a flush on the Gray tank like the black. I wonder how hard it can actually be?

Ron Lane
1 year ago

When I travel,I will empty both tanks, add about 5 gallons of clean water and then put in a cup full of liquid laundry detergent and a cap full of Calgon water softener into each tank. Travel to my destination, add additional water to both tanks and then flush both. Add about 5 gallons of clean water and use both tanks normally. I never have tank odors (from either tank), nor do I have any problems with draining the tanks. As an added plus, my factory installed tank level sensors work perfectly.
For what it’s worth, I never use any type of tank chemical and haven’t for years and years.

1 year ago

Hooking up a potable water hose to a sewer line is a good way to contaminate your water system. As a plumber better take my word for it than someone who’s trying to sell you a product that’s not needed. If you have problems smelling odors in your RV that means the trap under the sinks and shower need attention as that’s their job to prevent odors from coming into the living space.

Thomas D
1 year ago

I replaced the vents on the roof with a product called vent? 360 . No more smell in the rv. And I buy and use RidX septic tank treatment. Only thing there it should sit for a couple days. I put in a cup in each tank( mine has 3) not the fresh water tank! Fill it up and leave it set. You would be surprised to see what comes out. I usually do that at home and if you can’t dump at home, first stop out,dump. Cheap and really no extra work.

Tommy Molnar
1 year ago

We’ve never flushed our gray tank, but this looks like a LOT of work. We get the black tank to almost look like drinking water when we’re done though.

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