RV Black Water Tanks Definition

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What are RV Black Water Tanks?

Black water is the term used to described waste that flushes down a toilet. Therefore, black water includes human waste, toilet paper and the water or liquid that is used to flush the toilet.

In an RV that black water needs to go somewhere. It is collected in tanks aptly named “black water tanks”. These tanks, situated under the RV, are sometimes called holding tanks or waste tanks. (You can see the RV black water tank in the photo below. It is a photo of the underside of a 5th-wheel trailer RV.)

RV Black water tank - located under an RV
RV Black water tank under an RV

These tanks can only hold so much (different RV’s have different sized black water tanks), and they need to be emptied into a dump station or into a campsite’s sewer hookup using an RV sewer hose.

The gray water tank and the black water tank have their own plumbing system, but their outlet pipes collectively meet at one sewer outlet.

It is at this point that you connect your 3-inch sewer hose to empty the tanks.

Each waste tank has their own dump valve before they meet at the singular sewer connection so you can control which waste tank you are dumping at a time. (See the photo below showing the two dump valves. The sewer connection that your RV sewer hose attaches to is not shown in this picture.)

RV black water tank dump valve
Gray water tank and blackwater tank outlets merge into one pipe (sewer connection not shown in this pic)

This black water tank dump valve keeps the sewage inside the black water tank until you’re ready to empty it. It is a good idea to occasionally check this valve to make sure it isn’t leaking.

Emptying the waste tanks can be daunting task for new RV’ers, but once you learn and follow procedures, it’ll become just part of the RV’ing routine.

Black water tanks are not to be confused with gray water tanks. RV gray water tanks collect the waste water that empties down the drain of sinks and showers.




Key Takeaways

  1. Black water is waste water flushed down a toilet
  2. Blackwater tanks in an RV hold the sewage, toilet paper and liquid flushed down the toilet
  3. Blackwater tanks need to be emptied into a dedicated dump station or into a campsite’s sewer hookup
  4. Blackwater is not to be confused with gray water (which is water from sinks and showers)






Understanding RV Black Water Tanks

It can be liberating having a bathroom in your RV and being able to relieve yourself in the privacy of your own camper. But that human waste has to go somewhere, and you have to have plans to empty your black water tank.

The black water tanks on most RV have a waste connection that fits a standard 3-inch RV sewer hose. (Some older RV’s may have slightly different sizes.)

If you are staying at a campsite that has a sewer connection it may be tempting to leave the waste dump valve on your black water tank open the whole time. That sounds like a good idea right. That the waste will just flow out of the black water tank, down the sewer hose and into the sewer. But it doesn’t work like that.

What usually happens instead is that the watery contents of the waste will flow out of tank leaving heavier, solid material behind. These solids (aka fecal matter and used toilet paper) can stick to the walls of the tank and clog up the sensor that signals when the tank is full. Oh, and they can stink too!




Dumping your RV’s Blackwater Tank (and gray water tank)

Once you’ve securely connected one end of your RV sewer hose to your RV and the other end into your campsite’s sewer hookup (or into the dump station) it is time to empty the blackwater tank first. Pull the RV dump valve on your blackwater tank and its content will flow down the sewer hose and into the sewer.

Many campers choose to have a clear elbow fitting on the end of their sewer hose (like the picture below) so they can visually see if the tank is done emptying.

Camco RhinoFlex RV Bayonet Elbow Fitting with 4-in-1 Adapter (39736)

Flushing your toilet a few times will help flush out residue from the blackwater tank. And some newer RV’s even come with blackwater flushing systems. (Follow your RV manual on how and when to run this flush.)

Then after emptying your black water tank you close its dump valve.

Then open the dump valve on your graywater tank and let that waste water (from your sink and shower) flow down the sewer hose and into the sewer.

(In the picture above you can see the gray water dump valve and the blackwater tank dump valve. The outlet from each tank merges into one pipe which you can see goes toward the sewer connection in the photo below. There is no RV sewer hose connected in this picture, but it would attach to the fitting currently covered by the cap in the photo.)

RV galley gray water tank valve. It can be hard to find on some RV's

[Note: some RV’s may have a second gray water tank called a galley gray water tank that holds waste water from the kitchen sink. This article claims you should empty your waste tanks from the dirtiest to the cleanest. So empty the blackwater tank first, the galley tank second (which’ll have gunk from the kitchen sink) and then lastly empty the gray water tank.]

Above is a picture under a 5th-wheel trailer showing where the galley waste tank merges with the sewer outlet pipe right before the sewer hose connection fitting.


You empty the gray water tank last because this water helps flush the heavier blackwater residue down your sewer hose. (So it helps flush chunks of toilet paper and sewage from your sewer hose and into the sewer.)

Once you’ve emptied the gray water tank, disconnect your sewer hose from the trailer first, and lift it while walking back towards the sewer connection. This will help any waste water in the hose flow into the sewer.

Rinse your sewer hose with a dedicated water hose (don’t use your freshwater hose) to get the sewer hose even cleaner. And then store it…make sure it is stored away from your freshwater hose.

(Wondering how to store your sewer hose, click here for RV sewer hose storage ideas.)



After you’ve emptied your blackwater tank you should add some water back into the tank – along with some holding tank treatments – to prevent your blackwater tank from stinking up a poop storm.

Many campers choose a holding tank treatment that has enzymes and bacteria to break down the sewage waste in the blackwater tank. The product below from Eco-Strong has live enyzme-producing bacteria as well as added enzymes.

Eco Strong RV Toilet Holding Tank Treatment | Powerful Bioactive Odor Destroyer with Black Water Waste Digesting Enzymes [High Potency 17oz - Lavender]






Related Terms

RV Gray Water Tanks

Gray water is waste water from the drains of sinks and showers. In an RV this gray water is collected in gray water tanks.



Full Hookup Campsites

Full hookup campsites have fresh water, electrical and sewer hookups.



Partial Hookup Campsites

Partial hookups don’t have all the hookups of a full hookup campsite. They usually have fresh water and electricity but not a sewer connection.

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