How to Unclog an RV Black Tank: End the Nightmare Quickly

Have you ever had to unclog an RV black tank? Not sure how to go about it? Today’s post will have you well-prepared to handle what can become one of the grossest messes an RVer could face (especially if the tank is full). ????????????????

We’ll even let you know how to avoid black tank clogs in the first place, preventing you from getting to the gross part entirely.

While this isn’t the most pleasant topic, it can be a stressful RV black water tank issue that people do run into. So give us a few minutes and we’ll tell you how to treat it — and how to avoid it in the first place.

Anatomy of an RV Sewer System

Before we get to how to unclog an RV black tank, we have to take a look at the most likely causes of the clogs. And in order to do that, it’s important to understand how an RV sewer system works.

Not all RVs are set up the same way, and the route to RV holding tanks can be a bit different from rig to rig.

Your RV’s sewer system is likely set up in one of two ways:

“Direct Deposit”

Many RV black tanks sit directly beneath the toilet.

When the toilet valve is opened during flushing, the contents drop via gravity directly into the tank sitting under it. In fact, if you hold the valve open you can see directly down into the black tank itself.

Even with this direct path from the toilet to the black tank, it’s possible for a clog to occur here. Too much toilet paper is the most common culprit. Or throwing something other than RV-safe toilet paper into the bowl.

Clogging between the bowl and the tank is less likely in a “direct deposit” setup, but still possible.

Twists and Turns

In other rigs, the toilet isn’t located directly over the black tank. That requires a longer pipe to connect them together. And the path it takes may not always be a straight line.

In this case, when you flush, the valve opens and the toilet contents move through the drain pipe that leads to the black tank. This sewer pipe may have elbows or “bends” in it in order to direct the waste to the holding tank.

Either way, if there’s a pipe that leads from your toilet to your black tank, that pipe can get clogged. Of course, the issue is more likely if there are bends in the pipe system along the way from the toilet to the tank.

If the distance or route from the toilet to the black tank is more than a fairly short distance, a macerating toilet will commonly be installed. Think rear-bath rigs, where the black tank is mounted closer to the middle of the RV.

Why Is My RV Black Tank Not Draining?

Simple answer? There’s a clog either inside the tank itself or in the sewer line leading to or from it.

You may have seen our post on how to unclog an RV toilet. These two issues are similar and even share some remedies and prevention methods. But while the issues may be related, they’re actually two very different problems in a couple of important ways.

Let’s first take a look at what might be causing a clog in an RV sewer pipe that leads from the toilet to the black tank.

What Causes Clogs in an RV Sewer Pipe?

The main reasons for a clog in an RV sewer pipe (in addition to any twists & turns/elbows) are the following:

Not Enough Water Is Used When Flushing

With RV toilets, flushing allows water to clean the bowl of whatever has been deposited into it, but it also moves the contents through any existing sewer pipes (elbows included) and into the black tank.

We all try to conserve water, especially if we’re boondocking. But if you get too  frugal with water, you just might cause a clog.

Too Much Toilet Paper

As is the case in a home sewer system, using too much toilet paper can easily clog the system.

Now, this is where many RVers might say “You also need to be sure to use special RV toilet paper that’s made for RV sewer systems.”

We’ve been living in our RV full-time for nearly 20 years, and we’re here to tell you that if you’re buying special “RV toilet paper” you’re flushing money down the toilet! There’s nothing special about it (other than being overpriced and, ahem, crappy!????). In fact, we wrote an entire post on RV toilet paper to de-bunk this persistent myth.

We happen to use Costco’s Kirkland brand and we’ve never had an issue. But if you have another favorite brand of toilet paper, you can test it to see if it’s RV-friendly. Here’s how:

“Flushable” (or Non-Flushable) Wipes Are Used

While you don’t need RV-specific toilet paper, using anything BUT toilet paper is just asking for trouble.

Wipes, whether they’re advertised as “flushable” or not, shouldn’t be put into an RV’s waste system. Not only can they clog the pipes of your motorhome or travel trailer, but they can also cause a clog in the tank itself and can get caught at the sewer outlet.

If you prefer to use wipes, it’s best to keep a small covered receptacle near the toilet where used wipes can be tossed and deposited with your trash. (Some RVers actually even do this with toilet paper. We never have and don’t see the need, but whatever works for you!)

Large “Deposits” Are Made

And finally, as much as we hate to mention it, large deposits of human waste can clog an RV sewer pipe.

This is especially true if large deposits are made with little water added when flushing. Combine these two reasons for a clogged RV sewer pipe, and you’ve got a surefire way to clog your RV sewer pipe!

What Causes Clogs in an RV Black Tank?

It’s possible for the RV black tank itself to “clog up”, causing difficulty draining the tank when you attempt to dump the contents.

And again, it’s possible for a clog to occur at the tank inlet or outlet.

Beyond that, there’s even a possibility of your sewer hose clogging which is why using the best RV sewer hose you can find is a good idea.

But here are the main reasons why the tank itself may clog…

The Dreaded “Poo Pyramid”

The so-called “poo pyramid” can be fallout from too little water being used to flush or, even more likely, from keeping the black tank’s gate valve open when you’re connected to a campground or RV park sewer. That is one of RVing’s most awful mistakes! NEVER leave the black valve open while using your RV!

Back when we posted about whether to keep an RV gray tank open or closed when connected to a sewer system, we made clear that you should NEVER leave your black tank valve open while using your RV because the “liquids” will run out, leaving the “solids” behind to dry up and harden.

This is commonly known as “pyramiding” and is one of the worst self-inflicted wounds an RVer can create.

Leaving your black tank valve open when you’re connected to a sewer is a BIG MISTAKE that leads to the number one cause of a clogged RV black tank.

Storing Without Dumping

If you store your RV or let it sit for long periods of time without dumping the contents of the black tank, the liquid in the tank could evaporate, leaving you with solid matter that dries out and sticks to the tank.

That’s why you always want to dump and clean your RV holding tanks before storing your RV. And if you’ve got an RV black tank flush on your rig, use it before storing or parking it for an extended period of time.

Struvite

And finally, no matter how clean you keep your RV’s sewer system, tank included, over time struvite will build up.

If you saw our post on how to clean your RV tank sensors, you’ve heard of struvite.

Struvite is a type of scale that forms over time in waste pipes and tanks. We’re not just referring to RV holding tanks here. Struvite is a problem in wastewater treatment plants as well.

The scale (called struvite) accumulates on the insides of pipes and tanks restricting the flow. In RV tanks, struvite adheres firmly to the walls and floor and eventually may need to be professionally removed.

Keep in mind that struvite formation is a natural process in waste systems that happens regardless of how meticulous your tank maintenance is.

In fact, the tank sensors in our motorhome weren’t reading correctly and we’d done all we could to remedy the situation. We had our tanks professionally cleaned, which solved the problem (and is when we learned about struvite buildup).

You can watch the video here to see how it all played out:

How to Unclog an RV Black Tank

Now that we know what causes RV black tanks and sewer pipes to clog, let’s get down to the business of unclogging them once it’s occurred.

As we’ve noted above, when you try to dump and clean your RV black tank and it doesn’t drain properly, you’ve either got a clog in the sewer pipe or black tank valve leading to the tank, or the tank itself is clogged up.

Either way, you’ll want to take a look at a few treatments/remedies to get ???? flowing!

Hot Water

Sometimes all it takes to move a clog is a flow of hot water. 

This works best if your clog is above the tank itself either at the blade valve or in the sewer pipe.

Simply pour some hot water directly into the toilet and flush. Depending on the placement and degree of the clog, it may be all you need to do to break through and resolve the issue. So it’s a reasonable and easy first thing to try.

Note that some people advocate using boiling water to clear a clog. We’d suggest that you avoid pouring boiling water into your RV toilet as this could damage the seal. That seal holds a small amount of water in the toilet, which is what keeps odors down in the tank where they belong. So keeping seals healthy is important.

Very hot (but not boiling) water is often sufficient to clear a clog.

Black Tank Treatment

Black tank treatments are best used to prevent clogs rather than to treat them, but it’s possible that they can help clear a clog as well.

The purpose of black tank treatments is to break down and dissolve the contents of the tank.

We’ve been using Happy Campers for many years now.

Happy Campers is a septic-safe blend of minerals that are activated by the water. In addition to breaking down waste, the treatment also eliminates odors.

If you have an issue with your holding tank sensors or with odor, Happy Campers also makes a deep cleaning product to remove odors that persist even after dumping and rinsing your tanks.

Dish Soap

Liquid dish soap generally has crease-cutting agents and other properties that can cut through “stuck” things.

For this reason, pouring about a cup of liquid dish soap into the toilet and letting it sit for a bit before flushing may cut through your clog sufficiently to clear it.

Snake/Toilet Wand

This one is NOT an option if you have a macerating toilet. In general, it’s best to consult the owner’s manual for your macerator to learn what they suggest is best for clearing clogs.

However, if you have a regular RV toilet, a toilet wand or snake (or even a toilet want used AS a snake) could help to clear a clog depending on its location and composition.

The reason we mention using a toilet wand as a snake is that your RV’s sewer system is likely too short for a regular toilet snake. And again, most RV toilets drop directly into the black tank so a regular plumbing snake is much too long for use in most RVs.

But a tank-rinsing wand can be used to knock a clog clear and to clean the tank up a bit before dumping (if you’ve got a clog to fix) and after dumping (every time you dump your tanks to keep things clear).

And speaking of the tank running clear when dumping, we highly recommend using a clear elbow when dumping your RV tanks. This is the only way to know when your tank is rinsed clean.

When you see the liquid flowing through the elbow is clear, that’s the “all-clear” for you to stop rinsing. (Although following the steps in our original tank dumping & cleaning video will ensure you’re getting your tank as clean as possible.)

This is the elbow that works with our favorite sewer hose:

However, if your hose already has the ears/tabs built onto it and it attaches directly to your RV’s sewer port, that elbow won’t work for you. Instead, you’ll need a clear adapter like this one:

While you’re setting up your sewer hose, make sure it slopes downward to the park’s sewer connection as smoothly and continuously as possible. We use a sewer hose support that not only facilitates that, but keeps the hose up off the ground (required in some parks, and always a good idea for protecting the hose from damage).

Ice Cubes

When looking into methods for how to unclog an RV black tank, the “ice cube trick” is often dismissed by some RVers as ineffective. But that’s only because they don’t understand what it’s good for, and what it isn’t. Ice cubes cannot solve pyramiding! They can, however, work wonders on debris stuck down to the floor of the black tank.

Our own first-hand (and successful) experience using ice cubes in the black tank was during our very first year on the road. It’s perfect for removing dried-on debris from the bottom of a black tank that was left in storage without thoroughly cleaning the tank first.

With this remedy, you simply pour the ice cubes right into the toilet and down into the black tank. Then you take a drive and as the ice cubes slide around in the tank, they can help dislodge debris that’s stuck to the bottom.

Again, follow the steps at the end of our original tank dumping & cleaning video to see more about using ice cubes to clean an RV black tank.

“An Ounce of Prevention is Worth a Pound of Cure”

Coined by Benjamin Franklin in the early 1700s, this famous phrase applies very well to the topic of how to unclog an RV black tank. (Maybe Franklin was an early RVer?)

The best way to avoid having to Google “how to unclog an RV black tank” is to prevent them from occurring in the first place.

Keep your black tank as clean and clog-free as possible by taking the following steps:

• Use sufficient water to flush
• Dump and clean the tank regularly and thoroughly
• Don’t throw errant items into your RV toilet
• NEVER leave your black tank’s gate valve open, even when hooked up to a sewer connection
• Clean your tanks before your RV sits for an extended period
• Use a holding tank treatment like Happy Campers regularly

For more information about various kinds of RV toilets, and how to use them correctly, check out our video on the topic:

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