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How to Unclog an RV Black Tank

Last Updated on

January 11th, 2023 09:43 am

Like most people, you probably don’t think about your RV black tank until it’s too late. And by then, it can be a real mess. Nobody enjoys a clogged RV black tank. But clogs are avoidable if you know how to keep them from happening in the first place. In this article, we’ll show you how to unclog an RV black tank and keep it from happening again on your next RV trip.

We’re not talking about simple RV toilet clogs, we’re talking about the dreaded RV black water tank. Whether you have too much toilet paper in there or your blackwater holding tank needs a good black tank flush, we’re going to help you deal with the heavy duty black tank clogs. Nobody likes dealing with waste water, but it’s a necessary task to keep our camper’s toilet drain functioning.

What is the black water tank, and how does it work?

RVs have three holding tanks, one for freshwater, one for gray water, and one for black water. Some RVs have multiple black and grey tanks. However, what we’re discussing today will work for those with multiple RV black tanks as well.

The gray water tank stores water used in the sinks and showers, and the freshwater tank stores clean water used for drinking and cooking. This RV holding tank can cause some smells, but that’s about it.

But you’re not here for that; instead, you’re here for the black water tank. Fortunately, the gray tank doesn’t get clogged too often. It’s the RV black water tank that poses the most problems for RVers.

The black water tank is where all the sewage and solid waste goes. When you flush the toilet, the waste and water go into the black water tank. There are sensors in the holding tank that let you know how full it is so you can empty it when necessary.

Actually, each holding tank on your RV has sensors. They usually display how full the holding tank is on a digital panel near the door.

Although, from my experience, those sensors are often unreliable. Over time, they get covered with waste and don’t accurately depict the tank’s full size.

Based on that alone you might think you have a black tank clogged up. However, your tank valves are working just fine but the tank sensors are malfunctioning. So now you see why I don’t trust RV holding tank sensors.

The black water tank is made of thick plastic and has a small bottom hole leading to the sewage system. A vent on the top of the tank allows air to flow in as the waste is being flushed out. This prevents a vacuum from forming and makes it easier to flush.

Components and flow of your RV black tank.

The typical flow for waste in your RV sewer system follows the same flow.

  • Human waste goes into the RV toilet. When you press the flush handle, the toilet flapper opens the waste goes down to the black tank via the toilet pipe. This is why you’ll very rarely see a clogged RV toilet. Also, if your tank starts to develop a poop pyramid, this is where it’ll form, but more on that later.
  • The blackwater tank holds the waste. That’s all it does. It’s like a big plastic tote with an entrance hole at the top and an exit hole at the bottom. You’ll want to ensure you have more liquid than solids in the tank otherwise the toilet contents will turn to hardened solids which will be difficult to remove.
  • The waste continues to fill the holding tank until you pull the RV black tank valve open.
  • The waste then follows the sewer line down to your drain hose and into the RV dump station.

So what do you do when your black water tank gets clogged and won’t flush? Here are a few tips to unclog an RV black tank.

Treatments to unclog an RV black tank.

It’s time to drain the septic tank, and you pull the handle, but nothing comes out. Maybe just a tiny trickle of water. What gives? Your black water tank may be clogged.

The first thing I would try is a black tank treatment called Digest-It by Unique. It breaks down the waste in your black tank and can help to unclog a slow-draining tank. Pour the recommended amount into your toilet and flush it several times to get it into the tank. Then add a few gallons of water to help it along and let it sit for a few hours. If you want to add more and let it sit longer, your chances of success go up.

Keep in mind that RV black tank treatment is different than holding tank treatment. This treatment is designed to eliminate stubborn waste deposits and remove waste buildup. It’s also different than standard drain cleaner you would use one a clogged toilet in your home. So make sure you get the right stuff.

If you’re draining into your residential septic tank or the septic at an RV park, don’t worry. This stuff is safe for all types of septic systems and is suitable for them. They don’t use harsh chemicals like other professional-grade treatments. It helps to break down the solids in your tank and keeps everything flowing smoothly.

Once that has had a chance to work, try draining the tank again. If it’s still not draining properly, we have some more options at our disposal.

Flush it with a hose.

Whether you have a clog or not, I highly recommend flushing your black water tank with a hose at least once a week. This helps to break down the solids building up on the walls in the tank and keeps everything flowing smoothly. This will also help prevent the infamous poop pyramid.

That’s when solid waste forms a poop pyramid in your black tank from constantly piling up after each drop. Gross?

Many newer RVs have a built-in flush system with a hose connection outside the RV. If you have this, hook up your garden hose and turn on the water. You’ll usually see the flush input near your “tank fill” connection. The RV will do the rest, flushing the tank clean with pressurized water for you.

If your RV has no built-in flush system, you can still flush it yourself. You open the toilet as if washing it and put the hose into it. Turn on the water and let it run until the tank is full. This will take a while, so be patient. If you have the option to do so, consider using a pressure washer wand into the toilet. The high pressure water will clean the walls and breakup any clogs.

Baking soda or soap method.

Something else you might want to consider is adding baking soda or liquid soap to the tank as you fill it with fresh water. We prefer Dawn dish soap as it breaks down the tank’s contents pretty good. It will also handle some of the odor causing particles as well.

Once the tank is full, turn off the water and let it sit for a few minutes to allow the solids to break down. Then open the black water valve and let it drain. Leave your tank valves open for a while as the water rinses the black tank.

You even have options for installing a flush system if your RV doesn’t come with one. They’re relatively inexpensive and easy to install, so it’s worth considering if you often flush your tank.

I recommend the Quickie Flush by Camco. It has everything you need to install it properly and even includes a back-flow prevention valve.

Quickie Flush by Camco
The Quickie Flush is easy to install and will help keep your tank clean and prevent future clogs.

Soak with hot water.

You’ve tried it all, and nothing is working. You’re pulling the RV black tank valve open and still not draining. Nothing is flowing down the sewer hose.

Now it’s time to break out the big guns and try soaking the tank with hot water. There’s a good chance this will unclog your RV black tank.

Take a large pot of water (preferably 6qt or bigger) and heat it on the stove until it’s nice and hot but not boiling. Then slowly pour it into the toilet bowl through the toilet flapper, careful not to splash or spill.

Some campers will say to use boiling water, but we advise against that. Pouring boiling water into the black tank of your travel trailer can damage the gaskets around the dump valve. You don’t want warm water, but try to avoid boiling water as well.

If you still have some Digest-It left over, you could add a little to the water to help break down the waste. Now is a good time to add liquid soap if you want to do that as well. Soapy water tends to get everything moving.

Let the hot water sit in the tank for at least 30 minutes to give it time to work. You can add another pot of hot water if you want, but I find that one is usually enough.

This is a great method as it’ll work all the way down to the black tank valve. Remember, you’re not dealing with RV toilet clogs, you’re dealing with a draining problem.

Once the time is up, open the black water valve and let it drain into the sewer drain. You may need to repeat the process if it’s still not draining properly. This method also works on grey tanks as well if you want them smelling a little better.

Use a toilet snake.

This is the last resort because it’s pretty challenging, and there’s always the possibility of damaging your tank. But if you’re desperate, you may want to try it.

Going down from the toilet, send the snake into the black water tank until you feel it hit bottom. Start feeding it through the tank, working it back and forth to break up any clogs.

You’re trying to break up the masses causing the clog physically, so be as forceful as you can without damaging the tank. If the snake gets stuck, don’t force it. Try a different angle or direction and see if that helps.

Although you might be successful, I don’t recommend going in through the drain valve. Sure, the clog is probably at the top of that U-bend, but the tank is full of sewage, and you don’t want that to come at you once you unclog the black tank.

Borescope it to determine what the source is.

Are you still stumped? The next step is hiring a pro, but before we get to that point, there’s one more thing you can try. If the problem is a mystery, break out the borescope and look inside the tank.

A borescope is a tiny camera on the end of a flexible rod that you can insert into tight spaces to get a close-up look. It’s how mechanics check engines and how plumbers inspect pipes. You can find them at hardware stores or online. They’re fairly cheap and there’s even some that connect to your phone via Bluetooth.

Once you have your borescope, lower it into the black water tank through the toilet bowl. Look around and see if you can spot the source of the clog. It might be something as simple as a misplaced toy or an errant wad of toilet paper.

If you can’t find anything, it’s time to call a professional. Obviously there is a bigger problem than a clogged blackwater tank. But at least you’ll know you’ve tried everything you can before spending the money.

Borescope Going down Pipe
Here’s a shot of a borescope going down into a pipe. This isn’t from an RV, but it gives you an idea of what you should expect to see when using one.

Tips to prevent your tank from clogging in the first place.

If you have to unclog your black water tank frequently, then you need to take some steps to prevent it from happening in the first place. Here are a few tips to improve your tank maintenance—

Use biodegradable RV toilet paper.

The cheaper brands tend to be less effective and can cause clogs. They’re usually a common culprit for clogs. However, that’s an easy fix. I like to use Grentay Rapid-Dissolving toilet paper.

RV specific toilet paper breaks down almost immediately once it gets wet so it’s less likely to build up on the walls of your black tanks. And, it’s not just your black tanks on the line, it’s the sewer system that you’re dumping into. So using septic-safe toilet paper is the responsible thing to do either way.

Install a tank rinser.

This handy little gadget attaches to your water hose and gives your black water tank a good rinse after you’ve dumped it. It’s beneficial if you’re camped in an area with hard water. I recommend the Camco Quickie Flush.

Use a black water tank additive.

These products help to break down waste and prevent clogs. I like to use Digest-It, as mentioned above. It’s not just for clogs but good for maintaining a healthy tank in general. These additives are generally safe for RV septic systems as well as that of the dump station.

Clean the tank regularly.

As we mentioned, you’ll want to clean your black tank. If it’s been more than a week, you might want to flush it. Another option is to use the ice cube method.

This involves using ice cubes in water to slosh around breaking up the solids. Of course you’ll have to do this before you start driving as you want that movement. As the ice melts it’ll keep knocking off waste buildup off the black tank walls.

Be careful what you flush.

Only flush RV toilet paper and human waste down the toilet. Not just any toilet paper, you want the RV-specific bio-degradable toilet paper. No feminine products, paper towels, or anything else.

If you’re not sure, err on the side of caution and throw it in the small trash can instead.

As my wife likes to say – nothing more interesting than toilet paper. Remember, RV toilets aren’t designed for anything more than that even if you have full hookups.

Don’t wait to dump your tank.

The longer you wait, the more time waste has to build up and form clogs. Dump your tank as soon as it’s full or when leaving a campsite, whichever comes first. As an RV owner, you know that maintaining your rig is a lot of work.

Clean your sewer hose.

Not only is it gross to not clean your sewer hose after each use, solid matter and other solids can harden and create a clog inside your RV sewer hose. To clean the sewer hose and prevent buildup, always drain your RV black tank first, followed by the gray water tank, then give it a good rinse with enough water from your fresh water hose.

Following these tips should help to prevent your black water tank from clogging in the future. But if you find yourself needing to unclog your RV black tank, now you know how to handle it. When your RV’s toilet is clean and working properly, you’ll have happy campers.

Hit the road with a fresh and clean black tank.

Cleaning and unclogging your RV’s black tank to keep your camper toilets working is no easy task, but it doesn’t have to be a daunting one either. Just follow these tips next time you have a tank clog. You shouldn’t have many clogging issues with any other RV holding tank, usually only the RV black water tank, but most of these tips will work on the gray water tank as well.

By following the steps provided above, you’ll be able to tackle the job efficiently and safely. These are just a few methods to maintaining a healthy black tank.

As a reminder, always wear protective equipment, dispose of waste properly at a dump station and make sure to reduce solids entering the tank as much as possible.

With a little bit of planning and preparation, you can maintain an odorless and clean RV black tank for years to come. With that, it’s time to hit the road with a clean and odor-free black tank on your next camping trip and RV adventures.

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