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How to Unclog an RV Black Tank

Like most people, you probably don’t think about your RV’s black tank until it’s too late. And by then, it can be a real mess. 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.

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

RVs have three tanks, one for freshwater, one for gray water, and one for black water.

The gray water tank stores water used in the sinks and showers, and the freshwater tank stores clean water used for drinking and cooking.

But you’re not here for that; instead, you’re here for the black water tank.

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

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.

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 treatment called Digest-It by Unique. It breaks down the waste in your 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.

If you’re draining into your residential septic tank, don’t worry. This stuff is safe for all types of septic systems and is suitable for them. 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.

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. The RV will do the rest, flushing the tank clean 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.

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.

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 valve open and still not draining. 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, careful not to splash or spill.

If you still have some Digest-It left over, you could add a little to the water to help break down the waste.

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.

Once the time is up, open the black water valve and let it drain. You may need to repeat the process if it’s still not draining properly.

Use a 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. 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:

  • Use biodegradable RV toilet paper. The cheaper brands tend to be less effective and can cause clogs. I like to use Grentay Rapid-Dissolving toilet paper.
  • 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.
  • Be careful what you flush. Only flush toilet paper and human waste down the toilet. No feminine products, paper towels, or anything else. If you’re not sure, err on the side of caution and throw it in the trash instead.
  • 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.

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.

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