How Long Can You Leave Black Water in an RV?

Written by Jeff Campbell in RV Water & Sewer Last Updated December 4, 2022

If you have an RV, you know that it can be a hassle to empty the black water tank. But sometimes you find yourself boondocking or parked for an extended period of time where dump tanks aren’t an option. So how long can you leave black water in an RV?

Here’s what I know from owning mine:

You should not leave matter in your black water tank for more than a week. Your black water tank should be emptied once it’s ⅔ full and/or at the end of every trip. If that isn’t possible, make sure to add water to the tank and add a holding tank cleaning chemical to avoid odor and backup.

But that’s not everything you need to know about your black water tank.

Because we’re talking about black water, there are potential issues with odor, bacteria, blockage and more disgusting things.

Just keep reading to find out what to do!

How often should you empty a black water tank?

If you are traveling alone, you might be able to last 2 weeks before you need to empty it. Otherwise, if you are not parked at a full hook-up campground, plan to empty your tank once a week.

Your RV’s black water tank is the tank under your rig that holds the waste from your toilet. Because it’s toilet waste, it’s vital that you keep it working properly.

To make sure it’s working properly and to keep it from stinking up the joint, you need to keep it clean and empty it regularly.

But knowing the exact frequency depends on a few factors. 

If you are traveling with a lot of people or using a small rig, you may need to empty it as frequently as every other day.

Thankfully, most RVs take the guesswork out of when you should empty the tank. They come with sensors that tell you how full each of the tanks are, including the potable water, gray water, and black water.

Generally, you should empty your black water tank when it is ⅔ full. 

They should also have an owner’s manual. If your rig doesn’t have sensors, the owner’s manual will tell you what to look out for to empty the tank.

You should at least empty and clean your tank after every camping trip. You definitely don’t want to store your RV with waste in the tanks!

Should I leave water in my RV black tank?

Yes, you should leave a small amount of water in your black water tank. If you don’t leave a small amount of water in the tank, the solid waste will build up.

By maintaining a swishy environment, you ensure that solids don’t build up, get stuck to the walls of the tank, or harden over the tank sensor.

In fact, you should leave both your gray water and your black water tanks closed at all times, even when you are connected to a sewer.

If you leave your tanks open, the liquid will flow through freely, but solids will get stuck.

This can get super messy! When solid waste gets stuck in the hose, you may not know it until waste starts to back up into the RV. Then you have to figure out how to unclog it.

Luckily, many more modern RVs have the option to hook up the hose to a black water tank to flush the black water tank.

By keeping the gray water tank closed, you can use the soapy water to clean out any debris leftover from emptying the black water.

It’s also a good idea to add a few ounces of chemicals to your tank after each emptying. This will help break down any solid material and prevent any odors from seeping out.

How do I know when my black tank is full?

The digital display in your RV will indicate how full the black water tank is. But you can also listen when you flush the toilet to hear how full it sounds. An increased odor also is an indicator of how full it is.

Sometimes the factory-installed tank sensor is inaccurate. Things like toilet paper and solid waste can get stuck on the sensor, making it completely unreliable.

So how can you tell if the tank is full when the sensor isn’t working properly?

First, you can listen to how your toilet sounds when you flush it. Just like you can hear when a bottle or a glass is close to full, you can hear when the tank is almost full, too. 

But the better you can “hear” how full your tank is, the more it smells, too.

Another indicator that your black water tank is full, is “splashback.” When air gets trapped in the system, it’ll bubble back before it goes down the toilet.

If none of these things are warning enough, eventually, you’ll see a brown liquid coming out of the stool at the floor flange.

Then it will ooze onto the floor of the bathroom or into the basement. You definitely won’t be able to miss that.

Can you dump RV black water at home?

For many people, the answer will be no as there is not an approved and sanitary way to dispose of black water at home. However, if you have a septic system or grinder pump, it can be easy to run the RV drain hose into one of those.

So, yes, you can, but there are some things you need to know first. Dumping at home can be convenient, but it may not be the best option for everyone.

First and foremost, check your local ordinances.

If you are on city water, or if you live in a neighborhood with an HOA, it may be against the rules, or the law, to dump RV waste into a residential sewer system. 

If you have a home septic system, it’s super easy to dump your waste at home.

Just look for the system’s cleanout. It’s a big PVC pipe sticking out of the ground with a screw-on cap. If you don’t have one, then look for your system’s access port.

Make sure that you are on the solid waste side of your system’s baffle.

If you dump on the wrong side of your septic, you can cause significant problems. I’m talking about the kind of problems that can cost thousands of dollars to repair. That will make dumping at home inconvenient for sure.

Finally, when dumping at home, don’t use any chemicals in your RV black tank. Those chemicals can kill off any good bacteria in your septic tank.

If you are on city septic, and you can’t dump directly into the septic system, you can grind up the contents of your holding tank with a macerator. Then you can use a pump to flush it through a garden hose directly into your home toilet. It’s tedious, but it can be done.

I would just make sure to either mark the water hose or store it somewhere out of the way. You don’t want to use the same hose to play in the sprinkler or fill up your swimming pool in the summer.

Will it hurt my black water tank if I don’t empty it?

You should not store your RV with black water in the tank. It can get smelly, and dried fecal matter can get stuck inside the tank and stuck to the sensors and be very hard to later clean off.  

It should be emptied and flushed once it is about ⅔ full.

For two people, this may take a week or so. But if it takes longer because you use campgrounds and rest stops, it won’t hurt it to wait until it gets ⅔ full.

When you do empty it, be sure to empty it first, and then empty the gray water next. The soapy water will help to flush out any remnants. It will also help clean out the hoses.

Once you have emptied it, it is imperative to make sure the valve is closed completely.

If it’s not closed completely, the liquids can leak out, leaving the solid waste no way of draining out. It will build up into a stinky mess.

You also want to make sure the gray water valve is closed completely too. Some people like to leave it open and let it drain continuously. But if you keep the valve open, you won’t have that extra water to flush out the black tank.

Empty Your Black Water Tank at Home

Did I answer everything you wanted to know about how long you can leave black water in an RV?

Emptying your black water through the sewer hose is not glamorous, but it is a necessary part of RV ownership.

Your tank should be emptied after every trip or once it’s ⅔ full. Leave some water in it so the solid waste doesn’t build up.

If you have an easily accessible septic system, you can dump your black water for free at home. But if you don’t have an access port, it’s much easier and cleaner to use a dump station.

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