How Big Is An RV Septic Tank?

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If you’re new to the RV life or just considering it, there’s one thing that hands down scares off more people than anything else. That is, dumping the septic tank. When people think about having a small kitchen or short showers, it’s no big deal. But then, when they start thinking about the hassle of dumping out the “toilet” tank, that’s when they start doubting the whole thing. So that brings up the first obvious question – how often does it need to be dumped anyway? Just how big is an RV septic tank?

The size of the tank changes with the size of the RV. However, they’re often more accomodating than most people expect. In a small RV, you can expect at least 15 gallons for the black water and a gray water tank of 30 gallons. A larger RV might easily have tanks as large as 50 gallons each.

But if you aren’t sure what that means, keep reading. We’ll explain the difference between black water and gray water and cover the third tank your RV has – fresh water. We’ll compare what you need to know about the sizes, how and when to dump, how to know it’s time to dump, and more.

A camper van parked in a designated parking area for camping, How Big Is An RV Septic Tank?

What Are The Different Tanks?

It’s important to start by understanding the different tanks in your RV. The size varies a bit, depending on how big your RV is. But all RVs have more than one tank. There’s a difference between the fresh water, gray water, and black water tank.
 
Fresh water is exactly what it sounds like. It’s the tank that holds your useable water. When you turn on the sink or shower, this is where the water comes from. Unless, of course, you’re hooked up to a city water line at a campground. 
 
Gray water is where the dirty water goes after it has drained from a sink or shower. This is used water, no longer drinkable. But not the contaminated toilet water – that has its own place. 
 
Black water is the “toilet” water or sewage. It’s kept apart from the gray water because you need to be a bit more careful when disposing of it. Water that you used to wash your dishes isn’t the same hazard that water containing fecal matter is. Occasionally, an RV might combine the gray water and black water into one tank. This is still a black water tank since contaminated water is present. This isn’t very common, mostly only in very small RVs.
 
Charging the batteries of a camper van

How Many Gallons  Does An RV Black Water Tank Hold?

The size of each tank varies with the size of the RV. An RV made to house eight people will need larger tanks than an RV sized for two.

As a general rule, the fresh water tank is the largest. According to RVing Know-How, it holds anywhere between 20-100 gallons. The gray water tank averages 50 gallons. Black water tanks hold 18-64 gallons.

While a larger tank might seem ideal, it isn’t always. Having a tank that is sized “just right” for your RV is the best. A large tank will either be empty when it’s barely used or take too long to fill. Either one can cause problems with odors and make it challenging to flush the tank clean. A small tank is just frustrating – who wants to have to find time to dump every day?

It’s a good idea to be familiar with your specific RV and the tank size. This helps you get an estimate of just how many gallons you fill each day. This way, if you stay somewhere without a dump station or hookups, you can anticipate how many days the tank should last. It also can help if you take a longer trip than normal and need to plan time for dumping. 

Parking a camper van in the middle of the forest for camping

How Often Do You Need To Dump RV Waste?

Dumping the black water tank takes some amount of precision. You don’t want to go too long, or it might start to smell. But, believe it or not, according to Colton RV, the more common problem is dumping too quickly.

Breaking down the waste in the black water tank takes time. And the fuller the tank, the easier it is to flush out. If there’s not much in the tank, solid waste tends to stay behind when you dump it. There isn’t enough liquid to flush it out properly.

You should try to avoid dumping a tank less than 2/3rds full. The closer to full, the better. Many tanks have sensors that can help judge this. Just remember that these sensors may not be as accurate after a few years, particularly if there’s a lot of toilet paper sticking to the sensors. This is one of the reasons that you should use RV toilet paper, which breaks down easier than regular toilet paper. Avoid using excessive amounts of toilet paper as well, which can wreak havoc on your system.

And remember, the longer you can wait, the more time the waste has to break down in the tank. Aim for a few days to a week for the best results. This makes flushing it out easier.

An RV waste water dumping station

Can You Dump Black Water On The Ground?

Never dump the black water tank out on the ground. It’s full of bacteria and unsafe. It can spread disease and cause serious harm when it enters the ecosystem. Always dispose of black water waste properly at an RV dump station.

The gray water tank, on the other hand, isn’t quite so toxic. It can’t be dumped “anywhere” – there are still safety concerns to address. The soap, for example, can be damaging to the environment. Food particles are another concern. But with proper filtration, some RV owners find ways to use the water elsewhere, reducing water waste. 

Where Is The Black Water Tank In My RV?

The black water tank is located underneath the RV carriage, next to the gray water tank. Of course, if you have two separate tanks – some small RVs use just one.

Finding and dumping the black water tank can be intimidating for a first-timer. This helpful video can make you more comfortable using a dump station and emptying the black water tank yourself. Always start with the black water tank and then the gray water next.

How Long Will A Black Water Tank Last?

It’s tough to predict how long a black water tank can last. In time, the plastic making the tank deteriorates or grows brittle. At this point, regardless of how well you take care of it, the tank is simply reaching the end of its life span. However, it’s not unheard of for a tank to last 25 years or more before this occurs.
 
However, tanks don’t always make it to this point. Damage occurs, usually by improper care or storage. Clean the tank out often, flushing out the system regularly. You can do this with a simple garden hose.
 
Never store the RV with waste still in the black water tank. Dump the tank, then rinse the system before putting the RV away for the season. Any time the RV is out of use for more than a week, you should dump the tanks. Unique Camping and Marine suggests filling the black water tank with fresh water and digestive enzymes to break down any waste that could be left behind by accident. 
 
If there’s any chance that the water in the tank could freeze during storage, however, do not follow this advice. Instead, remove the water from all tanks and lines, or they could crack and break. Winterize your RV, like shown here:
https://www.youtube.com/watch?v=HdLs8kTd26I

What Are Black Water Tanks Made Of?

In most RVs, the black water tank is made of a special kind of plastic. This material, known as ABS plastic, is also used in plumbing. Some tanks are made of low-density polyethylene (LDPE). This is easy to spot as it has a milky, almost see-through appearance. 
 
Tanks made of LDPE are particularly hard to repair. They don’t respond to most solvents, and most methods to fix cracks don’t work. Chances are, if you have a problem with an LDPE tank, you have to replace it.
 
On the other hand, ABS plastic is sometimes repairable. However, this depends partly on how well your tank is taken care of. It may just not be worth it. Once things start to go wrong with the tank, it tends to accumulate. The material is just starting to fail – patch one hole, but in a few weeks, there’s another. And when it’s your black water tank, most owners don’t want to keep messing around. It’s just easier to replace than to keep dealing with leaks from your toilet tank, odors, and other nastiness.
 
Disposal of waste water and gray water inside a camper van

In Closing

Most RVs have three tanks. The fresh water tank holds the potable water waiting for use. The gray water tank collects the used water from the shower and sink. The black water tank is for the used toilet water. Some RVs do not have a gray water tank, and all the used water collects in the black water tank.

For best results, dump the black water tank when it is close to full – never less than 2/3rds. Dump the tank every couple of days to a week. Always dispose of black water at an RV dump station for legal and sanitary reasons. Keep your system in good shape by never letting waste sit – always dump before storage. Flush the tank regularly with a hose to keep waste left by accident from accumulating in the tank.

If you enjoyed this article, please consider these next:

How Big Is An RV Shower?

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