The Proper Way to Flush and Clean RV Holding Tanks

Today, we have to talk about a topic no RV owner like: your holding tanks. From the graywater to the freshwater and the blackwater tanks, they need flushing and cleaning from time to time. How do you do it?

To flush RV holding tanks, start with your blackwater tank, then the graywater tank. Don’t let the tanks get completely full, but rather, two-thirds of the way, and always use a dump station when emptying the tanks. You can sanitize holding tanks with a gallon of water and a quarter-cup of bleach. 

There’s a lot more to discuss on both flushing and cleaning your RV holding tanks, and we will ahead. From where to find each tank, understanding tank valves, and detailed sanitizing steps, make sure you keep reading! 

RV Holding Tanks: The Difference Between Graywater, Blackwater, and Freshwater Tanks

Although all three RV holding tanks require maintenance, there’s a difference from what’s in one tank to another. That’s why we thought we’d start by reviewing the three tanks. We’ll also tell you where to find each one on your RV.  

RV Gray Tank

Not all RVs have graywater tanks, but for those that do, the graywater tank collects runoff from your shower and sink. Toilet waste doesn’t flow here, but all the soapy, greasy, used water does. Food particles and dirt will float within this tank as well. 

Of the three tanks, graywater tanks are not the most sanitary, but they don’t hold a candle to blackwater tanks in the dirtiness department. 

Blackwater and graywater tanks share an outlet drain, so where your blackwater tank is, the graywater tank won’t be far behind. The tanks don’t use the same valve though. Your RV might have the graywater and blackwater tank valves clearly labeled so you don’t mistake one for the other. If not, then read your owner’s manual or contact the manufacturer. You need to know where one tank is versus the other. 

RV Black Tank

At the very least, your RV must have a blackwater tank. Exceptions would be if you have a small camper or trailer that lacks a bathroom, as the blackwater tank is where your toilet waste goes. Yes, that’s right, we have to go there.

An RV toilet is not like the one installed in your bathroom back home. Each time you or your fellow passengers flush the RV toilet via its pedal, freshwater enters the toilet tank, sucking away the waste in the toilet bowl. That waste doesn’t enter the freshwater tank, of course (you’ll see why that is in a moment), but instead goes into the blackwater tank.

Emptying this tank regularly should be a paramount concern of any RV owner. You don’t want to find out what happens if the blackwater tank backs up or begins leaking, as it won’t be pretty.  

For those RVs without graywater tanks, all the graywater waste goes into the blackwater tank as well. It’s a lot easier to locate your blackwater tank if you don’t have a gray one, but even if your RV includes both, remember that the two tanks will have their own valves. Both valves should be closed unless you’re dumping the tank contents. 

Freshwater Tanks

The third RV holding tank is the freshwater tank. Besides flooding the toilet bowl with clean water to remove waste, the freshwater tank also provides potable water to your RV for everyday purposes. When you turn on your RV sinks to wash your hands or you run your shower, this water comes from the freshwater holding tank. You can also use water from the freshwater tank for drinking and cooking.

It’s a misconception that because the freshwater tank holds water that there’s no need to flush or clean the tank, but that’s not true! It can get dirty and dingy just like the other holding tanks, although not nearly to as bad of a degree. 

The valve and drain plug of the freshwater tank may be on the tank itself depending on the RV model. That should make it easy to differentiate between your freshwater and blackwater and/or graywater tanks. 

How to Flush RV Holding Tanks

Now that you’re clear on what each of your RV holding tanks does and where to find them, it’s time to flush the tanks. How do you do that? Allow us to explain.

Step 1: Know When You Have to Dump the Tanks

We’ll talk about how often your RV holding tanks need flushing a little later in this guide, but for now, let’s say this. Your tanks should never reach more than two-thirds fullness without you dumping them. Remember what’s in a blackwater tank: toilet waste. Now think about that toilet waste spilling inside your RV, making a mess of everything. That will motivate you to dump your tanks more often.

Can you still dump if the tank is less full than two-thirds? It’s not recommended. The reason for this is that at two-thirds fullness, the blackwater tank is filled not only with bathroom waste, but water as well. The water keeps the waste floating while the tank drains out. If the tank isn’t two-thirds full, then there may not be enough water for easy draining. You can quickly fix this issue by pouring in some water yourself.

Check your RV’s tank sensors to see how full they are.

Step 2: Find a Dump Station

For the sake of the wildlife and greenery around you, please never dump your RV holding tanks anywhere you feel like. Just like you wouldn’t spill a bucket of raw sewage out in your yard, treat the area you’re in now with the same respect. Even if you never come back to that campsite or national park again, you don’t want to make a huge faux pas like dumping wherever you want.

The purpose of dump stations is to release the contents of your blackwater and graywater holding tanks. Many dump stations are found around the aforementioned national parks and campsites. Even better is that you usually don’t have to pay to use a dump station if you’re already staying at the park. If you do have to pay, the fee is usually minimal.

However, proper dump station etiquette is important. When it’s your turn to dump your holding tanks, do it quickly yet thoroughly. Avoid using dangerous chemical treatments that include formaldehyde and other damaging ingredients. This will keep dump stations open around the country and free for RVers to use.

Step 3: Start with the Blackwater Tank, Then the Graywater Tank, Then the Freshwater Tank

You shouldn’t dump the holding tanks in random order. Instead, to save water and time, there’s a process to dumping. Begin with the blackwater tank so you can get the grossest part out of the way. Then, if your RV has one, follow up with the graywater tank and then finish with the freshwater tank. The water in the latter two tanks cleans out your hoses so you don’t have to hold up the dump station line by cleaning the hoses manually.

Wait, what hoses? You’ll need a sewer hose that attaches to the holding tank opening. A translucent sewer adapter and sewer extension hose (30 feet long or more) are necessary too, as is a straight coupler fitting or one that’s 45 degrees. Oh, and please wear gloves and maybe goggles too. You don’t want any holding tank spills on your clothes or face!

When your RV rolls up to the dump station, your blackwater tank valve should be lined up with the dump station opening. This reduces messes, so park precisely when you arrive at the dump station. 

Related Reading: Dumping an RV into a Residential Septic Tank: A Complete Guide

Step 4: Connect the Hoses 

Get out and put on your protective gear, as it’s time to attach the hoses for dumping. Check that your blackwater valve is closed at current. Then connect your sewer hose to the dump station hole using your hose ring and elbow to secure the point of attachment. This ensures that the blackwater waste can travel freely but not splash. 

Go back to your sewer hoses and double-check that they’re not going anywhere. Next, connect your holding tank drain outlet to the other side of the sewer hose. To do this, take off the sewer hose cap and put the hose beneath the drain outlet, again to reduce messes. 

Step 5: Open the Holding Tank Valve and Drain

Double-check or triple-check your connections one more time to be safe. When you’re ready to proceed, open the tank valve, starting with the blackwater holding tank as mentioned above. The moment that valve is open, wastewater will begin traveling through your hoses. You should certainly be able to hear it, and if your hoses are translucent, you’ll see it as well.

Although it’s not something you want to watch, you do need to keep an eye on the hoses to ensure the wastewater can travel clearly. Obstructions can lead to backups and messes, which you don’t want. 

The rush of waste will eventually slow down. When what’s coming out is a mere trickle, you’re done draining the blackwater tank. Now you can flush. 

Step 6: Flush the Holding Tank

It helps if your RV comes with a black tank rinse system, in which you can source dump station water via a garden hose and then flush the tank. Let the water pass through the blackwater tank for two to five minutes to remove any residual solids. You would be surprised at how much waded up toilet paper gets stuck in the corners of the tank.

What if your RV lacks a convenient black tank rinse system? That’s fine. You can flush your holding tank by sucking up water from the toilet or diverting water from the freshwater tank to your blackwater tank.  

Please make sure that you close the blackwater tank drain valve when you’re finished! If you don’t, then as soon as you accumulate toilet waste, it will flow right out.

Step 7: Repeat with the Other Tanks

At this point, you’ve only drained and flushed the blackwater tank. You still have one or two tanks left, so repeat the steps above until you’re done flushing all three tanks. 

Step 8: Disconnect Your Hoses

Now that you’re finished flushing your tanks, you want to remove all your connections from the dump station. Take one sewer hose end and raise it so gravity can empty whatever’s in there, if anything. Running water through the hose helps clean it. When you’re finished, put the RV drain cap back on.

Your sewer hose should be able to attach to itself via the two ends. This keeps all leftover waste that might still be inside it contained. Take off your gloves and goggles, throw the gloves away, and wash your hands.

Step 9: Add Water to the Blackwater Tank

Before you go, pour between two and four gallons of water into the blackwater tank. Then you can consider your tanks flushed. 

How to Clean RV Holding Tanks

Flushing your RV holding tank is one thing and cleaning it is another. You’ll know you have to sanitize any of your three tanks when a distinct odor assaults your nostrils. Depending on which tank is the biggest offender or if more than one tank reeks, you might smell sewage, bathroom waste, and bacteria mingling. 

Dumping the holding tanks does not necessarily prevent these unappealing odors, and even using chemical treatments only keeps your tanks anti-stink for so long. Fortunately, you won’t have to go to nearly as much time or trouble to clean your RV tanks. Here’s what you do.

Step 1: Make the Cleaning Solution

We always recommend a simple sanitizing solution of water and bleach for cleaning the blackwater, graywater, and/or freshwater tanks. For every gallon of water, you need a quarter-cup of bleach. The average holding tank might require up to eight gallons of water, so add the appropriate amount of bleach.

If you don’t want to make your own, there are many tank treatments you can buy. The ones we use are Camco RV Toilet Treatment Drop-ins. Other options are RV Digest-It and Happy Campers RV Holding Tank Treatments.

Step 2: Pour the Cleaning Solution into the Holding Tank

Next, transfer your mixture to a bucket and pour the bleach-water mix in little by little until your holding tank is totally full. Close up your tank and then do nothing over the next eight hours. 

Step 3: Drive!

Well, nothing to the tanks, that is. You can help the mixture really get in there and clean the inside of the holding tanks by using your RV. 

Drive around on some bumpy terrain or on a twisty road, within safety limits, of course. All the movement of your RV lets the water and bleach bounce, swish, and move so it can do a more effective job of scouring the inside of your holding tank.   

Step 4: Flush the Bleach from Your Holding Tank

When the eight hours elapse, you have to clear the bleach from the tanks. To do that, head to your local dump station and follow the instructions in the last section for flushing the tanks. Most dump stations should allow you to dump bleach, but if you’re concerned, it doesn’t hurt to check in first. 

If you feel like you have stuck toilet paper or waste in corners of your black tank, which most you us to, then a RV black tank flush or wand is a great product. Its rotary actions dislodges and flushes out waste and particles stuck in areas.

How Often Do You Flush and Clean RV Holding Tanks?

Whew! All three of your RV holding tanks are flushed and cleaned, and it admittedly wasn’t easy. You’d like to not do that again if you don’t have to. Unfortunately, you will have to. We recommend flushing your tanks after every use. If your tanks aren’t 2/3 full after a weekend trip, consider adding water to each tank in order to give them a proper flush. 

As for cleaning the holding tanks, do it when they start to smell, but don’t let the odors get too bad. You certainly want to give the tanks a thorough cleaning before the offseason as part of your RV winterization process. 

Final Thoughts

Flushing and cleaning your RV holding tanks might not be pretty, but it’s a job you’ll have to get used to. You have to flush your tanks at your nearest dump station at least once a month and sanitize the tanks a few times per season. As you flush the tanks more and more, it becomes second-nature so you can get it done as quickly and painlessly as possible!

Geoff Southworth

I am a California native and I enjoy all the outdoors has to offer. My latest adventures have been taking the family camping, hiking and surfing.

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