If there’s one thing on RVs that we’ve almost seen couples come to fisticuffs on, it’s their holding tanks. We think we know what’s going on in there and yet there’s a lot of frustration surrounding them. So let’s get to the bottom of it. What’s really going on inside those things?

What is a holding tank?

Well, as you might guess, it’s a box. A large, plastic box in almost every case (some older ones were made of metal). Rectangular in shape, they tend to either be cube-like or long, flat, and not very deep. Both types have angled bottoms that tip toward the front. That’s how waste and water flows out.

Almost all RV plumbing systems dump into holding tanks from the top. In the best of cases, waste and water comes straight down and goes into the tank, but in most cases, there’s some bends and runs in the RV waste system plumbing.

At the front of the holding tank, a large, 3-inch or 1-1/2″ (black tank vs. gray tank) hole is drilled. This is how waste gets out of the tank when the valve is pulled to empty the tank. Tanks empty themselves using gravity (i.e. waste and water flow “down hill” out of the tank and into the flexible sewer hose, then out into the RV park’s sewer system).

Your tanks are for holding, not composting

As we’ve said before, your RV’s holding tanks are not a septic system. They were not intended to hold a mass of water waste and allow it to break down. In fact, by adding tank additives that contain harsh chemicals, enzymes, or biologicals, you’re just making things worse in your holding tanks. Why?

Those additives are doing exactly what they are intended to do: break down waste in your tanks. But, they’re breaking it down into a thick, sticky slurry that has a hard time flowing out of the tank. Plus, that sticky mess is making your tank sensors misread. And it never truly all comes out, unless you have your tanks professionally cleaned by a service like Kleen Tank or any of our sister All Pro Water Flow companies.

So what do you do?

There are some “best practices” to follow when caring for your RV’s holding tanks:

• Stop using tank additives, especially those that contain harsh chemicals, enzymes, or biologicals. If you really want to use something in your tank to improve flow and keep odors at bay, our homemade solution works well and is very affordable.
• Dump your holding tanks when they are 90% to 100% full. If you have to dump them before they are full, add fresh, clean water to raise the level in the tanks.
• Use a product like the Valterra Flush King Back Flush Valve. It’s a great way to really flush out holding tanks and get them clean. Plus, they flood your holding tanks faster than your in-tank system for your black tank and you can use it on both black and gray tanks!
• Once your tanks are empty, be sure to “prime the system” in each tank by adding three to five gallons (depending on your holding tank’s capacity) to give a base of clean water to start out with. It’s also good to add the same amount of clean water to your holding tanks when you are traveling with our RV.

Get your tanks professionally cleaned on a regular basis

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Want to know a secret? There’s a way you can give your holding tanks a “head start” when you’re about to use them, something that will keep them draining faster, reduce odors, and, possibly, help sensors from misreading. You want to know, don’t you?

How holding tanks work

As we’ve said before, your holding tanks are not meant to be a septic system. That’s why we don’t recommend tank additives that contain harsh chemicals, enzymes, or biologicals, meant to break down waste. In fact, as we’ve noted in other articles, it’s those additives that are causing a lot of problems.

Your RV’s holding tanks are meant to be just that — tanks that hold waste until they’re full and you can release it into the park’s waste processing system (which likely then goes to a municipal waste processing plant). So keeping “stuff” from breaking down too much helps keep your tanks working properly.

So what’s the secret?

You know you should only empty your holding tanks when they are 90% to 100% full (adding water to bring the level up if you have to empty them before they reach that critical volume). Emptying your holding tanks when they are full takes advantage of the “tornadic effect” that occurs with full tanks.

Once you’ve emptied the tanks (and flushed them out, using either your in-tank spray system in your black tank or something like the Valterra Flush King Back Flush Valve (that can be used on both tanks!), it’s a good idea to “prime the system” before using your RV’s tanks again.

“Priming the system” simply means adding some clean, fresh water to your empty holding tank after you have emptied it. Why would you want to do this? As you know, holding tanks work best when there’s a high ratio of liquid to solid waste. Adding in three to five gallons (depending on your tank’s capacity) before you starting using them again is a wise move.

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Doubtless you have some questions about your tanks and how to flush them out. Here’s some of the most common ones we hear:

Q. My RV has a tank flushing system. Why do I need something else?
A. The in-tank rinsing system in your black holding tank sprays in set pattern, either through a single spray head or a rotating array of nozzles. While it’s a good system, it can take a long time to fill your holding tank to thoroughly flush it out. Plus, it’s only in your RV’s black tank. We do recommend, however, that you turn the rinsing system on while you are emptying the black tank. The added turbulence and flow of water is helpful.

Q. I use the tank flushing system after I empty the tank. Isn’t that enough?
A. The PSI of your in-tank rinsing system is 100 PSI to 150 PSI, since it comes from the RV park’s pressurized water supply (usually 40 to 80 PSI). Reduction of pipe size and the nozzle spray head increases that pressure a bit, but the flow rate is reduced. It can take a long time (a half hour or more) to fill up your black holding tank for rinsing. Plus, again, there’s no similar system in your gray holding tanks.

Yes, your RV’s black tank has a rinsing system built in. But it rinses, not flushes, and there’s no similar mechanism for your gray holding tanks.

So what’s the solution?

We use as a part of our professional holding tank cleaning service — and recommend to our customers — the Valterra Flush King Back Flush Valve. It’s the best way to quickly fill your holding tanks so that they can be thoroughly flushed out. Here’s how to do it:

1. Empty each of your holding tanks as you normally would.
2. Make sure that the holding tank release valves are closed.
3. Disconnect your flexible sewer hose from the attachment on your RV, in your waste system bay, or where the flexible hose attaches to your main pipe fitting.
4. Attach the Flush King to that same main pipe fitting.
5. Attach your flexible sewer hose to the other end of the Flush King.
6. Attach a water hose from the pressurized water tap at your site to the fitting on the Flush King. Never use the same hose you use for your potable water supply.
7. Close the valve on the Flush King, preventing any water from your RV to travel down your flexible sewer hose.
8. Starting with your black tank, pull the release valve for that tank (so it is open).
9. Turn on the water supply knob on the Flush King. Pressured water should now be rushing into your black holding tank.
10. Depending on the pressure of the RV park water and the capacity of your holding tank, the tank should fill in anywhere from a minute or so to around three or four minutes. If your sensors work, you (or someone inside the RV) can monitor them to indicate the tank is full. You may wish to get a water meter and attach that to your pressurized water supply hose to monitor how much water is going into your holding tanks (that’s what we do).
11. When the holding tank is 90% full (be careful not to go over the tank’s maximum capacity), stop the flow of water into your holding tank by turning off the knob on the Flush King.
12. Open up the release valve on the Flush King (on the flexible sewer hose side). Water will rush out, down the flexible sewer hose and into the the RV parks sewage system or tank). Watch to see how clear the water is (we bet you’ll see quite a bit of waste).
13. Repeat steps 7 through 12 until the water from your black tank runs clear.
14. Repeat steps 7 through 13 for the gray holding tanks.

Get the Valterra Flush King and use it to keep your tanks clean and working properly.

Can we ask you a favor?

We normally have the Valterra Flush King Back Flush Valve for sale at our vendor booth when we attend a rally and always have one or two for sale on our service truck if anyone wanted to buy one at the time of service. You can certainly purchase them at some camping store locations like Camping World as well.

If you are interested, however, in ordering a Valterra Flush King online from Amazon.com, we ask that you click on this link. Purchasing the Valterra Flush King via this link won’t cost you anything, but Kleen Tank earns a very small commission from your Amazon.com purchase. Thank you!

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To be direct, no. Absolutely not. We were called the other day by an RV owner at a local RV resort. The customer couldn’t get their toilet to flush. “The stuff won’t go down,” she said. We knew exactly what the problem was.

What did we find?

We got the RV and took a look. All three gate valves were open and we knew exactly what to expect. The owner said she had her son “go and flush some water up into the tanks” but all that happened was the toilet overflowed and there was a gurgling sound from the shower and galley drains.

Long story short, the customer had a clog in the waste drain pipes from the holding tank to the flexible sewer hose. A big one. A monumental one. One that required we suit up in Tyvek. You get the picture.

So what happened?

Think about it. By leaving the black tank’s valve open, water and liquid waste seeps out. What’s left is a concentrated mix of solid waste and toilet paper. The more liquid that seeps out, the more compacted and dried out the solids become.

It took us nearly an hour just to clear the clog, plus another half hour to clean the insides of the tank with our hydrojetter. Eleven back flushes (one away from our record!) and the tank was finally clean.

The final word.

Many RVers leave their gray tank valves open. We’re not big fans of that either. Your RV’s holding tanks work best when there’s at least a 50:50 ratio of waste to water. Leaving the release valves open removes the water from equation. It’s not so bad in gray tanks, but those too can have solids drop out and accumulate in tank corners and other areas that don’t benefit from the rush of water when emptying the tanks.

Keeping the black tank valve open is just asking for trouble. Don’t do it and remember that your RV’s holding tanks are not a septic system, they are a holding system. Waste and water is intended to be retained in the tanks (essentially, large plastic boxes) until they are full and ready to be empties periodically.

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