What’s a pyramid mound in my black tank?

Quite literally, the pyramid mound is a pyramid-shaped mound of solid waste and toilet paper that has built up inside your black water holding tank. Pyramid mounds are very easy to form if you’re not following proper RV holding tank care procedures. A bit more on that later.

Pyramid mound or clogged pipe?

A pyramid mound is often mistaken for a blocked plumbing line (and vice versa). How do you know if you have pyramid mounds or a blocked pipe? Here’s how we diagnose the problem:

  1. Do you keep your black tank valve open? If so, you almost for certain have a pyramid mound as liquid in your black tank is seeping out, leaving solids like waste and toilet paper behind.
  2. If you keep your black tank valve closed until it’s time to empty but you’re pretty sure that there is “stuff” in your black tank, open the black tank valve. If nothing or very little comes out, you may have a pyramid mound, but you likely have a blocked pipe. Close your black tank valve.
  3. Next, has sewage water backed up into your toilet? This is a sure sign of a blocked pipe. It could also mean the pyramid mound has grown in your black tank to block the toilet line, preventing you from adding water to your tank.
  4. Finally, if there is no water in the toilet, take a look down your toilet with the flush valve open. Can you see into the tank? Does there seem to be anything visible there, like a wad of toilet paper or even liquid with things floating in it? If not, there’s probably a pyramid mound in there.

Clearing the clog

If you’re pretty sure there’s no blockage in the waste pipe and that your problem is a pyramid mound, there are a couple of steps you can take to try to clear it.

  1. Close your black tank valve.
  2. Try adding several gallons of hot water down your toilet. This may help to dislodge the pyramid mound, allowing it to exit your black tank when you pull that valve.
  3. If that doesn’t work, it’s time to get physical! Use a commercial-style toilet pluger, like the Corky, to push waste down out of the bowl or pipe. If the bowl is clear, use a toilet flush wand to try to poke the pyramid mound and break it up. You can attach a garden hose to the toilet flush wand by either snaking it from the outside through a nearby open window or through your RV or by using a water thief attached to your bathroom’s sink faucet. The added water may help to break up the pyramid mound. Try — carefully — adding another couple of gallons of hot water.
  4. If you have a tank rinse system on your RV (almost all do), turn it on and open your black tank valve. You shouldn’t skimp on time with this step. By running your rinser for 20-30 minutes, you make sure to clean all the extra waste out of your tank. When you first start your rinser, the water coming out of your tank might look clear, leading you to believe that your tank is clean, but if you let the rinser run for a while, the residual waste will often break loose and come rushing out of your tank!
  5. If you still can’t clear the clog, it’s time to call an RV tank cleaning professional, like Kleen Tank. We have special equipment and training to clear pyramid mounds and pipe clogs from both inside your RV as well as from the outside.

How to prevent a pyramid mound

A pyramid mound isn’t impossible to remove, but it can be quite a hassle! The best way to avoid this frustration is to prevent it from happening in the first place.

As we see, a pyramid mound most often happens when a black tank valve is left open or doesn’t seal completely. Keep your black tank valve closed unless you are dumping your tank. If you find there is seepage (more than an ounce of dark- or brown-colored liquid) every time you take off the cap to your sewage pipe, have your valves checked and replaced (if needed) by an RV repair facility or mobile tech. You may want to consider electronic gates, as these do a much better job of keeping your holding tanks sealed off.

Follow good practices in maintaining your holding tanks including:

  • Emptying tanks when they are 75% to 100% full. Add water to the tanks to get them to this level before emptying.
  • Run your black tank (and gray, if it has one) flush or rinse system when emptying your tanks. The added water and slight turbulence will help to keep pyramid mounds from forming.
  • Use a tool like the Valterra Flush King Reverse Flush Valve to keep your tanks clean after each emptying. It’s handy to add a water meter to the valve so you can add 75% to 90% of each tank’s capacity in fresh water to flush out any remaining waste after emptying.
  • Once your tanks are emptied and clean, prime the tanks by adding 1 to 3 gallons of fresh water to each tank. This helps to balance the waste-to-water ratio in your tanks and keep things moving when emptying.
  • When using your toilet, always use plenty of water. Start with a half bowlful of water, relieve yourself as needed, flush it down, fill the toilet bowl at least halfway full, flush that down, and then fill the bowl again halfway.

In conclusion

When you keep your black tank valve closed and use plenty of water in your tank, the water in your tank sloshes around and keeps the waste from forming into a pyramid mound.

A pyramid mound in your black tank is definitely frustrating, but it’s ultimately preventable. Even if you’ve already gotten a pyramid mound, however, it can be solved with a little work.


Jim Tome, Owner

Jim Tome, Owner

Hi, I'm Jim Tome and, along with my wife, Debbie, the owners of Kleen Tank LLC, the national leader in RV holding tank cleaning service. We've been RVers like you since 2004 and have traveled all over the U.S. in our Airstream travel trailer. We started the business about twelve years ago and have cleaned tens of thousands holding tanks in thousands of RVs. From tiny weekend travel trailers to monstrous fifth wheels to luxurious motorhomes, I've seen just about every situation there is with RV holding tanks and waste systems. I hope you enjoy our articles; I try to post at least one per week and we've got a great library of them to cover just about every problem. Enjoy!


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