Diagnosing, Clearing, and Preventing A Clog In Your Black Water Holding Tank

A clog in your black water holding tank sounds like a terrible nightmare that you would want to avoid at all costs. Fortunately, clogs are really not too terrible to get rid of and they are even easier to avoid. In this article, we will discuss the three most common types of clogs and how to diagnose them, then we will explain how to clear them, and finally, we’ll give you tips on how to stop them from happening in the future. 

Pyramid Plug

The first clog we will discuss is the pyramid plug, which typically occurs when you leave the black tank valve open. Whether you accidentally left the valve open or are new to RVing and thought that leaving the black tank valve open was common practice, you should be aware that an open valve can lead to a pyramid plug.

Here’s what happens: when the black tank valve is left open while you are camping at a full-hookup campground, all of the liquid will flow out of the tank and into the sewer port, leaving behind all of the solid waste and toilet paper.

As more waste and paper are added to the tank, they begin to stack on top of each other, making a pyramid. If left unchecked, this can stack up all the way to the 3-inch discharge pipe that connects the toilet to the black tank. (If this happens, breaking through the clog will be a little more difficult.) You will know that you have a pyramid plug clog when you look down the toilet and you can see waste and paper through the open toilet valve. You may also notice, when you go to dump your tank, that you have left the valve partly or fully open. And if the pyramid clog has been there long enough, you will see water backing up the pipe and into the toilet when you go to flush. 

Plumbing Clog

The second clog is a plumbing clog, which will be located in the plumbing that connects the toilet to the black tank or in the plumbing that is at the discharge end of the black tank.

Several things can lead to a plumbing clog. First, if you have a larger RV such as a Class A or 5th wheel, there can be a storage space between the living area and the location of the holding tank(s), necessitating longer pipe runs from the toilet to the black tank and 45-degree pipe fittings. Because of these longer sloped runs, it is much easier for water and paper to accumulate in the pipes. Over time they can become completely blocked and lead to backing up of waste and water into the toilet.

Also, clogs can form on the other side of the tank when waste and paper get knocked loose while the RV is in transit and pile up at the exit of the tank. Clogs can also form in this area when there is a mechanical problem with a black tank valve. For example, sometimes the track which the knife valve travels through can catch waste and toilet paper. When this happens the valve will still allow liquid to flow past, leading to more waste and paper accumulation which can compound, resulting in a full blockage. And finally, this type of clog can form when you deep clean your black tank, especially if you are going from a chemically-based holding tank treatment to a bacteria- and enzyme- based treatment. When you deep clean the tank before making the switch, it is not uncommon for waste and paper that have built up over time to break free and form a clog at the exit of the black tank. 

Diagnosing this type of clog is done by noticing water and waste backing up into the toilet, assuming the blockage is in the pipe before the black tank, which typically occurs in the larger RVs (often you can see the pipe and 45-degree pipe fittings in the storage area below the bathroom, which can help you in the overall diagnosis of the problem). Or on the other hand, you might notice either a very small trickle of water leaving the sewer discharge pipe or no liquid at all when you go to dump the black tank, indicating a clog at the exit end of the black tank. It is also possible that you will feel the knife valve catching or find it difficult to fully open or close along with the low volume of liquid flowing out of the black tank. 

Compacted Tank

The third and final clog that can occur in the black water holding tank is called a compacted tank. This type of clog forms when an RV is put into longer term storage such as over the winter or because it is being sold. When an RV is stored with waste, paper and water in the holding tank, over time the liquid will evaporate, leaving behind all of the solid waste, which will eventually harden into a brick. This can be a difficult clog to clear but certainly not impossible. You will most likely know that you have this type of clog by the smell. Since the liquid has evaporated, the waste will no longer be blocked by a liquid vapor barrier and the smell will be noticeable nearly as soon as you enter the RV. Also, no liquid will come from the discharge port. And if there are no 45-degree bends in the plumbing between the RV toilet and the black tank, you will most likely be able to see the accumulated waste at the bottom of the tank. 

Clearing a Clog

In order to clear any of these three types of clogs, you will need to use a high-quality bacteria- and enzyme-based clog treatment. We recommend using Clear-It. The idea is to try to surround the clogged area with as much water as possible. This may sound counterintuitive, especially if you’re concerned with overflowing the toilet. But in order for the bacteria and enzymes to survive and replicate, you will need water, and lots of it.

In order to get as much water as possible to the clog, you may need to use a stick of ½ inch PEX pipe to break holes in the waste. This can be useful if you have a pyramid clog that is stacked to the level of where it blocks the pipe from the toilet, or if you have a plumbing clog that is in between the toilet and black tank. PEX can even be used in the compacted tank to start the process of getting water to clog. Once you have gotten water to the clog, try to get as much water into the tank as possible. Since your sensors are most likely not working, you will know the tank is full when you see water backing up the pipe towards the toilet(s).

Add a full bottle of a high-quality bacteria and enzyme clog treatment and allow it to sit for as long as possible. No amount of time is too long for the product to work. The longer you can allow the bacteria and enzymes to break down the waste that makes up the clog, the more likely you are to fully clear the clog. Once enough time has elapsed (at least 12 hours, preferably 48 hours or more), dump the contents of the black tank.

After you have gotten all of the waste out of the tank and have cleared the clog, close the black tank valve and fill the tank completely full with water and dump the tank again. It is important to repeat this process as many times as necessary to  ensure that you have fully cleaned the tank out, including any waste that may have been left behind from the initial dump. There are instances where you will not be able to allow enough time to break through a difficult clog, such as with a compacted tank, which can take multiple cleanings to get fully broken down and cleared out. When this is the case, using a holding tank cleaning service may be your best option. These services are able to use high-pressure water to break apart clogs and clear them quickly. They are expensive but they can certainly speed up the process considerably if time is of the essence.

Clog Prevention

In order to prevent clogs from forming in the future, the steps are simple. First, start every camping trip with 3-5  gallons of water in your black tank along with a dose of a high-quality holding tank treatment. We recommend using a bacteria- and enzyme-based treatment, which we consider to be the gold standard of tank treatments. 

Next, be sure to always leave your black tank valve in the closed position at all times unless you are dumping the tank. We have already explained why this is important.

And we can’t stress this enough–make sure there is plenty of water in the take. Achieve this by, first, always using lots and lots of water when flushing the toilet. When you think you have held the flush pedal down long enough, hold it just a little longer. 10-15 seconds per flush is a good amount of time to aim for. 

Another way to ensure that you are adding enough water to the black tank is to always keep several inches of water in the toilet bowl in between uses of the toilet, which will add several ounces of water every time the toilet is flushed.

It will also have the added benefit of creating a vapor barrier between the black tank and the rest of the RV, helping to keep odors at bay. Be aware that using lots of water will fill the holding tank up relatively quickly. An average family of four using the bathroom will fill the tank to a level requiring dumping every 3 to 5 days. 

And speaking of liquid levels in the tank, to further help in preventing clogs, we recommend dumping the black tank only when the liquid level is around 2/3 full to full according to your on-board tank sensors. This will allow enough water pressure through gravity flow to move the broken-down solid waste and toilet paper out of the black tank. If you find yourself ready to break camp but the tank is not at the 2/3 full level or higher, just hold the flush pedal down on the toilet to add more water to the black tank to reach that level before dumping the tank. This will make rinsing the tank much easier. 

At the end of your camping trip, always dump your holding tank and rinse it several times if possible. We understand that there are sometimes lines at dump stations and this may not be possible every time, but if you were unable to  rinse on one trip be sure to do it after your next camping trip. We also recommend deep cleaning your holding tank periodically. If you are a weekend warrior camper, we recommend deep cleaning your black tank at the start and end of the camping season. If you are a full-time RVer, we recommend deep cleaning your tank once every 3-4 months. This will work as a reset for the black tank and clear away any waste or toilet paper that may have been left behind from normal dumping and rinsing. 

The final thing we can recommend to keep clogs from forming in your black tank involves when you have guests in your RV. Make sure to instruct guests on the best practices for using an RV toilet. It may seem like an awkward conversation to have with them but it may save you money and headaches in the long run. We’ve created some friendly signs that you can hang up as a light-hearted reminder.

The Takeaway

There are three major types of clogs that can occur in the black tank: pyramid plug, plumbing clog and compacted tank. All three of these clogs can be cleared with a high-quality clog treatment that uses bacteria and enzymes to break down the waste and toilet paper. Water and time are the other factors that make these treatments effective. If you do not have the time to wait for the treatment to work, then there are professional holding tank cleaning services that can do the job quickly and efficiently; just be aware  that they are quite expensive. And your best course of action to avoid clogs in the future is to follow the above instructions, which we call the Unique Method.

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