understanding rv black waterUnderstanding RV black water is not as difficult as the RV beginner would think. Everyone that has an RV will have some type of “poop story” or had an incident involving the “stinky slinky.”

What is Black Water?

Black Water is quite simple the water within the holding tank which comes from the toilet. Black Water will contain fecal material, toilet paper and anything flushed down the toilet. Black Water is relatively hazardous to your health. I should be treated with respect when touching anything involving the Back Water system.

Black Water Basics

The black water basics start with understanding the process within the tank, all the poop should we say. I think the best place to start is by looking at a residential septic system, particularly before 1992. That is when most residential septic tanks added baffles. Baffles were added to help separate the grey water and keep “sludge” for settling.near the outlets. These outlets were to distribute grey water into the leach fields or drain fields.

Before 1992 they did not have baffles. This would result in the possibility of sludge getting into the fields and creating a blockage or odor in the fields. This was caused by the incoming water mixing with the sludge that had already settled within the tank.

In the RV the obvious happens. Sludge settles while we are at a location. Then we decide to move or relocate. This is when the sludge gets mixed up. The big difference you will find is that a residential system has the vent at the top of the plumbing system. Where the RV will have a vent coming from the black water tank and another from the grey water tank. Another big difference is that the two waters are separate in the RV.

Hopeful now you will understand why it is important to drain your tank once you have the opportunity. We will learn more when we talk about Maintaining Black Water Systems.

The RV Black Water System

understanding the rv black water systemNow that we have explained a little about the black water system, let’s break in down into individual components. We will start where it all begins, at the toilet. The Toilet is the only appliance within the RV attached to the black water tank. It is similar to the residential toilet in the fact it restricts some odors from coming from the holding tank. That means some odors will come back from the tank. Unlike the residential toilet were odors are restricted by water in the trap of the toilet. The toilet should be located at the highest point of the holding tank.

Once you push down on the flush level. The waste is passed into the black water plumbing line. Unlike the residential system, these lines are completely separate from the sink, tub, and laundry. The waste then transferred directly into the black water holding tank. It’s really that simple.

The holding tank has one inlet from the toilet and two discharges. One will run vertically up from the top the tank to the roof line of the RV. This vent is like the vent in a residential system. It allows for odors to vent into the atmosphere. The vent has a cap on the top to prevent rainwater or rodent from entering the tank.

The second discharge is from the low point of the tank. Black and grey water tank are designed with a high point and a low point. At the low, this discharge will lead to a pipe with a dump valve. This dump valve should only be opened if you are dumping the sludge of the tank.

It really is that simple of a system. Keeping in mind it is a gravity system. This means that in order for it to dump properly and remove the sludge, it should be as level as possible.

Maintaining Black Water System

The maintaining of the black water system is not rocket science. As we just covered the system is really basic. One of the biggest misconceptions is that the system takes a lot of work. Follow a few simple rules and the system will be odor free.

  1. Keep at least one gallon of water in the tank. After you have dumped the holding tank. Allow for at least two gallons of fresh water back into the tank before using. This allows for sludge to not settle and stick to the inside of the tank. Also, if while traveling it allows for the fresh water to move around in the tank to clear sensor along with the inside of the tank.
  2. Drop in a toilet treatment tablets or drop-ins that are formaldehyde free every time you dump your tank. Toilet treatment drop-ins help reduce or eliminate odors along with helping break down sludge and toilet tissue,
  3. Use toilet tissues designed for RV or marine use. Residential Toilet tissue does not break down as well. This can cause a build-up of tissue within the tank. The build-up will eventually reduce the tank capacity and plug up sensors as well as discharge lines.
  4. After every few dumps, when you start to get some odor, after every trip. Add a gallon of RV or marina holding tank treatment. It may be a good idea to add this before you travel home. This will work with the water added in #1 to clean the inside sensors and the inside of the tank. The use of a toilet tank treatment will reduce the likelihood of odors if the RV will be sitting for an extended time.
  5. Finally, after the RV holding tank has been dumped off the sludge. Use the fresh water from the dump station to backwash the dump lines and hose down around all the exterior components.


Understanding the RV Black

Understanding the Black water system

Water System is not as difficult as one would think. It is a pretty simple system of the toil, a few plumbing lines, a holding tank, and a dump valve. If you maintain the system it will make the use much cleaner. This system can be maintained with little effort and about a half hour of time. Keeping a clean system will reduce odors for you and your neighbors.

Hope you learned a few things today about the RV Black Water System.

Please feel free to share and comment below.

Kenneth Sagendorph


  1. Todd P Matthews


    Definitely learned a lot here. It seems like the black water process sounds the hardest, but once one reads through step by step it’s really not at all as hard or as scary as it would sound. If I decide to live the RV life, I won’t be overwhelmed when it comes to emptying this tank.

    • Reply

      Great to hear from you Todd. Agreed the black water is not my favorite job in the cleanup process. But someone has to do it. You just cannot be afraid to it and dive right in. Once you have done it a few time it gets easier.

Leave a comment

Your email address will not be published. Required fields are marked *