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Why It's Crucial To Keep Your RV Black Water Valve Closed

  • 6 min read

Key Points:

  • Your RV sewer valve (black water valve) should always be closed no matter what type of camping you are doing because you want to keep liquid in the tank to help break down solid waste and create an odor barrier against sewer gases.
  • If you don't keep your black water valve closed, you could experience the following problems:
    • Waste buildup on the tank floor
    • Pyramid plugs
    • Odors
    • Ineffective waste breakdown

Leaving your black water tank valve open could be the mistake that steals your camping joy. It may seem like being on sewer hook-up means that you can leave your black waste valve open and all the waste will flow out as it's added to the tanks, and that would be true if it was all liquid waste completely drains; your tank could have an odd shape, be slightly out of level, or have low points where even urine can accumulate and increase odors. But the biggest reason you probably got your RV is to be able to do your number two business in a real toilet instead of on a log in the wilderness. Whether you consciously choose to use your black tank for liquid waste only or you use it for both liquid and solid, the black water valve should still remain closed. Commit right now to always keep your black valve closed unless dumping a full tank to avoid clog and odor issues. In this article, we will briefly discuss proper use of your black waste valve, what could happen if you do not keep your black tank valve closed, and resources for how to fix those problems if you are in the midst of them now.

Proper Use of the Black Water Waste Valve

Keep it closed unless you’re dumping; it’s that simple. There’s a misconception out there that the rules for the black tank mirror the rules for the gray. We always stress the difference in how you use your gray tank valve when on hook-ups as opposed to when you're boondocking; gray valves stay closed while dry camping/boondocking and open while on sewer hook-up with a p-trap in the discharge line. Some RVers assume the same goes for the black tank valve when in reality, nothing could be further from the truth. The black sewer valve should remain closed unless actively dumping a full tank regardless of which type of camping you are doing. Yes, even campers on sewer hook-up should keep their black water valve closed until they're ready to dump a full tank. 

Note: For a more comprehensive look at proper use of your wastewater valves, refer to The Definitive Guide to Using Your Wastewater Valves.

You might wonder if it's really that important to keep your black tank valve closed all the time even on sewer hook-up. In the sections below, we will go through what could happen if you ignore this very important rule and leave your black tank valve open while on hook-ups.

Waste-Eating Bacteria Don’t Function Well

How does a valve being open affect bacteria breaking down waste? Well, all bacteria (the good and the bad) need water to survive and be as effective as possible. When you leave the black valve open allowing liquid of any kind to immediately drain out, the good bacteria from Unique RV Digest-It Plus and even the stinky bacteria from your gut won’t break down the waste well. They both need water to function at their highest potential. No one wishes for clogs, and by impeding the efficiency of the bacteria, waste buildup can quickly accumulate and therefore pose a much higher risk of developing clogs. Retain the water you add to your black tank by keeping the valve closed, and give the bacteria what they need to to do their work.

Solid Waste Piles Up and Dries Out

Gravity is the naturally occurring pump that moves waste out of your black tank. The best way to ensure all waste makes it out of the tank using this natural pump is to get it as liquefied as possible. One of the things that helps waste become liquefied is a high-quality bacteria and enzyme tank treatment (like Unique RV Digest-It Plus) and the other is plenty of liquid (a mixture of urine and water) to help soften the waste so it can easily flow out when you dump. The problem with keeping your black sewer valve open all the time is that all the treatment product and liquid that helps soften and break apart the waste flows immediately into the sewer because of that pesky thing called gravity. This leaves all the solid waste sitting in a pile solidifying, not liquefying. The best way to avoid dried out poop that refuses to be pushed into the sewer is to keep your valve closed and plenty of water and treatment in the tank until you're ready to dump. This will make your dumping experience much easier and will help you avoid clogs and pyramid plugs, something that we will discuss in the next section.

Pyramid Plugs Can Form

A pyramid plug is exactly what it sounds like: a pyramid of poop reaching from the tank floor to the toilet pipe. Leaving your black valve open all the time just invites a pyramid plug to form because without liquid in the tank, solid waste can do nothing but fall right where the toilet line is positioned and build up on top of itself. It can't flow out and it can't soften without liquid. If you’ve left your black tank valve open for any amount of time and nothing goes down when you flush, you probably have a pyramid plug. This type of clog can be a pain to fix, but if you are dealing with a pyramid plug right now, we have an easy-to-follow guide on Clearing RV Toilet Clogs.

RV Pyramid Plug Diagram | Unique Camping + Marine

Leads to Toilet Odors

Odors are the arch rival of an enjoyable RV trip, and campers the world over are intimidated by the potential for odor intrusion into their RVs. Odor problems are caused by much more than just pooping a lot in your RV; there are many factors that can cause odors, but leaving your black waste valve open while on hook-ups is one of the most likely reasons for odors. As we discussed above, by leaving the black water valve open continually, all the liquid flows out before it can soften the solid waste, and a pile of poop is left behind to release its methane gases. Leaving your black tank valve open can also create an unsafe situation where dangerous gases from the sewer itself can start drifting back into the tank and eventually the living space.

We talk about water a lot when it comes to your black tank. Water is powerful; that is clear every time we see a flood or hurricane, but it also has positive super powers. One of them is that it is a great odor barrier. Think of a porta-potty that has water covering all the solid waste; it definitely doesn't smell as bad as one that has poop piling above the water line. Keep your black tank valve closed in order to control odors. It's also a good idea to add a high-quality bacteria and enzyme tank treatment (like Unique RV Digest-It Plus) to assist in both waste breakdown and odor control.

Guide: Why Using Lots of Water in Your RV Toilet and Holding Tanks is Crucial 

Guide: Eliminating RV Toilet Odors

Prevention is always best, and keeping your black water valve closed until you’re ready to dump is always the best first step. We always recommend following The Unique Method to enjoy all the benefits of problem-free camping, and we encourage you to start using it today!


There are a lot of reasons to always keep your black sewer valve closed; here's a brief review of what we covered:

  • Always keep the black tank valve closed unless actively dumping a full tank as opposed to the gray valve, which can stay open on sewer hook-up and closed while dry camping/boondocking.
  • Leaving your black water valve open can lead to the following problems:
    • Waste-eating bacteria don’t function well
    • Solid waste piles up and dries out in your holding tank
    • A pyramid plug can form
    • Odor issues can develop
  • Follow The Unique Method to avoid many of the issues discussed in this article

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