Wednesday, November 30, 2022


Good or bad idea: using sewer hose as a p-trap?


By Doug Swarts
In the last issue of RV Travel we commented on three sewer hookups at RV parks. No one took issue with photo #1 or photo #3. But photo #2 started a conversation regarding using the sewer hose at the sewer inlet end as a p-trap.

IF you feel adding a water trap in your hose will prevent sewer gases from entering your RV you must be leaving your gray tank or tanks open for it to have any potential effect.

IF you keep your gray tank closed, as most RV waste management experts recommend, and only open when using the clothes washer or dumping tanks, you have NO need for a water trap. Notice I say water trap, NOT p-trap. That’s because most p-traps are designed to equalize water in the pipe so NO air can get through unless the returning air is more than 3 to 4 psi, at which time the gases begin to filter through the water and back up the pipe into the sink.

Without actually measuring the trap in photo #2 it is difficult to say whether the hose in the droop is actually full with no air space at the top of the hose.

Does having a water trap in your hose cause any negative effects on the dumping process? Probably not, other than potential wear on the bottom of the hose and the need to “milk” the hose before storing it.

Standing water in the hose, however, will permeate it with odors that cannot be flushed away and decreasing the life expectancy of the hose. The water trap does little to prevent sewer “critters” from entering your gray tank, which is reason enough to keep the gray valve closed!

For the reasons stated above, I would not add a water trap in my hose, but then again I do not leave my gray tank open unless the washer is in use or I am dumping my tanks. I also feel that the less time water is in the hose the longer it will last and the cleaner it will stay.

Doug Swarts is America’s premier expert on RV waste management. His Waste Master Hose is the state-of-the-art hose for RV waste tank disposal. Visit his website,

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5 years ago

We see it all the time in our travels – both dump valves open all the time. What are they thinking? Of course there are sewer gases and odors that will get by those useless “P'” – anyone who knows much about water knows they won’t work. Then there are the sewer flies, and cockroaches, and we recently heard of a rat getting into someone’s grey tank. Black tank? Solids build up under the toilet in the tank – a mountain of “stuff’ – which then breaks loose and clogs the dump valve so you can’t dump. Probably more reasons but that should be more than enough.

5 years ago

I ‘m the one in the referenced article that brought up the idea of the trap in the hose. This is my fourth trailer in 22 years and it’s the only one I’ve had the issue of the bathroom smelling if the grey is open so I make a trap with the hose and problem solved.I leave the grey tanks open when on full hookups. I’m not worried about my hose building up sediment in the bottom of the hose. I always rinse my hoses before storing them. I have a built in tank flusher as well as the clear elbow that you can flush the hose and tanks with. After I’m all done dumping, I close all valves and let the fresh water run into the hose from the clear elbow. Also, I use hoses that have storage caps on the ends of each section and store the hoses in a water proof container in one of my compartments. I travel with my wife and three daughters, so when hooked up, I don’t want to be constantly opening and closing the grey valve.

5 years ago

You would think that Mother Nature created sewer inlets as no 2 seem to be the same. Keeping your tank valves closed at all times, other than to dump as Richard and Marvin do, is the key to minimal waste transfer issues.

Dr4Film ----- Richard
5 years ago

Frankly, I have been to so many RV Parks from Key West Florida to Fairbanks Alaska that most of them have no clue as to the proper way to build a RV dump port. In most of these places once the 90 degree connection and hose are installed I always have to “walk the hose” back to the dump end to drain the hose completely. So by the physics of the RV Parks setup there is always a “trap” in the hose. However, in the 14+ years of full-time RVing and now part-time RVing I have never left my valves open for any reason, AND always dump when they are full or close to full. How many RV parks have you been in where the sewer connection is below ground level surrounded by concrete to form a natural slope/drain into the sewer? Most of the time they are well above ground level or even higher than the RV site itself.

Marvin Thomasson
5 years ago

I have long since realized how much less gunk accumulates in the hose when you only open when your tanks need emptying. We’re fulltimers and often sit for months in one spot, but never have problems with accumulations in our tanks because we only dump full and rinse with several bowls of water each time. when we move, borax goes in with extra water and we dump as soon as we stop. after 4 years, tanks still clean with no permanent gunk.

5 years ago

Spot on Marvin. The process of transferring waste from your RV to an approved sewer inlet is very simple, only people make it complex by changing the simple procedure. Keep tank valves closed except when dumping, put water and some additive (endless choices here) in the Black tank before traveling and your issues with this aspect of RVing will be minimized.

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