theory only- The stuff in the black tank has reached a point where in order for new stuff to slide into the tank air needs to be displaced and since the toilet chute is the closest to where the stuff is depositing the burp occurs. Prior to being near full the air will release through the black tank vent so no burp up the toilet chute occurs.

I was waiting for Linda "The Toilet Whisperer" to reply, but can't hold it (pun intended). You guys figured it out. The toilet vents at the roof. The connector pipe is several inches below the bottom of the bowl. Once the vent connector is obscured, the only place for air to escape is the ball valve in the bowl. Howard and Linda explained the "burp" to us at a rally. When you hear that, you need to make dumping the black tank a top priority. On a related note, if you are back flushing your black tank and leave the dump valve closed, the tank will fill all the way up the vent pipe and you will have a small fountain coming out the top of your RV. Not as dramatic as Robin Williams portrayed in the movie "RV", but enough to get you some laughs from fellow RV'ers.

To take this conversation to another level, what do you place in your black and/or gray tanks to help keep it broke down so that you empty your entire tank completely?
You read stuff on line and half is advertising a product, but what really works. I would say no chemicals from what I have read but what about products like Tech Tanks Rx or other types of organisms that break down the solids.
Give me the short version of the TP education. We are going to the Spring Rally in 2017, but unexpectedly are starting FT next week. That is what happens when you sell everything way before you thought and purchase an RV sooner than expected.

Thanks,

Yes, as mentioned, the holding tanks are vented with a pipe going to the roof of the RV.  This diagram is a good example.

 

Gases tend to build up in the holding tanks, and these vent pipes allow the gases to release.  However, when the liquid in the black tank gets high enough to block off the vent pipe, the gases then have nowhere to go except between the tank and the pipe leading up to the toilet.  When you flush the toilet, the gases release through the bowl causing the "burp".

As for the other issue, I just wanted to clarify that Bill's comment related only to the backflushing and not the "burp".

"On a related note, if you are back flushing your black tank and leave the dump valve closed, the tank will fill all the way up the vent pipe and you will have a small fountain coming out the top of your RV. Not as dramatic as Robin Williams portrayed in the movie "RV", but enough to get you some laughs from fellow RV'ers."

---

Bill's quote:  "This mostly happens when someone hooks up a hose to their black tank flush and forgets they did so or gets distracted."  

Though this next part should really be in a different thread, it's partially related.

"To take this conversation to another level, what do you place in your black and/or gray tanks to help keep it broke down so that you empty your entire tank completely?
You read stuff on line and half is advertising a product, but what really works. I would say no chemicals from what I have read but what about products like Tech Tanks Rx or other types of organisms that break down the solids.
Give me the short version of the TP education. We are going to the Spring Rally in 2017, but unexpectedly are starting FT next week. That is what happens when you sell everything way before you thought and purchase an RV sooner than expected."

As full-timers, we use nothing in our tanks as far as products go.  You'll want to use plenty of water in your black tank, but always keep the black tank closed until it is nearly full.  Never leave the black tank open while hooked up to a sewer connection - liquids drain out and solids build up and harden.  As long as you are using plenty of water, the solids will break down naturally.  If you still want to use a product, we recommend either "Happy Camper" or "Digest-It".

As for toilet paper, use something that is septic safe and passes the "swish" test.  Take a few squares, put them in a container of water and swish them around.  If they dissolve completely, you are good to go.  If you are seeing toilet paper in your pipes when dumping, it's not the right kind - it should always dissolve.  Not a bad idea to put a few gallons of water in the black tank after dumping to "charge" it and let it swish around in the tank while moving.

That's the short version.  You'll learn more at the Rally.  :)

To break down the "stuff" I have always used Thetford Aqua-Kem powder. The little bugs (enzymes) in it do a good job of breaking everything up.

It seems to last through several full tanks. I'm guessing the bugs are able to hold on to the side of the tank while the "stuff" is draining out....tee heee or should I say Tee Peee

One note on products: So while some people really think Happy Camper is a good product (and it was heavily recommended at the Oregon rally a few years ago), we were told by people who run a business cleaning tanks that it's one of the worst products you can use. It actually causes calcification and you will end up with crystals in your tank.

Ask us how we know? We had the tanks professionally cleaned and it took twice as long as usual and they showed up the pile of calcified crystals they removed during the process. Since then we can extend our boondocking time without the need to empty the tanks as frequently. No more Happy Camper for us.

First ... thanks for all the feedback and discussion ... awesome. We have been using a product made by Walex called "Bio Pack" ... it's supposed to aid in breaking down the TP and Bio stuff, and to be honest, I have been pretty happy with it. However, when long termers like the Payne's and the O'Keeffe's say "just use a lot of water" and that they don't use any "additives" ... I think I'll change my plan. Their collective 25 plus years of experience trumps my 4 months.