Best way to help reduce black tank odors.

[Ned]

Most of the holding tank drains I've seen would make it impossible for the gray water to back up into the black water holding tank regardless of the valve settings.  The black tank on our coach drains straight down through the 3" pipe while the gray tank is teed into the 3" drain.  It is physically impossible for the gray water to back up into the black water tank.  That would require a 3rd valve be added to the 3" drain below the tee fitting, and even then I doubt more than a few gallons would actually get into the black tank.  A tank flush is much more effective.

 

[Rich D.]

Ned,

You must take into consideration the length of the sewer hose, the slope of the hose toward the sewer and the height of the sewer connection pipe. Trust me, this works in most every situation. The pressure to push the waste water down the hose is greater than what is required to send some back into the black tank. I've been doing this for years (20) with several different motorhomes ant it has always worked! Try it next time out if you don't believe me.

You only want a few gallons to backflush - about the same amount as shown in the video.

 

[Ned]

I can tell you it won't work with our drains.  The gray tank drain is only 1.5" while the sewer hose is 3" so there is no back pressure whatsoever when the gray tank is drained.  It all goes straight down.  Even if some would back flow into the black tank, it won't be nearly as effective for cleaning as a typical flush spray.

 

[Bob Buchanan]

My gray into the common 3" is only 1.5", Ned. That's why I would take the extra step of unhooking the 3" hose and recapping the outlet prior to the backflush. Then the gray will seek it's own level with no lose of head pressure. Leaving the main hose open "would" cut down the pressure a lot with only 1.5" coming from the gray. As you know, regardless of the size of the two tank hoses, if left on long enough, both tanks will wind up at the same level.

Of course, recapping will take a bit longer so needn't be done with every dump. Sometimes we have more time to dump than others. 

As mentioned previously here, my backflush is with a macerator connected so the recapping is not necessary.

 

[Ned]

And how do you remove the cap after the drain pipe is full of liquid without making a mess?

 

[Wigpro]

And how do you remove the cap after the drain pipe is full of liquid without making a mess?

You can't....trying to use the grey water to clean or flush the tank would be fruitless on most RV's, maybe this is a unique instance posted here, but my black water tank is above my grey water inlet into the dump outlet and I find it hard to believe that it would back up into the black tank, no matter how full it is.

I dump black first and use the grey water to "rinse" the dump hose!

I have no tank smell and use plenty of water in my black tanks so it has enough fluid to break down the waste and enough pressure to dump clean. Water is the key to no smell. A shot od dawn dishwashing liquid in both tanks helps keep them clean as well. I have found the grey water with food waste and grease can get more smelly that the black tank from time to time....the soap helps cut that.

 

[Ned]

You can't....trying to use the grey water to clean or flush the tank would be fruitless on most RV's, maybe this is a unique instance posted here, but my black water tank is above my grey water inlet into the dump outlet and I find it hard to believe that it would back up into the black tank, no matter how full it is.

Thank you, that's precisely my point.

 

[Bob Buchanan]

And how do you remove the cap after the drain pipe is full of liquid without making a mess?

Oops, guess  you have to buy a small bucket, Ned.  Or, use a smaller hose with a valve as part of the hose. My macerator 1" has such a hose valve, tho not needed for backflushing. Sorry, I use a macerator and don't have that problem. There is no cap involved.

Unless the sewer hose stays connected as posted by Rich D., one would have a bit of a blackened gray water mess to deal with.

 

[Bob Buchanan]

You can't....trying to use the grey water to clean or flush the tank would be fruitless on most RV's, maybe this is a unique instance posted here, but my black water tank is above my grey water inlet into the dump outlet and I find it hard to believe that it would back up into the black tank, no matter how full it is.

I'm on my 5th RV - and it would work on all of them. It's not necessarily the position of the black tank to the outlet - but rather the position of the high water line in the gray tank relative to the bottom of the black tank. If the bottom of the black tank is below the high water line of the gray - it would seem to me that it will backflush until the two water levels are equal.

 

[Rich D.]

I'm on my 5th RV - and it would work on all of them. It's not necessarily the position of the black tank to the outlet - but rather the position of the high water line in the gray tank relative to the bottom of the black tank. If the bottom of the black tank is below the high water line of the gray - it would seem to me that it will backflush until the two water levels are equal.

Exactly. Thank you.
You don't have to wait until they equalize...  just long enough to put a few gallons of gray into the black because any heavy residue is on the bottom of the black tank.