Mystery dirty water in toilet

[grantsoo]

I dumped my tanks this morning before moving to another location about 2 hours away.
I hooked up water to the black tank flush. Opened the black tank dump valve and then walked about 40' to turn on the water. I figured by this time the tank would probably be empty. I let the tank flush for about 10 minutes while I did some other things. Then shut the flush down, closed the black tank valve and opened the grey tank and finished everything off as normal. When we got to the destination we discovered dark colored water with a slight odor in the toilet bowl, some of which had splashed around on the floor.
Since no one had been in the bathroom after I started dumping, the only thing I can imagine is that somehow some of the flush water had worked its way back up into the toilet. I don't know where else this water could have come from.
Does anyone have any ideas?

Thanks
Grant

[dalenoel]

Black Tank while flushing.

[3'senough]

Sounds like backwash. Do you have a clear elbow on the hose to confirm that your tank is draining fully? Could have had a block when you thought it was fully drained and while the added water was more than what was going out your tank was filling higher and eventually got to the bowl. If your tank fills higher than the vent stack air will come up through the bowl and create a whoosing sound if the ball valve is closed. Don't ask how I know this...

Sounds like you avoided a huge cleanup by a few mins.

[grantsoo]

Originally Posted by 3'senough

Sounds like backwash. Do you have a clear elbow on the hose to confirm that your tank is draining fully? Could have had a block when you thought it was fully drained and while the added water was more than what was going out your tank was filling higher and eventually got to the bowl. If your tank fills higher than the vent stack air will come up through the bowl and create a whoosing sound if the ball valve is closed. Don't ask how I know this...

Sounds like you avoided a huge cleanup by a few mins.

Thanks, this had me really puzzled. But I can see with a blockage, even partial, this could happen. Once I read your post I remembered going to the hose and feeling it and thinking, there doesn't sound like much water flowing. (duh)
For it only to fill the toilet bowl, I'm thinking the back pressure probably got sufficient at that point to free the blockage. You are right, I was moments away from disaster. From now on my wife will be monitoring the bathroom while the flush is on and I shall not go off wondering around.
Much appreciated.

Grant

[Ladywendolyn]

From now on my wife will be monitoring the bathroom while the flush is on and I shall not go off wondering around.

Brave wife!

[norty1]

Never let yourself be distracted when the black tank flush is running.

Don't ask me how I know.

[jal0710]

We just bought a FW with the first black tank flush and when trying it, noticed that I didn't seem to get the same flow as what seemed to be going in. Is there a chance this new FW might have a blockage? What kind of flow should a guy be getting? I would life the drain hose up and take the bend out of it and the flow would increase, but not a lot. Do I have a problem that I need to address?