where did black water come from?

[CHICKDOE]

we moved from one rv park to another today. when we left I drained the black and gray tanks. I flushed the black tank using the flush hose connection. I turned off the flush water and let the black tank drain until there was no water coming out of the tank (watched it via a clear plastic adapter). the trailer was level when I did this. when the water stopped flowing out I disconnected the sewer hose and put the cap on the sewer hose output connection. apparently I forgot to close the black tank drain valve. when I got to the new rv park I went to hook up the sewer hose and I took off the cap on the connection. a surge of black water came out of the drain before I could get the sewer hose connected. that is when I realized is forgot to close the black tank drain earlier in the day. also, I had already leveled the trailer at the new site when this happened. my question is, where di this water come from? I had left the drain open earlier in the day until there was nothing coming out. does this mean that even if you drain the tank there is still some water left in it? does the waste drain not drain the tank completely?

[rockfordroo]

Here are my guesses:

1. Level is relative. Level on the frame may not be level in the tank, and vice-versa. So while you thought you were level, you weren't.

2. Even though you flushed, you still could have had a small "dam" of poo, etc. holding back some water. After sloshing around on your way to the next CG, it entered the drain line, and came out when you removed the cap.

3. All of the above.

[French]

X2 on that dam poo!