According to Stormin Mormon :
I'll likely be flamed for t his, but what occured dto me is to put an
inverted P-trap. Something like:

!--------!
! !
SUMP ! !__________________discharge

So that the outlet pipe goes UP a wasys before going over. Don't know if
that will help. But it makes sense. The other thought, is that if you can
run a pipe way up (fifteen feet or so) you can put a vertical vent. Half
inch or so. That way, you will not get siphon effect drawing the water back
to the sump pump.

OK, flame me now. Your turn.

Flame thrower set ......

The inverted trap-like section of pipe will NOT prevent siphoning.

There, satisfied? ;-0

If you put a vent (or anti-siphon) valve on the pipe at the high
point, it will prevent siphoning.

No need to go up 15 feet per-se, just go up enough to be higher than
anywhere else on the line by a few inches.

It boils down to something like this:

- If the outlet is lower than the pit, you really don't need a check valve
or anti-siphon. The water will simply flow out by itself.

- If the outlet is a bit higher than the pit, but lower than your head-room,
you can use an anti-siphon valve. You can still use a checkvalve if you
wish, but this would be bad if the pit is subject to freezing.

- If the outlet is much higher than the pit, you need a check valve, and
an anti-siphon won't help. If freezing is possible, you have a considerably
harder thing to solve.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.