I hope this helps - The problem is when the ATO reservoir's water level is higher than the pipe that delivers the water (as shown in the first part of the image) this creates a siphon - not a reverse siphon like normal, but a siphon in the same direction as the flow.
The fix is the raise the pipe so that it is above the ATO reservoir level. I use the PVC
(as shown in the second part of the image) in order to reduce splash. The water just runs down the side of the PVC
and mixes at the bottom of the sump.
I hope this is helpful.