and if you have a durso but have the outlet under water without a reverse durso you may still get the flushing effect
So even with a durso on top be sure to NOT submerge the outlet pipes in the sump or else the effect of water/air will create the flushing.
You can submerge it if you build a reverse durso for the outlet and prevent spray that way
(do a search in the DIY -I just posted a video on the reverse durso)