Well in theory if the water level in the sump is above the grade of the drain you could install a 90 facing up on the outside of the bulkheads and then a 90 at your desired water level and would act as a trap. It would be a similar setup to the way I have my drain setup on my sump. On the inside I would use a t at the desired water level and add a small pice of pvc to act as vent on top.
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Not the best photo but you can see what I’m talking about. I can take a better one later if you would like and post it up.