When you backwash your pool filter where does the water go?

This could be the tie in to the sewer. Check next time you backwash.

I doubt it is goes to the sewer line. I don't believe you can have an open air sewer line connection. I got a notice from the city years ago because a clean out wasn't properly capped (it was open air). And per the op, this doesn't go down to the depth of the sewer line.

This. I'm currently in the process of getting a pool installed and they ran the overflow/backwash clear around to the other side of the house exactly like that. It's the same for my neighbor's pool as well. Some municipalities require that you run overflow/backwash to the sewer because they dont want chlorinated or salt water in the storm drains.

Looks exactly like a pool backwash drain. Same look/set up i had on my old house. It did feed into the sewer but it had its own backflow preventer of some sort underground.

ItsA&InotA&M said:

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I doubt it is goes to the sewer line. I don't believe you can have an open air sewer line connection. I got a notice from the city years ago because a clean out wasn't properly capped (it was open air). And per the op, this doesn't go down to the depth of the sewer line.

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If it has standing water below grade, it probably has a trap on it.

It's an air gap. It's the cheap version of a backflow preventer. Since your pool can be lower than the sewer, you don't want the sewer backing up into your pool if your neighbor (or you) pours too much grease down the drain. Backups will surcharge out of the air gap instead of filling your pool with shi+.