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I have a semi-outdoor drain that looks like this (I have removed the grate):

picture of drain

The container of the drain is plastic, and there is a good 6 inches from the bottom of the drain to the pipe where the water leaves the drain.

This water never evaporates, and it has eventually turned into compost. This is bad right? I try to empty it every so often.

I would like to fill the bottom 6 inches of the drain with concrete so that the amount of standing water is minimized.

Is concrete the right material to use? It feels wrong filling a plastic container with concrete. Was there some good reason this drain was designed this way? If I don't use concrete, what material should I use?

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Sumps typically have standing water, as Jasen mentions, part of the reason of having them is a sediment trap to prevent debris from going into the storm drain. All that "compost" didn't end up in the storm drain. You also likely don't want that compost entering your pipe and possibly getting a clog. A deeper sump pit means you have to clean it out less often.

Is there something specific you worry about?

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    If the bottom of the drain is solid plastic, drill it full of holes so the water will eventually seep out. In a "traditional clay tile drain sump" the bottom would be completely open. The space between the bottom and the pipe keeps the pipe from filling with silt, if you clean that space out regularly. If you fill it with concrete or let it fill with silt, you will eventually plug the pipe, which is a lot harder to clean out. Voice of Experience, having one inherited a drain sump nobody had cleaned out for years.
    – Ecnerwal
    Commented Jan 26, 2020 at 15:13
  • I didn't mention this in my question (I think I will post another one), but I plan to cover the drain with artificial grass. So I will probably have very little sediment come through after this. But your answer makes perfect sense.
    – Nacht
    Commented Jan 27, 2020 at 0:35
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Last time I checked the installation instructions on a similar drain pit it said to fill the bottom with concrete.

standing water, depending on the climate, can harbour mosquitoes, filling the sump prevents this.

conversely the sump can trap debris preventing pollution of waterways so if there's a lot of crud that gets washed in there it might be a good idea to fit a screen to trap that stuff before you fill the sump.

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