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I believe the building code has to have the drain connected to your house sewer system.
I’m guessing that the pit is designed to catch any chemical spills that may leak, with the drain pipe elevated.
If you have a back flow preventer, you can run fresh water through that pipe and watch it drain out from your residence.
An illegal trap would not have that pipe, it would “French drain” under the garage.
Once clear, you can use household bleach at about 10% solution to clear out the bacteria creating the odours. The remaining water in the pit can be removed and dumped in your toilet.
Don’t flush any residual oils, fuels into the sewer.
They can be dropped off a city depots who handle that kind of waste.
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