I have a septic system. There is a pump for laundry gray water in the basement. When I do laundry there is a strong septic smell in a first floor bathroom. I assume the basement pump is so strong it is somehow drawing the water out of the shower trap. What can I do?

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I assume the basement pump is so strong it is somehow drawing the water out of the shower trap.

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I have a different take on this.
I think the pump either the laundry discharge hits a partial clog or simply is too much volume for your pipes to handle. This will set up a back pressure situation that pushes a bubble of sewer gas backthrough the shower trap and that's what you smell. If you think it might be a clog then snake the drain line for pipes that can't handle the volume either increase the size of the pipes or see if you can cut down on the pumps output. Good luck, Tom