I am trying to drain a dehumidifier in my basement. It is stubbed (sort of) for a bathroom. There is the main drain (3 1/2" Pipe) that toilet waste as well as sink waste drains into embedded in the concrete floor, and I assume drains into the septic tank. There is another pipe (PVC, 1 1/2" ) that runs around the wall of the "bathroom" and also is embedded in the concrete about 4 inches from the main drain. The only thing from upstairs that drains into this pipe is the washing machine and the kitchen sink. I would assume that this pipe drains into the septic system also. If I drain the dehumidifier into this pipe, will I need a P-trap or can I simply use a T-connector and insert the drain tube from the dehumidifier into the T?

If I've left something out, please let me know.

Thanks.

Don't use a Tee because without a trap you'll get sewer gas. You could rig a p trap but do you have a sump pump? You could drain to that.