I have a laundry tub in my basement and recently I have noticed that there is some black water coming up through the drain. The drain is hooked up to a pump that pumps the water from the tub up into the septic system. Water from out AC unit and humidifier are what drains into the tub and the previous owners had their washing machine draining into the tub. The line going from the sink which has a pump attached also has a check valve. We are unsure the age of the pump. Could this be water from the septic system or is it old water from the laundry tub's drain line backing up into the sink? Nowhere else in the house has black water backing up. I hope someone can help.

Thanks!

Sounds like either the check valve on the pump, or the pump float switch is failing, or both.