More often than not, the sewage smell is caused by a faulty U-shaped pipe underneath the sink, called a P-trap. At some point, you’ve probably identified that that’s where the smell is originating from. After wastewater travels from its starting point (whether it be from the toilet, shower/bath or the sink) to the P-trap, if it’s working as it should, a slight amount of water should stay in there to prevent the gas smell from reaching the air. If the water isn’t there to block the scent from traveling, though, you get the sewage smell , drifting up and into your home.