Ask Shell's Tip Of The Day - Kitchen Drain Pipe MaintenanceJuly 4th 2018
The foul smells coming from sink drains are caused by soaps, shampoos and food going down the drain and clinging to the side of the drain pipe between the drain hole and the trap.
Eventually, this residue builds up, leaving a small hole for water to drain away. This will be noticeable as the sink will take longer to drain.
These odours cannot be masked by using deodorizers or hot water.
To correct the problem, remove the centre drop drain tail piece from the trap adapter. In kitchen sinks, it is normally brass or ABS plastic and in the bathroom probably chrome.
Using sudsy ammonia and a bottlewashing brush, scrub the residual from the inside and outside of the tail piece.
Replace in the drain and you will notice that the water will drain more freely and the odour will be gone.