If the odor is emanating from only one sink, then the problem is not with the water supply. It is most likely a small buildup of hair, lint, and dirt caught in between the spillover and the sink drain. The buildup attracts bacteria that carry a bad odor. To correct the problem, you must block the drain below the point where the spillover meets the drain, about 5 cm (2 in.) down the drain. Once it is blocked with a rag or small towel, fill the rest of the drain with a 50/50 mixture of water and bleach. As you fill the drain, it will enter the spillover. Leave it there for an hour and the bacteria will be killed and the odor will disappear. If the odor is coming from all hot water faucets in the house, then the problem likely lies with sediment buildup in the hot water tank. If drained every year, the hot water tank should not create any foul smelling water.