Very few odors are as unpleasant and embarrassing to a homeowner as the smell of sewage. While they are disruptive, sewage odors from bathroom sinks are relatively common. A variety of issues can cause the smell of sewage to emanate from a bathroom sink. Some causes are minor -- you can deal with them in seconds -- while others may indicate serious and potentially costly problems.
Function of Traps
The most common cause of sewage odors in sink drains comes from a lack of water in the traps. All sinks have a U- or P-shaped dip in them called a trap. Besides preventing items from entering a home's in-wall pipes and causing blockages, sink traps act to keep gasses and vermin from coming up through the sink. Traps keep gases in the pipes by maintaining a constant barrier of water in the bottom of the curve, which acts as to seal out the sewage smell.
When you don't use a sink regularly, water that sits in the trap begins to evaporate. This leaves an air gap in the top of the trap that sewer gases can flow through, resulting in a sewer smell emanating from the drain. Evaporation in the trap most often occurs in secondary and guest bathroom sinks, but using all bathroom sinks one or twice a week will prevent evaporation and leakage of sewer gas.
Another source of foul odor in a bathroom sink comes from a layer of slime that can consist of dirt, mildew, hair, skin particles and mold that can grow in the section of pipe that connects to the trap or the inside overflow drain of the sink bowl. Regular cleaning of this pipe can keep foul odors at bay. For homes connected to a sewage system, pouring an even mixture of bleach and water down the pipe periodically will loosen and remove this slime. Homeowners with homes connected to septic systems should not use bleach, since it can damage the delicate pH of the system, but should use a small bottle brush instead.
Every home has a vent pipe to help vent gases from the home. Issues with this pipe can cause sewage odor issues. This pipe can get clogged with leaves or debris, or may become home to a bird or squirrel nest, causing a backup of sewer gases that will leak out through the drains. While this may affect a bathroom sink, it will also affect other sinks throughout the house once the blockage is bad enough. Placing a piece of screen or a vent cap over the pipe will usually avoid blockages. Even in pipes that don't have blockages, wind blowing across the pipe can create downward pressure, leading to leakage of sewage gas into the home. This is a temporary problem that ends when the wind shifts, although vent caps can reduce its occurrence.
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