Sump pumps are plumbing devices used to remove excess groundwater from the basement to outside the home. Without sump pumps, basements could be susceptible to potential flooding, which would cause major water damage to a home's furnishings and structural design. Most quality sump pumps come equipped with a backflow preventer valve to ensure that it pumps water in one direction. However, if the valve is missing or becomes clogged, the water will return to the sump pump pit, creating a breeding ground for odors.

Mildew

When sump pumps are not able to filter groundwater or rainwater effectively, the pit can accumulate mold, or simple small plants that form black or white growths on damp surfaces. Mildew causes the musty odor that comes from the pit.

Sulfur

The sump pump pit may emit a noxious sulfur or rotten egg smell. In addition to the accumulation of mold and mildew, the pit of the sump pump may have bacteria, dirt and grime that have passed through the pump's valve. Over time, a mixture of all these elements create a thick layer of slime--or sludge, which starts to collect not only inside the pit, but also inside the valve pipe.

Sewage

Odors that reek of must and old water are most likely caused by a combination of bacteria growths, mildew-water saturated debris and even detergents or cleaners that have mixed in from household plumbing fixtures. According to the Evansville Courier and Press, bleach can kill the bacterium that causes the odors when it is poured down plumbing pipes, but this is only for a short time. Eventually, the sewage odor resurfaces.

Solution

Clean out the sump pump pit, which holds accumulated dirt, grime and bacteria. First, disconnect the sump pump from its power source or discharge pipe to prevent electrical shock, and remove the pump from the pit. Use a water hose to wash away sludge and slime—wear protective clothes and gloves if you need to dig or scrape sludge from the pit. With a bucket and a wet-dry vacuum, remove standing water. The sump pit should be cleaned every six to 12 months, depending on the amount of groundwater it pumps.

Prevention

Installing an airtight lid over the sump pit can decrease the amount of noxious smells emitted by the sump pump. In addition, installing a vent pipe allows built-up gases to be released outside the home, which allows fresh air to circulate inside the pump and the pit.