When a sump pump smells the first thing that we need to do is determine whether it is a sump pump or sewage ejector pump that we are talking about. Many people do not know there is a difference between the two types of pumps yet the treatment for odor would be very different on the sump pump and sewage ejector pumps.
Sump pumps are used to pump relatively clean water such as storm water, or ground water that enters the sump pump pit. The pit may have a cover or it may be left uncovered. In most cases a sump pump will not smell or, may only have a somewhat musty smell. Sump pumps may discharge outside on the ground, into a storm sewer in most cases.
Sewage ejector pits are used to pump dirty water such as wastewater from sinks, tubs, showers, toilets and laundry lines that are located too low in a home to drain by a gravity drain up to a sewer line. The sewage ejector pits although they handle smelly waster still should not smell when properly installed and maintained. The pit should have a cover that is tightly sealed to the pit, with all pipes and wires coming up through the cover sealed to prevent sewer gases from leaking out of the pit and into your home. The sewage ejector pit should be vented through the roof and the waste from a sewage ejector only goes only to a sanitary sewer.
By now you have probably discovered that what you were calling a sump pump is indeed a sewage ejector pump. The things you want to look at when confronting an odor problem on a sewage ejector pit is the sealing of the cover and all the pipes and wires coming through the cover. The integrity of the cover and pit should also be examined. Sometimes older pits and covers were constructed out of steel, which has corroded and no longer is airtight. Other times gaskets are missing or, not properly installed. A vent through the roof is essential because the sewage ejector pit is so tightly sealed the air needs to be able to escape when waste enters the pit. A vent is required which goes out through the roof to allow the free flow of air in and out of the pit. Air admittance valves also known as Studor Vents or, cheater vents will not work in this application because they only allow the flow of air to go into the pit but not escape. The improperly vented ejector pit almost always smells as it is filled the pit pressurizes and air under pressure forces its way past seals that would not normally leak. Every thing on a sewage ejector pit needs to function properly to prevent odors from sewer gases from entering your home. If you have sewer gases from an ejector pit entering your home I would give a high priority to getting this problem addressed out of concern for the health of your family.
If you have a sump pump in many cases they are odor free with a good circulation of ground water and or, storm water that stays fresh via the circulation. In some cases during low water conditions the water in the pit may become stagnant. Sometimes it is as simple as running a hose into the pit for a little while to allow the pump to run a few cycles while other times you may want to put a loose cover on the pit. In most cases this is adequate for odor control. If you have water entering the pit that smells you want to examine if you have leakage in your sewer lines or, septic system if you have one which is allowing sewage water to enter the ground water reaching your sump pump pit. One of the ways that may check for sewage water to be entering a sump pump pit is to use dye to test the sewer lines. If the dye shows up in the pit you have leakage but it may not always show. Another way is to watch for water to suddenly start entering the pit when you drain water. If you have sewage leaking into the ground the leaking sewers should be addressed for the health and safety of your family and others in your neighborhood. In the worst case scenario you have smelly ground water entering the pit in which case you may just go to a sealed lid on the sump pump pit with venting to the outside.