Quick Answer
Sump pump odors usually come from one of three causes: a dried-out basin releasing trapped sewer gases (sulfur or rotten egg smell), stagnant water breeding mold and bacteria (musty or mildew smell), or in more serious cases, a cracked sewer line or failed ejector pump (sewage smell). The first two are typically fixable yourself with water or a bleach cleaning. A sewage odor always warrants a call to a licensed plumber.
Last Revised: 5/8/2026
You walk into your basement and the smell stops you in your tracks. Maybe it’s sulfur, like rotten eggs, or it could be that damp, musty odor that clings to old laundry rooms. Or, worst of all, it smells unmistakably like sewage. Whatever it is, it’s coming from the sump pump pit, and you want to know why.
The good news: most sump pump odors are fixable, and many you can handle yourself. The better news: knowing which smell you’re dealing with tells you almost everything about what’s wrong and what to do next.
At Mattioni, we’ve been helping homeowners across Chester, Delaware, and Montgomery counties with plumbing issues for over 75 years. Here’s what we want every homeowner to know about sump pump smells.
What Does a Sump Pump Do? And Why Would It Smell?
If you have a basement in southeastern PA, you almost certainly have a sump pump. It sits in a pit at the lowest point of your basement floor and collects groundwater before it can seep into your living space. When the water level in the pit rises high enough, a float switch triggers the pump to kick on and push that water up and out through a discharge line, usually to a storm drain or away from your foundation.
The pit itself is a dark, damp environment, and it’s connected to the ground surrounding your home. That combination (moisture, limited airflow, and proximity to soil gases and drainage) is why odors happen. They’re common. They’re often not serious. But they do tell you something worth paying attention to.
The 3 Types of Sump Pump Odors (and What They Mean)
1. Sulfur or Rotten Egg Smell
This often seems alarming, but it’s often the most straightforward to fix. That sulfur or rotten egg smell comes from hydrogen sulfide gas, a naturally occurring gas found in groundwater and soil. Under normal conditions, standing water in the sump pit traps those gases and prevents them from escaping into the air. But if the pit dries out, there’s no water barrier, and the gas releases directly into your basement.
The first thing to try: pour a few gallons of water into the basin to restore that water seal. If the smell clears up within an hour, it’s likely nothing serious and was caused only by a stretch of dry weather.
If the sulfur odor is persistent, very strong, or doesn’t improve after adding water, it could point to something more serious, like a nearby sewer line crack that’s allowing sewer gases to migrate into the pit. That’s a job for a plumber.
2. Musty or Mildew Smell
A musty, wet, or mildew-like odor is extremely common around sump pits, and it’s almost always the result of stagnant water and organic debris. Leaves, dirt, and sediment accumulate in the basin over time, and when they sit in standing water in a dark, poorly ventilated pace, mold and bacteria grow. The smell is the byproduct.
This isn’t extremely dangerous, but it needs to be addressed – both for air quality and because mold growth in a basement can spread to walls and framing if conditions stay damp long enough. A thorough cleaning of the basin usually resolves it completely.
3. Sewage Smell
This is the one to take seriously immediately. If the odor from your sump pump area smells distinctly like raw sewage (not just musty or sulfuric) it could mean a few different things, none of them small DIY fixes.
The most common culprit is actually a sewage ejector pump, not the sump pump itself. Many homeowners confuse the two, but if you have a bathroom, laundry, or other plumbing fixtures in your basement, you likely have a sewage ejector pump pit as well as a sump pit. The ejector pump handles wastewater, and the sump pump handles groundwater. A failing ejector pump or a cracked seal on the pit is the most common source of true sewage odor in a basement.
Other possibilities include a cracked or disconnected sewer line running under or near your basement, or a dry floor drain trap nearby that’s allowing sewer gas to back up. Any sewage smell that lingers or returns after cleaning needs professional inspection and diagnosis.
How to Clean a Sump Pump Basin
If your sump pump smells musty, mildew-y, or like standing water, cleaning the basin is usually the right first step. Here's how to do it safely and effectively.
Tools Needed
- Wet/dry vacuum
- Stiff-bristle scrub brush
- Garden hose or buckets of clean water
- Rubber gloves & eye protection
Supplies Needed
- 1 cup household bleach
- 1 gallon warm water
Unplug the sump pump from its outlet before doing anything else. Never work in or around the basin with the pump energized.
Use a wet/dry vacuum to pull out as much water, sludge, and loose debris as possible. Scoop out any visible leaves, gravel, or buildup by hand — gloves on.
Combine 1 cup of household bleach with 1 gallon of warm water. This dilution is effective against mold and bacteria without damaging pump components.
Pour the solution into the pit and use a stiff brush to scrub the interior walls, floor, and the exterior of the pump itself. Pay extra attention to dark or discolored areas where mold is most likely hiding.
Flush the basin with clean water using a garden hose or several buckets. Vacuum out the rinse water completely — you don't want bleach residue sitting in the pit.
Plug the pump back in and slowly pour water into the basin to trigger the float switch. Confirm the pump activates and discharges properly before replacing the lid.
Is a Sump Pump Smell Dangerous?
It depends on the odor. A musty or mildew smell is unpleasant and worth fixing, but it poses no immediate health risk. Mold in a basement is a long-term concern if left unchecked, but a smelly pit alone isn’t a crisis.
Hydrogen sulfide (the rotten egg smell) is worth taking more seriously at higher concentrations. At the levels typically found in a home basement, it’s more of a nuisance than a hazard. But if the smell is very strong, persistent, or if anyone in your home is experiencing headaches or nausea, ventilate the area and call a plumber to assess whether there’s a sewer gas issue.
A true sewage odor is the one that warrants the most urgent response. Raw sewage contains pathogens, and sewer gas contains methane – both are legitimate health and safety concerns. Don’t wait on that one.
When Should You Call a Plumber for Sump Pump Odor?
Most musty or sulfur smells are manageable at home. But reach out to a licensed plumber if any of the following apply:
- The smell is unmistakably sewage
- The odor returns quickly after cleaning
- You see murky, discolored, or waste-like water in the pit
- The sulfur smell is very strong and doesn’t clear after adding water
- You believe there’s a sewer line issue
- If you’ve tried everything and it still smells
Mattioni’s plumbers have seen every variation of this problems across southeastern Pennsylvania homes. We’ll find the source, explain what’s going on in plain language, and give you a clear plan – whether it’s a cleaning, a repair, or a replacement.
Frequently Asked Questions: Sump Pump Odors
Why does my sump pump smell like rotten eggs or sulfur?
A rotten egg smell usually means the sump pit has dried out, allowing hydrogen sulfide gas — naturally present in groundwater and soil — to escape into the air instead of being trapped by standing water. Refilling the basin with water is typically the first fix. If the smell persists or is very strong, it could indicate a cracked sewer line nearby, which requires a licensed plumber to evaluate.
Is sump pump smell dangerous?
Most sump pump odors — musty or mildew smells — are unpleasant but not immediately dangerous. A strong sulfur or sewage odor, however, can indicate hydrogen sulfide gas or raw sewage contamination, both of which are health hazards. If you smell something resembling a sewer or gas, ventilate the area and call a plumber promptly.
Why does my sump pump smell musty or like mildew?
Musty and mildew odors come from stagnant water and organic debris sitting in the pit. Mold and bacteria thrive in dark, damp environments. Cleaning the basin with a diluted bleach solution and ensuring your basement has adequate ventilation will typically resolve this.
Can a sump pump smell like sewage?
Yes — and this is the most serious scenario. A sewage odor in the sump pump area often signals a cracked sewer line, a failed sewage ejector pump, or a cross-connection between your drainage and sewage systems. Don't attempt to fix this yourself — call a licensed plumber right away.
How do I get rid of sump pump odor?
Start by identifying the type of smell. For sulfur odors, add water to a dry pit. For musty or moldy smells, clean the basin with a bleach-and-water solution and scrub thoroughly. For sewage odors, call a plumber. In all cases, make sure the pit lid is sealed and your basement has good airflow.
What is the difference between a sump pump and a sewage ejector pump?
A sump pump handles clean groundwater and stormwater — it keeps your foundation dry. A sewage ejector pump handles wastewater from basement plumbing fixtures like toilets, sinks, and washing machines. If you're smelling something that resembles sewage, it's more likely coming from the ejector pit than the sump pit.
How often should I clean my sump pump to prevent odors?
Most plumbers recommend cleaning the sump pump basin once a year. In the Greater Philadelphia area, late winter or early spring — before the heavy spring rains — is the ideal time. Regular maintenance also gives you a chance to check that the pump, float switch, and discharge line are all working properly.
When should I call a plumber for sump pump smell?
Call a plumber if the smell resembles sewage, if the odor returns quickly after cleaning, if you see discolored or murky water in the pit, or if a strong sulfur smell doesn't clear after adding water. These are signs of a problem beyond routine maintenance.
Mattioni is Here When You Need Us
A smelly sump pump can feel like a mystery, but it doesn’t have to stay one. Here’s all you need to do: identify the smell, try the fix that matches it, and call us if it doesn’t clear up or if you’re dealing with anything that smells like sewage.
With over 75 years of experience serving homeowners across Chester, Delaware, and Montgomery counties, Mattioni’s licensed plumbers have the tools and expertise to diagnose the problem accurately and fix it right. No guesswork. No upselling what you don’t need. Just honest answers and quality work from a team your neighbors have trusted for generations.
Ready to get rid of that smell for good? Schedule service with Mattioni online or call us at (610) 400-8510. We offer same-day service and flat-rate pricing so you can put the issue behind you fast, with no surprises.
