Common Causes of Sump Pump Odours
1. Stagnant Water
If the sump pit collects water that doesn’t drain properly, bacteria can build up and cause a musty or sour smell. This often happens when:
- The pump isn’t cycling regularly
- There’s debris blocking discharge
- There’s no check valve and water flows back in
2. Dry Sump Pit
On the flip side, if your pump hasn’t run in a while and the pit dries out, any remaining residue or bacteria can create a sewer gas smell. This is common in dry months or basements with low water tables.
3. Mold and Mildew
Warm, damp areas like sump pits are ideal for mold and mildew growth, especially if the lid isn’t sealed. Mold spores can create a musty odour that spreads through the basement.
4. Sewer Gas Leaks
If your sump pump is connected to a perimeter drain or a combined sewer system, foul odours may be sewer gas backing up through the pit. This needs to be addressed immediately—it’s a health and safety concern.
5. Decomposing Organic Material
Leaves, dirt, or other debris that find their way into the pit can decay over time and produce a rotten smell. A missing or broken sump pit cover can make this worse.
How to Get Rid of the Smell
Clean the Pit
- Unplug the pump and remove any standing water
- Scoop out sludge or debris at the bottom
- Use a diluted bleach solution or vinegar to scrub the interior
- Rinse and dry completely before reconnecting the system
Check the Check Valve
If your sump pump doesn’t have a check valve—or if it’s faulty—water may be draining back into the pit and going stagnant. A proper valve keeps discharged water from re-entering.
Seal the Lid
Installing a sealed sump pit lid helps contain odours and limits debris from entering. If you already have a lid, check the gasket or rubber seal for damage.
Keep the Trap Wet (If Connected to Drain Lines)
If your sump system connects to other plumbing, make sure there’s a properly installed P-trap that stays wet. Dry traps allow sewer gases to vent into your home.
When to Call a Professional
Contact a licensed plumber or waterproofing contractor if:
- You smell sewer gas or anything resembling rotten eggs
- Your sump pit fills unusually fast or slow
- The smell returns after multiple cleanings
- There’s visible mold growth or water leaking from the pit
MT Drains & Plumbing has over 30 years of experience diagnosing sump pump issues across the GTA. We offer full service from sump pump installation to mechanical repairs and backwater valve installation.
Tips for Preventing Future Smells
- Test your pump every few months—especially before spring thaw or heavy rain
- Install a lid if you don’t already have one
- Keep the area around the sump clean and dry
- Avoid dumping anything down basement floor drains that could reach the sump
- Use an enzyme-based cleaner every few months to break down organic residue
FAQ
Is a smelly sump pump dangerous?
It can be. Sewer gas exposure is a health hazard. Mold spores also reduce air quality. Always investigate unusual smells promptly.
Can I pour bleach down the sump pit?
Yes—in small amounts, diluted. Avoid overuse, as it can damage pump components and harm septic systems if applicable.
How often should I clean my sump pit?
We recommend a full cleaning once or twice a year, or more frequently if your pump cycles often.
Should my sump pump have a lid?
Yes. A sealed lid reduces odours, improves safety, and helps keep the system clean.
Need help with a smelly sump pump?
MT Drains & Plumbing provides sump pump inspections, cleanings, and full repairs across Toronto and the GTA. Book a service call today and restore clean, dry air to your basement.
