industrial slurry pump and equipment

Why Does My Basement Sump Pump Smell?

Common Causes of Sump Pump Odours

A foul smell coming from your sump pump isn't just unpleasant—it can point to a maintenance issue, a drainage problem, or even potential health risks. Here’s what causes that smell and what steps you should take to eliminate it.

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

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?