CLEARMASTERS
The short version
Why does my septic tank smell?
Most septic tank smells come from one of four things: a dry trap or failed seal inside the house, a tank that’s full and overdue emptying, a drainage field that’s failing, or, on a treatment plant, an air blower that’s stopped. A healthy system shouldn’t smell beyond the odd whiff from the roof vent. A smell that’s strong, constant or getting worse needs looking at.
Interactive
Smell finder: where is the smell coming from?
Tap the number nearest to where you notice the smell to see the likely causes and what to do.
Smell inside, in the bathroom
A sewage smell indoors is usually a problem with the pipes or seals inside the house, not the tank itself.
- A dry trap: a basin, shower or bath that hasn’t been used, so the water seal has evaporated
- A loose or failed seal at the base of the toilet
- A blocked or missing vent, so flushing sucks traps dry
- A cracked waste pipe or soil stack inside a wall or boxing
What to do: Run water into every basin, bath, shower and floor drain for a minute to refill the traps. If the smell stays, check the toilet seal and call us to find the cause.
Smell from the kitchen sink or plugholes
Kitchen smells are often the drain itself rather than the tank.
- Food and fat built up in the trap or waste pipe
- A slow or partly blocked drain holding dirty water
- An appliance (dishwasher or washing machine) connected without a trap or with a kinked hose
What to do: Clean the trap and pour plenty of hot (not boiling) water through. Don’t use caustic drain cleaners if you have a septic tank or treatment plant.
Smell around the roof vent
The vent pipe lets gases out above the roof line, so a faint smell outside on still or windy days can be normal.
- Weather pushing vent gases down into the garden
- A vent that’s too short, or close to a window
- A full tank making more gas than usual
What to do: A short-lived smell on still days is usually nothing to worry about. If it’s strong or constant, the tank may need emptying, or the vent may need extending.
Smell around the manhole or inspection chamber
A smell at a manhole between the house and the tank usually means something isn’t flowing.
- A blockage, with sewage sitting in the chamber
- A cracked or badly fitting cover
- Root ingress or a sagging pipe holding waste
What to do: Lift the cover if it’s safe. If the chamber is full, there’s a blockage further down. Our guide to blocked drains explains what to do next.
Smell around the tank itself
The tank is the most common source of outdoor smells, and the cause is often simple.
- The tank is full and overdue emptying
- A cracked lid, or a lid seal that’s perished
- A blocked outlet, so the level has risen
- A treatment plant whose air blower has stopped, so the bacteria have died off
- A smell for a day or two after emptying, which is normal
What to do: Check when it was last emptied: if it’s more than a year, book an empty. On a treatment plant, listen for the blower humming. If it’s silent, call us.
Smell over the drainage field
A smell, soggy ground or extra-green grass over the field means effluent is reaching the surface.
- A drainage field that’s clogged or failing
- Ground waterlogged after heavy rain or by a high water table
- Solids carried out of a tank that hasn’t been emptied
What to do: Keep people and pets off the area, cut your water use and get it checked. Our guide to drainage field problems explains the fixes.
Smell at the ditch or outlet
If you have a treatment plant discharging to a ditch, a smelly or cloudy outlet means it isn’t treating properly.
- The air blower or diffusers have failed
- Chemicals have killed the bacteria
- The plant is overdue desludging or overloaded
- A septic tank discharging to a watercourse, which isn’t allowed
What to do: A dirty discharge can cause pollution, so call us. A septic tank discharging to a ditch needs upgrading under the rules.
Timing matters
When does it smell?
When the smell appears is the second big clue.
After rain
A high water table or rainwater getting into the system can flood a drainage field or fill the tank. Check gutters aren’t connected to it.
Only on still or windy days
Usually the roof vent. Short-lived and mostly harmless, unless it’s strong.
Straight after flushing or a bath
A trap being sucked dry or a blocked vent. Common when a big flush pulls water out of other traps.
Since it was last emptied
A day or two is normal while the bacteria re-establish. Longer than a week, call us.
Slowly getting worse for weeks
Usually a tank that needs emptying, or a drainage field starting to fail.
In summer
Warm weather speeds up the bacteria and lets traps dry out faster. Keep traps topped up.
Before you call
Five checks you can do yourself
Refill the traps
Run water into every basin, bath, shower and floor drain, especially rooms you rarely use.
Check the toilet
Look for movement, staining or damp around the base, which means a failed seal.
When was it emptied?
Find the last waste transfer note. Over a year ago, it’s due.
Listen to the plant
A treatment plant’s blower should hum quietly all the time. Silent means a fault.
Walk the field
Look for soggy patches, puddles or lush grass over the drainage field.
Smells that are usually normal
A faint smell near the roof vent on still or windy days; a smell close to the tank while it’s being emptied; a mild smell for a day or two after emptying while the bacteria build back up.
Smells that need looking at
A smell indoors that doesn’t go after refilling traps; a constant smell around the tank or the field; soggy, smelly ground; a smelly or cloudy outlet to a ditch; any smell that’s getting steadily worse.
If you have a treatment plant
Why does my treatment plant smell?
A treatment plant that’s working properly smells far less than a septic tank, because air is pumped in to keep the bacteria healthy. When it starts to smell, the usual reasons are:
- The air blower has failed or been switched off, so the bacteria die off
- It’s overdue desludging, so sludge carries through
- Heavy use of bleach or antibacterial products has killed the bacteria
- It’s overloaded by more people, or by rainwater getting in
Our guide to treatment plant problems has a fault finder for alarms, smells and dirty water.
Health and safety
Is a septic tank smell dangerous?
The gases
Sewage produces gases including hydrogen sulphide (the rotten egg smell) and methane. In the open air and at the levels you’d notice in a house or garden they’re unpleasant rather than dangerous, but they build up in enclosed spaces like tanks, chambers and pump wells, where they can be deadly.
What that means for you
Never enter or lean into a tank or chamber, and keep children and pets away from open covers. If you smell sewage strongly indoors, open windows, find the cause and get it fixed. Our crews hold confined space entry certificates for exactly this reason.
Prevention
How to stop septic tank smells coming back
Do
- Get the tank emptied at least once a year
- Run water in unused rooms every week or two
- Keep a treatment plant switched on and serviced
- Keep the lid sealed and the vent clear
- Keep rainwater out of the system
Don’t
- Pour bleach, disinfectant or drain unblockers in heavily
- Flush wipes, fat or sanitary products
- Rely on air fresheners or additives to mask it
- Ignore a smell that’s getting worse
See also our guides to what not to flush and how often to empty a septic tank.
Questions answered
Septic tank smell FAQs
Why does my septic tank smell?
The most common reasons are a tank that is full and overdue emptying, a cracked lid or seal, a blocked outlet, a failing drainage field, or on a treatment plant, an air blower that has stopped. If the smell is indoors, it is more often a dry trap, a failed toilet seal or a blocked vent.
Why can I smell sewage in my house when I have a septic tank?
Indoor smells usually come from the pipework rather than the tank: a trap that has dried out in a rarely used room, a failed seal under the toilet, a blocked vent pipe or a cracked waste pipe. Run water into every outlet first. If the smell stays, get the pipes checked.
Is it normal for a septic tank to smell after emptying?
A mild smell for a day or two after emptying is normal while the bacteria build back up. If it lasts more than a week, or is strong, there may be another problem such as a blocked outlet or a damaged lid seal.
Why does my septic tank smell more after rain?
Heavy rain can raise the water table and waterlog the drainage field, or get into the system through cracked pipes or connected gutters. The tank fills faster and the field cannot take the water, so smells rise to the surface.
Why does my septic tank smell in summer?
Warm weather speeds up the bacteria in the tank and makes traps in unused rooms dry out faster. A faint smell from the vent is common on still summer days. A strong or constant smell is not normal.
Can I use something to stop my septic tank smelling?
Masking products and additives rarely fix the cause and some can upset the bacteria. It is better to find out why it smells, whether that is emptying, a seal, the vent or the drainage field, and put that right.
Are septic tank smells dangerous?
At the levels you would notice in a house or garden, sewage gases are unpleasant rather than dangerous. Inside tanks, chambers and pump wells they can build up to deadly levels, so never enter or lean into one.
Why does my treatment plant smell?
A treatment plant should smell very little. A smell usually means the air blower has failed or been switched off, the plant is overdue desludging, chemicals have killed the bacteria, or it is overloaded.
Got a different question? Call 03333 233566 or send us a message.