Septic Tank Smells

What's happening?

Sefton Council explains that a drainage system should not vent smells at ground level or inside the property: external gullies and internal sinks, baths, toilets and showers should each have a water trap as a barrier, and the soil vent pipe lets foul air vent above the eaves. A blocked or missing soil vent pipe can create a vacuum that draws water out of a trap, and once a trap is empty the smell comes in. Outside, Natural Resources Wales (for Wales) says that if sludge builds up too much it can flow into the drainage field and clog it, and that a healthy outlet should be free-flowing and clear. In England, GOV.UK says that if you do not desludge often enough, solids are likely to be carried into the drainage field or soakaway.

What usually causes it?

• Sludge building up in the tank because it is not emptied often enough
• A clogged or waterlogged drainage field, with effluent backing up towards the tank
• A water trap that has run dry or been drawn out, so smells enter the house
• A blocked or missing soil vent pipe, which can create a vacuum that draws water out of traps
• Pollution from a failing system, which in England you must not cause

Is it dangerous?

Low for an ordinary smell. Leave the tank lid closed: opening or entering a tank is a job for the emptying contractor. A strong smell with wet, sewage-stained ground near the tank may be pollution, and in England GOV.UK says you must not cause it.

Can I fix it myself?

Run water into any rarely used sink, bath or floor drain to refill its trap. Look at the soil vent pipe from the ground: Sefton Council says it should run above the eaves, so tell a drainage engineer if it looks damaged or missing, rather than climbing up. Check the date the tank was last emptied: in England GOV.UK says you should desludge at least once a year or as the manufacturer instructs. Check the drainage field for waterlogging, as Natural Resources Wales (Wales) suggests monthly. If the smell is outside and the ground is wet, have the tank and field checked.

When should you call a drainage engineer?

Call a septic tank engineer when the smell is outside and strong, when the ground near the tank or field is waterlogged, when the tank has not been emptied for a year or more, or when indoor smells continue after refilling the traps. A drainage engineer can check the soil vent pipe and the traps.

What will the drainage engineer actually do?

• Empty and check the tank and its outlet
• Inspect the drainage field or soakaway for waterlogging, blockage or failure
• Check the soil vent pipe and the water traps, and trace leaks
• Service a treatment plant and check its pump and alarm
• Advise on emptying intervals and on the general binding rules

Who fixes this

• Septic tank emptying
• Sewage treatment plant servicing

Questions people ask

Sources & References

1. Sefton Council: smelly drains, internal drain venting
2. Natural Resources Wales (Wales): running your household sewage treatment system correctly
3. GOV.UK (England): general binding rules, small sewage discharge to the ground
4. GOV.UK (England): report an environmental incident