Septic smell? Here is what it means.

Septic systems produce gases as bacteria break down waste, mainly hydrogen sulfide (the rotten-egg smell), methane and carbon dioxide. A healthy system vents those gases out through the plumbing vent on your roof, so you should rarely notice them. When you start smelling sewage, something is letting gas escape where it should not, or the system is not handling the wastewater properly. Here is how to narrow it down.

Smell inside the house

1. A dry drain trap

Every sink, tub, shower and floor drain has a U-shaped trap that holds water and blocks sewer gas. Guest bathrooms, basement floor drains and utility sinks that are rarely used can dry out. Fix: run water in every drain for 30 seconds. If the smell disappears within an hour, that was the problem.

2. A loose toilet or failed wax ring

If the smell is strongest around one toilet, the wax seal under it may have failed. A toilet that rocks slightly or has staining at the base is a strong clue. This is a plumbing fix, not a septic one.

3. A blocked plumbing vent

The roof vent lets gas escape and lets air in so drains flow. Leaves, a bird nest or snow can block it. Signs include gurgling drains, slow drains throughout the house, and a smell that comes and goes when fixtures drain.

4. Sewage backing up

If drains are slow on the lowest level, toilets gurgle, or water comes up in a shower when the washing machine drains, the tank may be full or the line to it may be blocked. Stop using water and call us. See signs your septic tank is full.

Smell outside in the yard

Near the tank lids

A faint smell right at the lids can come from a cracked or poorly sealed riser lid. A strong smell, especially with damp soil around the lids, usually means the tank is overfull or the outlet is blocked.

Over the drainfield

A sewage smell over the drainfield, especially with wet or spongy ground, bright green grass or standing water, is one of the most serious signs. It means wastewater is coming to the surface instead of soaking into the soil. Keep kids and pets away and read signs of drainfield failure.

Near the roof vent

Smelling the vent occasionally on a still, damp day is normal. Downdrafts in our Northwest weather can push vent gas into the yard. A taller vent pipe or a carbon filter on the vent can help. If the smell is constant, have the system checked.

Smell after heavy rain

We get a lot of calls in the wet months. Saturated soil slows down how quickly the drainfield can accept water, and high groundwater can push into tanks with poor seals. If the smell shows up mainly after big storms, it is worth an inspection to see whether the drainfield is struggling or groundwater is getting in. See our post on septic water in the yard.

Is septic gas dangerous?

In the small amounts you smell from a dry trap, it is mostly unpleasant. In high concentrations, especially in enclosed spaces, hydrogen sulfide and methane can be dangerous. Never lean into an open septic tank or climb into one. If the smell is very strong indoors, open windows, leave the area and call for help.

What you can check before calling

1. Run water in all rarely used drains to refill traps.
2. Look for a rocking toilet or staining at its base.
3. Walk the yard: wet spots, standing water, or soggy ground over the tank or drainfield.
4. Check whether your septic alarm panel is lit or sounding (what the alarm means).
5. Think back to when the tank was last pumped (how often to pump).

When to call us

Call if the smell is in the yard, if drains are slow or gurgling, if you see wet ground over the system, or if refilling the traps did not fix it. We service homes across King and Snohomish County, day or night. Members of our maintenance plan get emergency fees waived and next-day service guaranteed.

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