Septic tank smells: the most common causes and what to do

Where you notice the smell is where to look for the cause

What to do right away

1. Don't pour chemicals into the tank Chlorine and other aggressive substances kill the bacteria and make the system work worse. BIO plant manufacturers warn about this too.
2. Check the traps If it smells in the house, first pour water into all the sinks, the shower and the floor drains. Often that's enough.
3. Open the lid and look Is the water level above the outlet pipe? Is there a thick floating layer on top? Don't climb into the tank and don't use an open flame, because sewer gases are dangerous.
4. For a BIO plant - look at the controller Is the compressor humming, and are there bubbles in the aeration chamber? Is there an alarm on the display?
5. Request pumping or maintenance In your request, say where you notice the smell, when the system was last pumped and its volume.

Why a full septic tank smells

In a septic tank, sediment builds up at the bottom, while fats and light matter form a floating layer on top. When both take up a large part of the chamber, the wastewater no longer has time to settle, more gas is released and solids escape to the drain field. That's how the smell appears both at the lid and above the field.

That's why municipalities set a minimum pumping frequency: for a septic tank, at least once a year in Riga and Ķekava Municipality, and twice a year in Mārupe Municipality. Read more on the page how often to pump a septic tank.

Sewer smell in the house: ventilation and traps

Every sink, shower and toilet has a trap (a water seal) that holds back the gases. If a sink isn't used for a long time, the water evaporates, and the gases come into the room. The second cause is ventilation: the sewer stack must vent the gases above the roof. If it's clogged, frozen in winter or missing altogether, the gases look for a way out through the traps, and you may hear the toilet gurgle when you flush.

If the traps are full and the vent is open but the smell remains, a joint in the house or underground may be loose. Then it needs checking; if pipes have to be relaid, see sewer installation.

BIO plant smells

A BIO plant that is working correctly has no strong smell. The AUGUST technical passport says that a strong unpleasant smell is not normal and that you should contact a service provider. Most often a stopped compressor is to blame: without air the aerobic bacteria can't work, and the process turns anaerobic. Other causes are too much sludge, overload after a party, chemicals or a long power cut.

For what the controller signals mean, read biological plant alarm. Whether bacterial products will help, we explain on the page bacteria for biological treatment plants.

When a smell means damage

If a holding tank smells, it isn't watertight enough or isn't being emptied on time. The regulations require that the contents of a holding tank don't leak out, don't seep into the ground and don't spread a smell. If wastewater is visible around the tank or above the field, it is also a health risk: keep children and pets away and act without delay.

Old concrete-ring tanks crack over time, and the joints come loose. Then the answer is septic tank repair or old septic tank replacement.

What not to do

• Don't pour chlorine, bleach or solvents into the tank.
• Don't cover or plug the vent pipe to “stop the smell”, because the gases will then come into the house.
• Don't wait until the septic tank overflows: a clogged drain field costs far more than pumping.
• Don't mask the smell with air fresheners, because the cause remains.