First: septic tank or holding tank?
A holding tank has no outlet, so it fills up as fast as you use water, and that is normal. If a family uses 9 m³ a month and the tank holds 8 m³, it will be full about once a month. A septic tank, on the other hand, has an outlet to the infiltration system, and it is always “full” up to the outlet pipe. The problem is when the level rises above it.
If you don't know which one you have, look at the technical passport or the building design, or ask the pumping contractor during pumping. The differences are described in more detail on the page holding tank or septic tank.
The most common causes
| Cause | Sign | Solution |
|---|---|---|
| Clogged drain field | Level above the outlet, wet ground or a smell above the field | Pumping, checking the field, restoring or rebuilding it |
| Groundwater flowing into the tank | In spring and after heavy rain, the level rises faster than you use water | Sealing the joints or replacing the tank |
| Rainwater downpipes or drainage connected | The tank is full after every rain | Disconnect the rainwater from the sewer |
| Leaking toilet or tap | The water meter turns when nobody is using water | Fix the leak |
| Tank too small for the consumption | More people live in the house than at the start | A larger tank or a BIO plant |
| Built-up sediment | The septic tank hasn't been pumped for a long time | Pumping as often as the municipal minimum requires |
How to check it yourself
- Compare with the meter Write down the water meter reading right after pumping and again when the tank is full. If there is more water in the tank than you used, outside water is getting in.
- Overnight test In the evening, turn off all the taps and write down the meter reading. If it has changed by the morning, something is leaking.
- Toilet check Put a few drops of food colouring in the toilet cistern. If the colour appears in the bowl without flushing, the valve is letting water through.
- Watch after rain If the level rises sharply right after rain, look for the way the rainwater gets in: the lid, the joints, the downpipes.
- Look inside the tank If the level is above the outlet pipe, the water isn't getting to the field: the outlet or the field itself is to blame.
Clogged drain field
The most common culprit. A field clogs when sediment and fats escape from the septic tank. This usually happens because the septic tank wasn't pumped on time or because wipes and fats are going down the drain. Over time the soil around the drains becomes impermeable, and the water no longer soaks in. High groundwater also makes it harder for the field to work.
Pumping helps for a while, but if the field is clogged, it has to be restored or built anew. Read more on the page drain field. If the soil or the groundwater isn't suitable for a new field, consider converting the septic tank into a biological treatment plant.
Water from a septic tank may be discharged into the environment only through a purpose-built infiltration system, and a holding tank must be watertight. Source: Cabinet Regulation No. 384, points 4.4 and 4.5 (likumi.lv). Checked on 30 October 2026.
Septic tank fills up after rain: groundwater and rainwater
Old concrete-ring septic tanks are often not watertight: groundwater flows in through the joints between the rings and around the pipe entries. If the lid is at ground level, rainwater runs into it. Sometimes rainwater downpipes or drainage are connected to the sewer, and then every rain fills the tank.
A septic tank isn't designed for rainwater: it overfills the tank and flushes the sediment out into the field. The solution may be sealing the joints, raising the lid, draining the rainwater separately or replacing the old septic tank. If the groundwater is high, see septic tank for high groundwater.
If the tank is simply too small
If more people live in the house than when the septic tank was installed, or a washing machine, a dishwasher or a sauna has been added, the tank may be too small. Then the sediment escapes to the field faster, and the problem keeps coming back. Read how the volume is chosen on the page septic tank installation.