In Riga and the Riga region, we assess the soil and install solutions that work in clay: a septic tank with a sand-gravel filter, a biological wastewater treatment plant (BIO plant) or a holding tank. A septic tank including the tank and installation costs from €2,200; in clay soil, building the filter or outlet comes on top.
How to tell that your plot has clay
- After rain, puddles stand for days, and the lawn stays wet for a long time in spring.
- The excavated soil is sticky, grey, bluish or brownish and can be squeezed into a plastic lump.
- A simple test: dig a hole at drain depth, pour in water and watch. If the water stands in the hole for hours and is still there the next day, a standard drain field will not work here.
- For a new build, the soil layers and their permeability are given in the geotechnical survey report.
What happens to a septic tank and field in clay
A septic tank only settles the wastewater; the treatment takes place in the soil beneath the drain field. In clay soil, water does not drain away from the drains, the field fills up, and the septic tank no longer flows. The result is a smell, a wet patch on the lawn above the field and a tank that has to be pumped more and more often.
Clay has two more properties that matter for the tank. First, a pit in clay acts like a bathtub: rainwater that soaks into the sandy backfill stays in the pit and can lift a plastic tank even when the groundwater is deep. Second, wet clay heaves when it freezes, so the tank and pipes are backfilled with sand, not with the excavated clay.
Solutions for clay soil
| Solution | How it works | What it needs |
|---|---|---|
| Septic tank with a sand-gravel filter | Water seeps through an imported layer of sand and gravel, and collection drains at the bottom carry the treated water away | Space for the filter and an outlet point (a ditch or a rainwater system) |
| Biological treatment plant | Treats the wastewater in the plant itself; the treated water is discharged into a ditch or a shallow filter | Electricity, regular maintenance, an outlet point |
| Filtration well down to a sand layer | If there is a sand layer beneath the clay, the water is discharged into it through a well | A borehole or survey confirming the sand layer, and enough distance to the groundwater |
| Holding tank | Nothing enters the ground; everything is taken away | Frequent pumping; anchoring in clay |
The main question in clay is not the tank but where the treated water will go. If there is a ditch nearby, a biological treatment plant or a sand-gravel filter with collection drains are realistic solutions. If there is no outlet point, a holding tank is what remains. Read the comparison on the page holding tank or septic tank.
LBN 223-15 (Cabinet Regulation No. 327), point 151: for wastewater up to 5 m³/d, septic tanks are used together with filtration fields, underground filtering drains, sand-gravel filters, filtration ditches and wells (151.1), or factory-made compact plants discharging into water bodies, drainage ditches or the ground, depending on the degree of treatment (151.2). Cabinet Regulation No. 384, point 4.4: septic tank discharge into the environment only through a purpose-built infiltration system. Sources: likumi.lv, checked on 30 October 2026.
What changes in the work in clay soil
- The excavated clay is not used for backfilling: it is taken away or levelled elsewhere, and sand is brought in for backfilling.
- The tank is anchored even if the groundwater is deep, because a pit in clay collects rainwater.
- For a sand-gravel filter, filter materials are brought in and collection drains are laid with a fall towards the outlet.
- The work is harder in wet weather: clay is better dug in a dry period, when the pit does not flood and machinery does not get stuck.
If the plot has high groundwater as well as clay, see septic tank for high groundwater.
How much a septic tank in clay soil costs
The base price for a septic tank including the tank and installation is from €2,200 in standard conditions. In clay soil, it is increased by bringing in sand and filter materials, removing the excavated clay, collection drains and a pipe to the ditch (from €25/m), as well as anchoring. The alternative is a biological treatment plant for 4–6 people from €1,750 plus installation work from €800, but it needs regular maintenance (from €160). We give an exact price after inspecting the soil.