Septic tank or biological treatment plant - which to choose

How a septic tank and a BIO plant work

A septic tank is a tank with two or more chambers. Heavy particles settle, fats float up, and the settled water flows on. It may be discharged into the environment only through a purpose-built infiltration system: a filtration field, underground drains, a sand-and-gravel filter, a filtration ditch or well, or a reed bed. So a septic tank always comes in two parts: the tank plus a drain field.

In a BIO plant, a compressor supplies air continuously, and aerobic bacteria (activated sludge) break down the pollutants in the wastewater. The sludge then settles, and the treated water flows out. Manufacturers state that it can be infiltrated into the ground, discharged into an open body of water or a ditch, or used as technical (non-drinking) water; which option is permitted depends on the specific site.

Comparison table

When a septic tank is the better fit

• The soil is sandy or gravelly, and the groundwater is deep even in spring.
• There is room on the plot for a drain field while keeping the required distances from the well, the buildings and the neighbour's boundary.
• The house is a summer house or is used irregularly: a septic tank has no compressor that has to keep running with nothing to treat.
• You don't want to pay for electricity and annual technical maintenance.

Distances and other site requirements are set by the building regulations and the local spatial plan; they must be checked with the building authority before construction. Read more about the work on the page septic tank installation.

When a biological treatment plant is the better fit

• The soil is clayey or dense, and water doesn't soak into it.
• The groundwater is high, and there is nowhere to build an ordinary drain field. See also septic tank for high groundwater.
• The plot is small, and there isn't enough room for a field.
• There is a ditch nearby where the treated water can be discharged, if this is permitted.
• The house is lived in all year round, and there is a lot of wastewater.

It's important for a BIO plant to receive wastewater regularly. For a summer house, choose a model with a holiday mode: the AUGUST controller, for example, has HOLIDAY and WEEKEND HOUSE programmes, and AQUABLOC has a holiday mode in which the plant only aerates and uses about 75% less electricity. See the plant comparison on the page BIO plant comparison.

When neither a septic tank nor a BIO plant will do

If there is nowhere to discharge the water (no permeable soil, no ditch, no space), what's left is a sealed holding tank that is pumped out regularly. It's cheaper to install but more expensive to run; the comparison is on the page holding tank or septic tank.

If there is a mains sewer in the street along the property, it's often better value to connect to it. National regulations provide for an obligation to connect if the existing system doesn't meet the requirements and the mains sewer is in the street or road next to the property. Read more on the page connecting to the mains sewer.

Can an existing septic tank be converted into a BIO plant?

If the drain field is clogged or the septic tank doesn't meet the requirements, it's often better value to install a BIO plant in place of the septic tank than to restore the field. Read how this works on the page converting a septic tank into a biological treatment plant.

How we help you choose

1. Request Give the location, the number of people, whether the house is lived in all year round, what the soil is like and whether water stands in spring, and where the treated water could be discharged.
2. Assessment Based on this information, we tell you which option is realistic at your site. If needed, we arrange a site visit.
3. Quote If both options are suitable, we prepare both so that you can compare the installation and annual costs.
4. Installation After installation, the system must be registered with the municipality. For how to do this, see the page registering a decentralised sewage system.