Septic tank, cesspool, retention tank: what are the differences?

Three tanks buried in the ground that look similar and are constantly confused. The difference is not in the construction, but in the... what flows into them and what happens next.

Retention and accumulation tank

It flows into it clean rainwater from the roof. It is used to retain and utilise water for irrigation, flushing and washing. Water is pumped back up from it.

In practice, the terms are often mixed up, but the difference is: accumulation the tank collects water for further use retention it retains it to prevent sudden discharge into the sewerage system, and either releases it in a controlled manner or allows it to soak away. On a detached house you typically install a tank that performs both functions simultaneously.

Septic tank (cesspool)

It flows into it wastewater from the building - from the WC, bathroom and kitchen. It is non-draining; nothing flows out of it anywhere and it must be watertight. The contents are removed by a faecal waste vehicle and you pay for both collection and disposal. You must be able to present documents proving removal. This is checked.

If someone offers you a "rainwater sump", they most likely mean an accumulation tank and are using the term colloquially. Ask for clarification, because the construction differs: a sump is built to withstand aggressive environments and ensure complete watertightness in both directions.

Septik

Wastewater also flows into it, but unlike a septic tank, it partially cleanssettling in several chambers. Pre-treated water flows out of it and must be directed to a soil filter or soakaway, yet even today, a septic tank alone often fails to meet the quality requirements for discharged water. Therefore, new buildings are more commonly equipped with a domestic sewage treatment plant.

Why they cannot be interchanged

Structurally, many concrete tanks could handle both functions, but practically they cannot. A rainwater tank requires a calm inlet (to prevent sediment from swirling), an overflow with a siphon, a filter for coarse impurities, and water extraction from the middle layer where it is cleanest. A septic tank needs none of these; instead, it must withstand aggressive contents and have a lid for pumping out.

Crucial is also separation from drinking water. If you discharge rainwater into the building, the distribution system must be clearly distinguished and must not be connected to the water supply except via a separator prescribed by the standard. Cross-connections are an issue that hygiene authorities deal with very reluctantly.

And what about the soakaway structure?

This one does not belong to this trio, but it is often installed alongside them. Infiltration pits, tunnels or blocks are not water storage tanks. They are spaces filled with gravel or hollow elements through which water passes into the subsoil. They handle overflow from the tank and drainage of paved areas. For more details, see the article on infiltration.