ᐅ Rainwater Cistern: Useful? Necessary? Costs?

@Bieber0815 try searching online for buying a cistern. You'll find plenty of online stores offering cisterns with various accessories (pumps, filters, covers, pipes, etc.). This will give you an idea of what to expect. Additionally, you need to consider the costs for excavation and backfilling, as well as the piping.

Hello,
we have a 6sqm (65 sq ft) concrete cistern. The cistern itself cost us around 900€. It supplies the garden faucets. We have a fresh water supply connected via the garden water meter. Additionally, there is a domestic water pump, pipes, excavation work, etc. I estimate around 500€ for these.
We only installed it because the local authority pays an additional 160€ per sqm. So it didn’t cost us much, and we save the amount usually charged for roof drainage (which we have to pay here and is gradually increasing).
The unit is buried in the garden with a lid covered by about 10cm (4 inches) of soil, with grass growing on top. It is no longer visible and is also load-bearing.

Bieber0815 schrieb:
Hi,

in a construction contract, we are being offered:

What do you think?
- Are the cistern and the wastewater connection directly related?
- Do we need the cistern? For what purpose? What if we don’t have it?
- What portion of the cost might be attributed to it (not listed separately, of course, I will ask the supplier as well)?

ypg schrieb:
In our case, the cistern was required. Other municipalities charge a monthly fee instead. The water from your roofs has to go somewhere!

To my knowledge, there is no direct connection between the cistern and the wastewater pipe. Only the overflow from the cistern could be connected to the wastewater line (due to the height difference between the overflow and the wastewater pipe, contamination of the cistern water by potential backflow of wastewater should be prevented). (Layman’s understanding )

The most important point for you (as @ypg already mentioned): you need to find out what your municipality (state) requires. In our case, one option was infiltration by installing a soakaway, basin, pond, or cistern (I might have missed something). Since a pond or basin on the property was not an option for us due to small children (risk of drowning) and also because of potential mosquitoes, and since installing a soakaway would have cost quite a bit, we chose a 10 m³ (about 2,650 gallons) concrete cistern for garden irrigation. I can’t give you the exact price (it was included in the overall construction contract), but the general contractor told us the price difference between 5 and 10 m³ (about 1,300 gallons) is only a few hundred euros.

My father has been irrigating his garden with a cistern for years. He has (I think) 7.5 m³ (about 2,000 gallons) which often gets completely emptied during the summer. His entire property is about 650 m² (7,000 sq ft) including the house. So I believe it pays off over time to water the garden using cistern water instead of drinking water—especially since wastewater fees are usually based on the amount of drinking water consumed.

By the way: a cistern with an overflow connection to the sewer is not recognized here for reducing the “stormwater discharge fee” (according to the local authorities). For that, all water on the property would need to soak into the ground. Whether this works during heavy rainfall and when the cistern is already full, in my opinion, depends a lot on the local soil conditions.

f-pNo schrieb:
A cistern with an overflow function into the sewer system is not recognized here for reducing the "stormwater discharge fee" (according to the authority).

That makes sense, a cistern does not primarily serve an infiltration function (otherwise it would be an infiltration shaft, not a cistern).

The information here is definitely helpful for me; now I can ask the right questions! I will provide an update once I know more.

Bieber0815 schrieb:
Obviously, a cistern does not initially have an infiltration function (otherwise it would be an infiltration shaft, not a cistern).

The information here is helpful to me, at least I can now ask the right questions! I will provide an update when I know more.

As far as I know, it is possible to install the cistern within a larger gravel bed and direct the overflow to infiltration. In that case, theoretically, no connection to the sewer system would be necessary. This would essentially create a cistern with an infiltration function, potentially allowing for the avoidance of rainwater discharge fees.

However, I consider this problematic because, when the cistern is already full and there is heavy rainfall, the soil may not be able to absorb the water quickly enough. This could lead to the cistern being pushed upward (despite being made of concrete and filled with, for example, 10m³ (350 cubic feet) of water).

f-pNo schrieb:
However, I consider this problematic because with an already full cistern and heavy rainfall, the ground may not be as absorbent as required.

This requires proactive water management (and appropriate level monitoring), then it should work fine. Anyway, a sufficient infiltration plan must be demonstrated, with or without a cistern. Infiltration on the property is one thing, the cistern is another (although both can be presented together).