Cistern size and wastewater fee

Hello,

we are planning the installation of a concrete cistern. I am still somewhat unsure about the size, but currently I would tend towards 8m3 (compromise between construction size / storage volume). A colleague has now told me that he has to pay sewage fees for his cistern because it is supposedly too small and not sufficient for garden irrigation and therefore fully or partially uses the sewer. However, according to research, this applies to every cistern connected to the public sewer. Exceptions are special retention cisterns. Here I just did not quite understand whether this reduces the usable volume or what the "retention volume" is about? Does it mean that the retention volume is considered as "lost" water volume, basically like overflow and therefore not usable - except, of course, during a rainfall event while the retention volume "forms"?
If that is the case, then I am actually "saving" at the wrong end, since you choose a cistern so large in order to bridge drought periods, and if you now apply this retention (time-delayed sewer drainage), then you reduce this "buffer for bad times" again.
An alternative would now be a cistern with overflow into a soakaway pit. But somehow I do not really like this idea, as a heavy rain event would certainly be too dangerous for our soil (clayey) ("local flooding")...

Is there possibly a way to size the cistern so that one can argue that no sewer fees will be incurred? We will only discharge about half of the roof area.

According to research, this applies to every cistern connected to the public sewer.

That is crucial,
if the cistern has an overflow to the sewer, you pay fees.
Our cistern of about 10m3 has an overflow into a soakaway pit of about 4m3, if it overflows there is just a puddle (which we have never had in the last 3 years,
maybe knows more

That is crucial, if the cistern has an overflow to the sewer you pay fees. Our cistern of about 10m3 has an overflow into a soakaway pit of about 4m3, if it overflows there is just a puddle (we never had that in the last 3 years, maybe knows more

No, unfortunately she doesn’t know ;). We are allowed/must let it seep on the property, therefore we have no connection to the sewer. We also don’t have a cistern because I think it’s not worth it. With 8 m3 you don’t last long if it’s really dry. We only have large barrels buried with many holes, gravel around, some gravel inside, fleece all around. It’s almost always enough, if it rains really hard sometimes it overflows, then a few square meters are just under water. But it seeps away again shortly after with our sandy soil. That happened twice in the last years.

Retention means that you temporarily store the water in the cistern and then slowly release it (e.g. 1l/min) back into the sewer system. You only want this if it is required to do so, because effectively you are burying a lot of air and you can only use the volume if you, for example, tie the float up.

As far as the wastewater fee is concerned... it is actually about stormwater. You generally have to pay if the cistern has an emergency overflow, because sometimes rainwater still flows into the sewer system. However, you usually still pay less stormwater fee than if you had not installed a cistern.

...We also don't have a cistern because I think it's not worth it. With 8 m3, unfortunately, you don't have enough for long if it is really dry. ...

commercially certainly correct. And you are right about the 8m³ as well. Watering about 80m² of roll lawn and some smaller beds for 3 weeks resulted in about 25m³ on the water meter. They were certainly not in the 5m³ concrete cistern. For nature, I still hope for a positive effect; there are outdoor faucets, a TOI, and a washing machine connected.

About 3 weeks of watering approximately 80m² of turf and some smaller beds resulted in about 25m³ on the water meter.

At first, that seemed like a lot to me, but then I did the math. It's only about 10 liters per square meter per day, and for the current hot weather (or is it different where you are in northern Germany?), not so much anymore.