Hello everyone,
We are actually finished with everything. The house is built, and the cistern is connected. However, it seems there are fees related to the overflow of the cistern if it is connected to the sewer system. Since I will need an excavator again for the landscaping, I might consider installing an infiltration chamber.
The only question is: how large should it be?
The roof has a footprint of about 100 m² (1,076 sq ft). The cistern holds 4,500 liters (1,189 gallons). What rainfall amount should I expect? How do I properly integrate the cistern, or should I expect that in the worst case it will always be full and overflow?
Best regards
We are actually finished with everything. The house is built, and the cistern is connected. However, it seems there are fees related to the overflow of the cistern if it is connected to the sewer system. Since I will need an excavator again for the landscaping, I might consider installing an infiltration chamber.
The only question is: how large should it be?
The roof has a footprint of about 100 m² (1,076 sq ft). The cistern holds 4,500 liters (1,189 gallons). What rainfall amount should I expect? How do I properly integrate the cistern, or should I expect that in the worst case it will always be full and overflow?
Best regards
G
Gartenfreund12 Feb 2015 06:37Hello hg6808
It’s not easy to say exactly how large the area needs to be for water to infiltrate, as it depends on the soil conditions—whether the water can seep away easily or not. It’s best to consult a soil expert for this. Regarding rainfall amounts, there are various websites where you can find such information. I looked into it myself last year. However, these sources should be used cautiously, as different websites sometimes provide varying data for the same region. I can’t recall the exact figures now, and I’m not inclined to search for them again, but if I remember correctly, the values for my area ranged between 700 and 1000 mm per m² (27.5 and 39.4 inches) annually.
Additionally, you should check with your local authorities whether infiltration is permitted and what regulations apply. In my case, the local water authority had to be involved, which also incurs some costs. A simpler option here was to pump the water directly into the garden without requiring further permits.
Enjoy your gardening time!
It’s not easy to say exactly how large the area needs to be for water to infiltrate, as it depends on the soil conditions—whether the water can seep away easily or not. It’s best to consult a soil expert for this. Regarding rainfall amounts, there are various websites where you can find such information. I looked into it myself last year. However, these sources should be used cautiously, as different websites sometimes provide varying data for the same region. I can’t recall the exact figures now, and I’m not inclined to search for them again, but if I remember correctly, the values for my area ranged between 700 and 1000 mm per m² (27.5 and 39.4 inches) annually.
Additionally, you should check with your local authorities whether infiltration is permitted and what regulations apply. In my case, the local water authority had to be involved, which also incurs some costs. A simpler option here was to pump the water directly into the garden without requiring further permits.
Enjoy your gardening time!
B
Bauexperte12 Feb 2015 10:36Hello,
To determine the size of the infiltration trench, a hydrogeological report is required (this is different from a standard soil survey). In this report, the expert analyzes the soil properties regarding infiltration capacity and provides recommendations on how the infiltration trench should be constructed. However, the first issue to clarify is whether the municipality permits an infiltration trench or if you are obligated to connect to the sewer system.
Regards, Bauexperte
hg6806 schrieb:No one here can answer this for you, nor can you simply calculate it yourself.
The only question is, how large should this be?
To determine the size of the infiltration trench, a hydrogeological report is required (this is different from a standard soil survey). In this report, the expert analyzes the soil properties regarding infiltration capacity and provides recommendations on how the infiltration trench should be constructed. However, the first issue to clarify is whether the municipality permits an infiltration trench or if you are obligated to connect to the sewer system.
Regards, Bauexperte
You need the soil permeability value (kf-value, which indicates how well the soil absorbs water). You can roughly determine this yourself, though not as professionally—just try searching for it online. Also consider the type of roof covering, the roof area, and your local rainfall amount (including the 100-year rainfall event), which you should also be able to find online. Based on these factors, you can calculate the necessary size of the infiltration trench (surface infiltration using a pipe trench is always better than point infiltration like an infiltration shaft). The cistern is not included in this calculation because if it is full (which happens more often than you might think, from my own experience), water just flows straight through. During heavy rain, water often passes through the installed in-tank filter and directly into the overflow (unless you only have a leaf catcher installed).
It is important to provide adequate retention volume, as a lot of water accumulates once the cistern is full. We have 150 m² (1,615 sq ft) connected to our cistern, which has a capacity of 4800 liters (1,268 gallons). The overflow goes into a pipe infiltration trench with six DN 150 (6-inch diameter) fully pervious pipes surrounded by a 30 x 30 cm (12 x 12 inch) gravel bed. Our soil is a silty clay loam.
When you see in real time during a heavy rain how much water there is while the cistern is full, you start thinking, "Wow, that’s a lot of water." When you consider how slowly it infiltrates, it can be a bit alarming—but everything works fine.
I recommend going to your local building materials supplier with the data you have. They can usually commission a sizing calculation from their manufacturer (for example, for pipe or block infiltration systems with companies like Hegler or Fränkische, or for tunnel infiltration systems with companies like Graf). They generally do this service for free—that was my experience at least.
By the way, I left an open overflow on our system in case the infiltration system can no longer handle the volume or if the soil’s permeability decreases over time. In that case, the system could be connected to the public sewer (the connection pipe is already installed—you would just need to dig a 1.5 m (5 feet) deep hole in the garden and connect the two pipe ends).
If you do connect to the public sewer, please don’t forget the backflow valve. You definitely don’t want the sewage backing up into the cistern during a backflow event.
By the way, the situation with the precipitation fee is complicated here. If we were connected to the public sewer, we would pay the full fee despite having an infiltration system, minus the cubic meters of water withdrawn from the cistern.
It is important to provide adequate retention volume, as a lot of water accumulates once the cistern is full. We have 150 m² (1,615 sq ft) connected to our cistern, which has a capacity of 4800 liters (1,268 gallons). The overflow goes into a pipe infiltration trench with six DN 150 (6-inch diameter) fully pervious pipes surrounded by a 30 x 30 cm (12 x 12 inch) gravel bed. Our soil is a silty clay loam.
When you see in real time during a heavy rain how much water there is while the cistern is full, you start thinking, "Wow, that’s a lot of water." When you consider how slowly it infiltrates, it can be a bit alarming—but everything works fine.
I recommend going to your local building materials supplier with the data you have. They can usually commission a sizing calculation from their manufacturer (for example, for pipe or block infiltration systems with companies like Hegler or Fränkische, or for tunnel infiltration systems with companies like Graf). They generally do this service for free—that was my experience at least.
By the way, I left an open overflow on our system in case the infiltration system can no longer handle the volume or if the soil’s permeability decreases over time. In that case, the system could be connected to the public sewer (the connection pipe is already installed—you would just need to dig a 1.5 m (5 feet) deep hole in the garden and connect the two pipe ends).
If you do connect to the public sewer, please don’t forget the backflow valve. You definitely don’t want the sewage backing up into the cistern during a backflow event.
By the way, the situation with the precipitation fee is complicated here. If we were connected to the public sewer, we would pay the full fee despite having an infiltration system, minus the cubic meters of water withdrawn from the cistern.
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