ᐅ Rainwater Harvesting with a Cistern: Rainwater Retention

Created on: 7 Mar 2014 08:13
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PhoenixDH
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PhoenixDH
7 Mar 2014 08:13
Hello everyone,

we are currently looking for a suitable cistern that we will use exclusively for rainwater and subsequent garden irrigation.

Since we need to retain rainwater and ensure it drains off slowly, I came across retention tanks. These have a usable volume and a retention volume. If we need a cistern of 3000 liters (790 gallons) based on the sealed surface area, does that refer only to the usable volume? Or is it the usable volume plus the retention volume?

And one more technical question:

The picture shows the usable volume and the retention volume separated at the level of the float. What happens if there is heavy rainfall and the retention volume is also filled, causing water to overflow at the top? When the rain subsides, does the water level then drop back down to the maximum usable volume, meaning the float allows draining? Or does the retention volume remain stored until it is effectively used?

Do you know what I mean?
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waldorf
7 Mar 2014 23:04
The water always drains down to the level of the usable volume. Otherwise, it wouldn’t make sense.
I just don’t understand why this thing needs a float.

Only 3000 liters (790 gallons)? That’s about what we need just for a carport here.
Do you know what I mean?


No, no idea.
For some unknown reason, a shut-off valve was installed before the throttled outlet on my system. But it seems to be clogged or somehow "jammed."
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PhoenixDH
8 Mar 2014 08:47
Judging by your explanation, you understood what I mean.

That’s exactly what I thought as well.
Normally, you have the usable volume, and if it rains heavily, it fills up to the retention volume and then overflows at the top.
It then slowly drains down.

What’s the purpose of the float? That’s what I’m wondering too. Although, is it a float or just a restrictor?
Because the delayed discharge is already ensured if you set the restrictor accordingly.

According to my information, we need 1,000 liters per 100 sqm (1,076 sqft) of developed area.

However, I’m still clarifying whether it’s 1,000 liters per 100 sqm (1,076 sqft) of retention volume or usable volume.
The fact is, we have separate sewer systems for rainwater and wastewater and must ensure delayed discharge.
This is guaranteed by the system. Therefore, I think the 1,000 liters refer to the retention volume, since the usable volume probably doesn’t matter to the authorities and is only of interest to me.

We have a large garden and will probably go for 5,000 liters of usable volume plus 2,000 liters of retention volume.