I was wondering as 400 liters is not a lot of water (but 4000 is!). Include diatomaceous earth as the most fine filter (on top of the sand). Put in about 3-4 inches of thickness.
DE (as it is known) is cheap, very effective and traps bacteria, pollen as well as all algae. You do not need the food grade. Get the kind that goes into swimming pool filters.
After looking at this, you might want to reverse the trapping layers. The last filter element might be the DE, then the sand, then the pea gravel.
Think about it.
Let us know how this works out!
Steve
Steve
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consulting on geothermal heating/cooling & rational energy use since 1990
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