So I planned to dig a hole in the grond and to establish a natural swimming pond, not for Koi but for kids. I read a lot of articles about filtration and water circulation and my worry was the energy consumption to turn around the water through the filtering plant beds.
Until I stumbled on this forum which gave me a bit of eureka feeling.

My plan is to have a deeper swimming zone of 3X9m and 1,5m deep resulting in nearly 40m3 plus an undeep plantbed filtration zone of 24m3.
Now I will have no fish so the recirculation requirements are less severe as with a koi pond I read. If I turn around the water once every 4-5 hours this should be enough to maintain equilibrium... so a 15m3/hr waterflow would be sufficient. Unless anybody with more experience tells me it is not so

The pond would be all communicating vessels, with no height difference (no waterfall) and just a 300micron screen sieve before the airlift coming from the water suction collector and a skimmer. Although there are no trees around I want to avoid organic debris to collect in the pond and remove it as soon as possible. So water will be collected from the bottom of the swimming zone + the skimmer, pumped drought the sieve and a 2m long airlift per this forum's design to a diffuser in the filter bed with plants, whereby the water will flow through the gravel bottom-up to the surface, which is in direct communication with the swimming part, and where it will be flowing direction pump again through the swim zone.
I attached the principle in a drawing.
I got the idea to suck the water at the bottom of the pond through a collector made of many drilled PVC pipes from an American pond enthusiast who built this set-up in the gravel at the bottom of his pond, and who emptied his pond after 22years (!) with the intention to clean the sludge out of his old gravel, to find out at his surprise that it was perfectly clean !
The same principle applies to the filterbed: if you have the water migrate from the top through the bottom, organic debris will collect at the bottom requiring -every so many years- to clean the whole gravel. And I do not want to do this! So reverse filtration (bottom-up) has a "self-cleaning" action on the bottom of the gravel which should reduce the sludge build-up.
As there is a height difference in bottom between the swim area and filter area my question is how this might affect the working principle of the airlift?
I thought the efficiency might be impaired but looking at the full plexiglass airlift demo pump, it also works with a height difference in return pipe...
Any feedback would be greatly appreciated before I start digging !