It gets quite busy, so to balance the numbers of people swimming vs how the natural system can cope with cleaning the water, there is a countdown of people allowed in at any time. Your maximum time in there is about 45 minutes, but nothing to stop you going back in of course. As this is Belgium there are also change rooms and lockers (bring a euro coin to use these).
The pool itself is very natural – a little mossy, and with the freshest of water. I’d almost say you could drink it but that is probably not advised.
The filtration system is “natural” – meaning that the water runs through a series of feeder ponds.