This is the waterbassin " Lange Vlieter" Today it was clear in colors how it works.
The water is entering in the left bassin ( 8 ha) from he canals. It is purifying and when it meets the average standard purification, it is transferred to the right side bassin ( 120 ha) where it stays for about 2 years. In the right bassin they control the progress of purification till it is ready as drinking water. The barrels in the middle controle the process..Of course no swimming, no boats but a peaceful " lake" were only ducks, rare birds and nature exists. I love this place and it is about 7 km to walk around.
A great project of Dutch watermanagement.