There are plenty of cities in the world with a big ol’ dirty artificial lake in them. Usually brown in colour, not great to swim in. I was thinking about how you could create a clean pool for swimming in a great big dirty lake – sort of similar to the rock pools you find on the coast.
One thing I was thinking about is the amazing terracotta water purification systems they use in 3rd world countries. It’s literally a terracotta bowl with water in it, and a tub underneath. The terracotta is porous so it lets the water through but amazingly keeps the bacteria out. Can we build one of these at scale? A massive terracotta (or some other modern equivalent) which is essentially porous, you drop one in, it fills, via water seeping in through the sides, there is a pump in the middle of it which is constantly pumping water out at the same rate that it seeps in through the sides, hence it is always clean.
Another method is to have a ziggurat of biological water filtration systems built to enclose a bay of the lake. Water gets into the bay by being pumped to the top of the ziggurat and going through levels of biological filtration, and then returns back to the main body of the lake through the sand. Over time the water on the inside of the enclosed area gets cleaner and clearer until you’ve got this little pristine bay.