As the title says, something that can turn badwater into clean water instead of just diverting it. I think it would be a great addition as it further satisfies that desire to fix/clean the Earth.
Building qualities can be one or few of listed below:
-modular 1x1 filtration blocks. Possibly like a sluice but cannot allow the same amount of water(lower cms) to pass through
-a big building that can take in pumped badwater and release it as clean water
-a chain of buildings to allow such water processing
-would require resources and power to keep in operation

I think such a tech would be in theme with the game. Also, it would be a great addition as it rewards the player with even more water, specially in hard mode, using fully vanilla methods