Cities: Skylines

so i managed to get this achievement just by placing four of them down meaning it is deffo bugged

but if you just look under the water tab you should see the new inland water treatment plants

simply delete your old water outlets if you are using a current city , then place the new ones and pretty much just have to wait from there

here is a video guide showing you how to do it

Sounds like a simple guide, but I'll write one anyway!

Instead of placing down sewage outlets that flow into your rivers, use the new inland ones. Be warned that they create ground pollution, so I would place these within your industrial area or next to a recycling centre (from Green Cities DLC).

Alternatively, if you have a previous save, load this up and convert your water-based treatment into inland ones. With a city of 60-70k, this should unlock quickly.

As a side note, this may be bugged and simply unlock immediately even if you haven't met the requirement.