Ok, so you have people willing to work at the wastewater treatment plant. What happens when the Reverse Osmosis pump gives out? Costs $500,000 to replace. Whose going to pay for that? Wait, sorry I forgot we're in an anarchist society so supposedly no money (if there is money, add on a whole other layer of complexity to the following questions).
So who's going to build the pump? People willing to work at the pump factory? Ok, where do they get the materials to build it? I'm assuming none of this is local because logistically that's practically impossible, so who delivers the materials to them? The pump factory is unlikely to be next door to the wastewater treatment plant, so how is the pump delivered? Who is the specialist that installs the pump? Who makes sure it's done safely and correctly? Are there consequences if it's done in a way that doesn't result in clean water?
That's the thing, anarchism seems great whenever everything is working and everything is already in place. The moment something big breaks, anarchism just doesn't provide enough resources to get it fixed. We would need a post-scarcity society before we could move to something like that.