To reiterate, your argument was that people in an anarchist society will, as a whole, not take on an ounce of extra work even if it means helping not poisoning their neighboring towns.
My argument was that there are social and material benefits to taking on that extra work of cleaning their waste water, and I believe most people would not feel good poisoning their neighbors unless they were extremely uneducated or sociopathic.
You believe that the only solution to that scenario is to have a hierarchical government with a big stick to force that group to clean their waste water.
I believe that most places under an Anarchist society would, on the whole, not need the stick. There may be some infrequent specific scenarios where even under Anarchism, that a settlement acts as you suggest. In those cases, especially if the harm was severe or could not be mitigated, the community(s) effected down river may opt to resort to other means to resolve that conflict, such as social shaming, or potentially even sabotage. Then that population would have to decide between the extra time and work that defense against them would require, or just clean the damn water.
Perhaps reread Chapter 9 of The Conquest Of Bread.
I don't see how it conflicts with what I've been suggesting?
In short, the five or seven hours a day which each will have at his disposal, after having consecrated several hours to the production of necessities, will amply suffice to satisfy all longings for luxury however varied. Thousands of associations would undertake to supply them. What is now the privilege of an insignificant minority would be accessible to all. Luxury, ceasing to be a foolish and ostentatious display of the bourgeois class, would become an artistic pleasure.
Every one would be the happier for it. In collective work, performed with a light heart to attain a desired end, a book, a work of art, or an object of luxury, each will find an incentive, and the necessary relaxation that makes life pleasant.
In working to put an end to the division between master and slave we work for the happiness of both, for the happiness of humanity.