It's possible to have a fish tank where you never need to change the water simply by having plants that consume the fish excrete and balancing how much food you put into it. What if we applied this same concept to human cities? Instead of "treating" the sewage with the current facilities, we'd have a massive flood plain filled with duckweed and other fast growing aquatic plants. If there was excess nutrients --- nitrites, nitrates, phosphates, etc --- on the duckweed pond output, we'd simply have a mandatory population-wide fasting period, which would inevitably decrease the amount of nutrients being dumped into the pond. The duckweed pond would also serve to capture atmospheric CO2.