Incremental infrastructure transitions towards cities as forces for good in the environment

As source-separation has been proposed as a sustainable alternative to the current wastewater treatment strategy, the improvement in sustainability associated with such transition needs to be evaluated. However, given the difficulties and uncertainty in both technological and social aspects, such transition will most probably be happening gradually, step by step. Here with a computational case study based on the city of Atlanta within the Upper Chattahoochee watershed in the southeastern United States, we simulated three steps of transition, i.e. 50%, 70%, and 100% ANS-separation (Anthropogenic Nutrient Solution) is reached. The economic sustainability of these transition strategies was evaluated with total annual economic cost (TAEC), and the environmental sustainability was evaluated with three indicators, i.e. ecological footprints (EF), flux of materials passing through the city in its context of global material cycles, and pulse rate in terms of the spectrum of disturbance frequency to which the city is subject. The simulation results showed that compared with the current strategy, the ANS-separation has significantly lower TAEC, lower EF, lower pollutant discharge, higher recovery of nutrient and energy, and more beneficial manipulation of perturbation regimes of the city´s environment. These advantages increase with the rate of ANS-separation.