*2.1. System Configuration*

Based on a load shift approach, a DR and net zero energy (NZE)-oriented system was proposed for WWTPs. As previously introduced, the aeration during biological wastewater treatment accounts for more than 30% of the plant's total energy consumption. Instead of shifting wastewater treatment to periods characterized by lower energy costs (leading to the need for large storage tanks or equalization basins, not always practically feasible) [1], the possibility of storing the air necessary for the biological treatment was considered. This solution (schematically represented in Figure 1) allows one to store energy in a compressed air tank during low energy cost (o ff-peak) periods, utilizing it when the cost increases (peak periods). Furthermore, the possibility to use the electricity from biogas (locally produced in AD process) by cogeneration was analyzed. This second scenario would allow one to cover a significant share of the process electrical load, while the heat production could be used to entirely cover plant heat demand.

**Figure 1.** Wastewater treatment plant process flow.

As a first-approach study, the hypothesis of steady state process was made along the whole plant and throughout the di fferent proposed scenarios.
