**3. Results**

As previously introduced, two main scenarios were investigated, the first one involving only introduction of the air storage unit, composed of an air compressor and a storage tank, to allow air accumulation during the off-peak periods and air utilization from the air tank during the peak periods for sustaining aeration in the biological basin. The second approach includes AD introduction, with electricity production in a CHP unit and integration of the produced electricity in the storage system to increase both total energy savings and the use of renewable energy sources. This solution is particularly indicated in medium and large-scale plants, where AD is typically already implemented.

The simulations were carried on as explained before using a 32 GB RAM, i7 4770 3.40 GHz PC. A population of 500 individuals and 250 generations was adopted, resulting in 125,000 total evaluated designs for both scenarios, sufficient to obtain the convergence of the process in about 2–3 h.

#### *3.1. Scenario 1: Compressed Air Storage without Anaerobic Digestion*

In Figure 3, the influence of the selected input parameters (Table 1) on the economic convenience of the proposed storage system is summarized. It can notice that air tank volume has a strong impact on NPV, with a significant increase in the economic income as the volume increases (up to 1000 m3). The air storage pressure is shown to have a limited influence on NPV, with a slight increase in the economic convenience as a higher air pressure is selected. As for wastewater characteristics, the influent COD concentration and the wastewater flowrate have a mild effect: a more polluted effluent (meaning a higher internal energy) and a higher plant potentiality are slightly favorable for storage tank installation. Moreover, a linear behavior was encountered by analyzing NPV variation with respect to the influent COD concentration, for the wide chosen range of wastewater flowrates. In the most favorable conditions, PB time was lower than 1 y for Scenario 1, highlighting a significant convenience of air storage system installation, given the actual market economic conditions and the investigated plant characteristics.

**Figure 3.** Influence of the main input parameters (wastewater flowrate and influent COD, air storage tank pressure and volume) on the NPV of the proposed compressed air storage system (Scenario 1).

The detailed analysis of NPV variation with respect to air storage pressure, reported in Figure 4, interestingly highlights that NPV increases to a maximum at an intermediate pressure value, while a

decrease is observed for a further pressure augmentation (in particular considering larger tank volumes); this is due to the fact that for a higher vessel pressure, the specific compression power increases, leading to higher compression costs.

**Figure 4.** Influence of air storage tank pressure and volume on NPV for different wastewater flowrates (Scenario 1).

#### *3.2. Scenario 2: Compressed Air Storage with Anaerobic Digestion*

In the second scenario, AD introduction is included, with biogas valorization through the CHP unit, able to supply a share of the needed electricity. The analysis of the influence of single input parameters on NPV value (Figure 5) shows that a similar behavior to that encountered in Scenario 1 (Figure 3) is obtained, even if influent wastewater flowrate has a stronger influence, due to the fact that AD becomes more convenient for a higher plant potentiality.

**Figure 5.** Influence of the main input parameters (wastewater flowrate and influent COD, air storage tank pressure and volume) on the NPV of the proposed compressed air storage system (Scenario 2).

Scenario 2 is particularly representative of medium and large-scale plants, where AD is already applied on full-scale, with biogas valorization: the integration of locally produced electricity with air storage system would optimize energy saving. In Figure 6, NPV and primary energy saving (PES) variation with respect to the influent flowrate was depicted: the maximum obtainable NPV was in the range of EUR 6–10 M, while PES was in the range of 36–108 toe/y for the different analyzed plant potentialities.

**Figure 6.** Influence of the treated wastewater flowrate on NPV and PES indices (Scenario 2).

It could be seen that the investigated scenarios had a comparable behavior with respect to the considered input parameters. The proposed CAS system was shown to be technically and economically feasible. Given the cost-effectiveness of the solution, it is now possible to move on to a more in-depth study to analyze the commercially available technical devices for further system optimization.
