*2.3. Biological Treatment Unit*

The oxygen injected in the biological phase (*O*2,*cons*) was supposed to coincide with the oxygen consumed by biomass, estimated by considering a simplified approach (Equation (1)), as proposed in [30]. The operating parameters of the activated sludge process in Equation (1) were set as follows, considering typical full-scale plants in the analyzed territory, where a significant influent dilution is observed in sewers due to aquifer infiltration: mean influent COD concentration *(CODin*) = 250 mg/L, mean effluent COD concentration (*CODout*) = 50 mg/<sup>L</sup> (corresponding to 80% COD abatement), hydraulic retention time (*HRT*) = 7 h, solid retention time (*SRT*) = 15 d, biomass concentration in the biological basin (*X*) = 3.5 g volatile suspended solids (VSS)/L. The factor 1.42 represents COD conversion factor for biomass [30].

$$CO\_{2,cons} = \left(COD\_{in} - COD\_{out}\right) - \frac{HRT \times X}{SRT} \times 1.42\tag{1}$$

From the calculated oxygen need and considering oxygen share in the air (20%), it was possible to determine the total injected air flowrate. To evaluate the energy consumption of the biological treatment, the specific operating capacity of oxygen diffusers (fine bubbles) was estimated as 1.2–1.5 kg O2/kWh (data given from specialized companies in the field). Finally, total plant energy consumption was calculated considering that the electricity consumption for oxygen insufflation is 30–70% of the total plant energy need, as emerged from relevant literature studies [18].
