## 3 Results
### Cost-Benefit Analysis  
An economic analysis of the RWH system was carried out in order to investigate the balance between the investment/cost for system purchase and installation, and the benefits obtained by the rainwater use for indoor hydroponic lettuce cultivation. To this aim, a schematic underground installation of a RWH system was considered, consisting of a pre-fabricated concrete tank provided with a first flush device, a manhole with a rainwater filter, a pumping system, and its Programmable Logic Controller (PLC) equipment, the drainage piping system inlet and outlet, the tank, and the piping distribution system to supply the rainwater for the analyzed uses. Table 1 summarizes the costs of the RWH system elements for each tank capacity. These costs were obtained by means of a market survey. In the case of the combined system, the costs are higher, since the installation of water collection from the exchanger must be included. Figure 6: Mean monthly characteristics of individual rainwater recovery system. For tank capacity \(S=12\) m\({}^{3}\). (**A**) Water demand, rainfall volume, and yield. (**B**) Monthly variation of system efficiency. Figure 7: Mean monthly characteristics of combined water recovery system. For tank capacity \(S=12\) m\({}^{3}\). (**A**) Water demand, rainfall volume, and yield. (**B**) Monthly variation of system efficiency. In the present analysis, the costs related to system maintenance were considered negligible when compared to purchase and installation costs [27]. With regard to the benefits related to the RWH system installation, only the financial benefits due to the potable water saving were considered. In this analysis, environmental and social benefits were not accounted for. Two performance indicators, the Net Present Value (_NPV_) and the payback period (_PBP_) were evaluated [27; 28]. In the analysis, the following assumptions were made:  
* The evaluation period to assess the _NPV_ was set equal to 20 years [28; 29];
* according to [30], a discount rate of 5% had been assumed;
* the inflation rate had been assumed equal to 4.5% (on the basis of the inflation rate of potable water price in Wroclaw in recent years);
* the actual price for potable water had been set equal to 1.2 \(\upxi\)/m\({}^{3}\) (the actual prices adopted by water utilities operating in Wroclaw). The effect of the variability of annual yield had been accounted for in the _PBP_ and the _NPV_ appraisal, considering the minimum, the maximum, and the mean annual yield. Results of both systems analysis are shown in Tables 1 and 2 for each tank size. It can be seen in Table 1 that from the perspective of saving water when collecting rainwater, the smaller reservoir had the lowest annual average \(E\) = 36.8% and the largest, the highest \(E\) = 39.8%. Table 1 also summarizes the average payback period of a tank, which equaled 13 years (bolded) both for 6 m\({}^{3}\) and 9 m\({}^{3}\). By using those reservoir sizes over 20 years, respectively \(\upxi\)2292 and \(\upxi\)2279 may be saved. The performed analysis showed that tanks larger than 9 m\({}^{3}\) were associated with a longer payback period and a lower _NPV_, which in the case of a volume 18 m\({}^{3}\), the minimum value was \(\upxi\)1841. The annual \(E\) and water-saving increased with the tank size increment, hence for only rainwater recovery, including the economic indicators, the reservoir 8 m\({}^{3}\) was proposed. The selected economic efficiency indicators for the combined water recovery system are presented in Table 2. In general, the combined system reached much more favorable values of _NPV_ and _PBP_ than the system with only rainwater recovery. According to the presented data, the most economically profitable solution in terms of _PBP_ was using tanks with smaller volumes (e.g., 6 m\({}^{3}\)). The cost of the tank depends on its size and the smaller tank has a cheaper price, hence a system with a 6 m\({}^{3}\) tank over 20 years saved \(\upxi\)615, and it had the shortest payback period equal to 7 years (bolded).