## 4 Discussion  
On the basis of the performed rainwater harvesting system analysis, the use of rainwater may be a feasible alternative to tap water for hydroponic lettuce cultivation located in Wroclaw only during the summer months, in particular in July and August. For example, in the case of a tank size of 12 m\({}^{3}\) during those months, the system efficiency equals 69.6% and 56.3%, respectively. In the winter months, the rainwater harvesting system efficiency is 23.6% (in December), 29.5% (in January), and 23.1% (in February). The smaller tank provides lower efficiency and minimal payback period, which equals 13 years both for 6 m\({}^{3}\) and 9 m\({}^{3}\) tanks. By using those reservoir sizes, respectively \(\copyright\)2292 and \(\copyright\)2279 may be saved over 20 years. The larger tanks will provide greater buffer capacity and reduce the amount of discharged water to the sewer. The final decision should be made on the basis of a trade study taking into account all the aspects (economic and ecological analysis, the location of the indoor farm, etc.). One must emphasize here that for such selected water reservoir parameters, in practice, the rainwater only goes into the sewage system when there is very heavy precipitation, with daily totals exceeding 40 mm. This allows the reservoir to hydraulically relieve the rainfall drainage system, in this way reducing its hydraulic overload. Such a solution significantly reduces the pluvial flash floods phenomenon [31; 32; 33], as rainwater management is one of the most efficient methods of limiting city floods occurring in Poland during the warmer half of the year. In this study, for the rainwater harvesting system, the 6 m\({}^{3}\) tank volume is recommended to obtain low \(PBP\) and the highest \(NPV\). Due to the rainwater recovery system giving the best results in the warmer half of the year and the exhaust air water recovery system working best during the colder half of the year, an attempt has been made to combine these solutions. An initially selected volume of a 12 m\({}^{3}\) reservoir allows, on the one hand, during heavy precipitations, for the accumulation of water in the reservoir for a few days, but on the other hand, due to a \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Tank** & **Costs, \(\mathbf{\xi}\)** & **Water-Saving,** & **Annual \(E\), \%** &
\begin{tabular}{c} _NPV_ \\ **(20 Years), \(\mathbf{\xi}\)** \\ \end{tabular} & _PBP, Year_ \\ \hline \multirow{3}{*}{6} & \multirow{3}{*}{3099} & 301 (min.) & 80.0 & 7644 & 8 \\  & & **328 (av.)** & **87.1** & **8615** & **7** \\  & & 349 (max.) & 92.5 & 9356 & 7 \\ \hline \multirow{3}{*}{9} & \multirow{3}{*}{3307} & 311 (min.) & 82.6 & 7789 & 8 \\  & & **336 (av.)** & **89.2** & **8686** & **8** \\  & & 355 (max.) & 94.2 & 9379 & 7 \\ \hline \multirow{3}{*}{12} & \multirow{3}{*}{3461} & 316 (min.) & 84.0 & 7820 & 8 \\  & & **340 (av.)** & **90.4** & **8698** & **8** \\  & & 358 (max.) & 95.1 & 9346 & 8 \\ \hline \multirow{3}{*}{15} & \multirow{3}{*}{3806} & 319 (min.) & 84.8 & 7583 & 9 \\  & & **344 (av.)** & **91.4** & **8488** & **8** \\  & & 361 (max.) & 95.9 & 9108 & 8 \\ \hline \multirow{3}{*}{18} & \multirow{3}{*}{3960} & 322 (min.) & 85.6 & 7536 & 9 \\  & & **348 (av.)** & **92.4** & **8458** & **9** \\  & & 364 (max.) & 96.7 & 9061 & 8 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: \(NPV\) and \(PBP\) values related to each tank size for the combined system. large water demand (1030 L/day), there is no possibility of the water remaining in the reservoir for a dry period longer than 11 days. Such a solution allows us to maximize the volume of the accumulated water, simultaneously reducing the operational issues related to the quality of the accumulated water. The proposed combined system ensures efficiency of 100% during 7 months of operation (from Jan to May and in Nov and Dec). During those months, the water recovery exceeds the yield significantly. In January, the system presents a maximum water recovery (about three times higher than the yield and water demand).