# Towards Sustainable Water Use in Two University Student Residences: A Case Study
## 2 Materials and Methods
### Water Use Efficiency Measure Impact  
Considering the obtained results for water consumption, suggestions for more efficient devices were proposed to achieve higher water efficiency, according to the Portuguese Labeling System [36], which promotes water-saving technologies as a crucial aspect of national water management strategies. According to this system, water devices are classified from A to E, with \"A\" indicating the highest efficiency and \"E\" representing the lowest. The proposed measures were organized into scenarios (Table 2).  
Energy consumption by showers and taps depends on the amount of hot water consumed by the users [37; 38; 39; 26; 31]. According to [37; 38], the percentage of hot water consumed in relation to all the water used can be considered 100%, whereas for kitchen taps, it corresponds to 56% of all consumed water. Therefore, before and after the simulation of Scenario 1, the energy consumption associated with hot water consumption by showers and kitchen taps was calculated according to the following equation:  
\[Energy\ consumption\ (kWh)=\frac{\rho\times V\times C_{p}\times\Delta T}{3.6 \times 10^{6}\times\eta} \tag{3}\]  
* \(\rho\): water density (\(\rho\) = 1000 kg/m\({}^{3}\)).
* \(C_{p}\): specific heat of water (\(C_{p}\) = 4187 J/kg \(\times\) K).
* \(V\): consumption of hot water/resident (L).
* \(\Delta T\): Denotes the difference between the cold water inlet temperature and the hot water outlet temperature (\({}^{\circ}\)C). According to the long-term climate series [40], the average annual water temperature in Braganca was estimated at 12.3 \({}^{\circ}\)C. The water heating system raised the temperature to 60 \({}^{\circ}\)C, resulting in a temperature variation of 47.7 \({}^{\circ}\)C.
* \(\eta\): efficiency of the heating system, which in the present study is 2.97 for both residences.  
In Scenario 2, the inclusion of a RWHS was simulated to recharge flush cisterns, as the water used in these devices can be sourced from non-potable supplies. Since this is a hypothetical scenario, an approximate sizing method was applied, as suggested by [PERSON] et al. [41], assuming uniform discharge over time and excluding considerations of heavy rainfall or prolonged droughts [42; 43]. The design of the rainwater storage structure adhered to the criteria and procedures outlined in the Technical Specification ETA 0701 [42], with the volume of usable rainwater estimated using Equation (4).  
\[V_{a}=C\times P\times A\times\eta_{f} \tag{4}\]  
* \(V_{a}\): volume of rainwater in the reference period (L);
* \(C\): runoff coefficient (dimensionless);
* \(P\): average precipitation accumulated at the site (mm);
* \(A\): catchment area (roof) (m\({}^{2}\));  
\begin{table}
\begin{tabular}{c c l} \hline \hline \multicolumn{1}{c}{**Scenario**} & \multicolumn{1}{c}{**Measures**} \\ \hline \multirow{3}{*}{1} & \(\bullet\) & Washbasin mixer tap replacement with more efficient ones (5 L/min) \\  & \(\bullet\) & Kitchen tap replacement by one more efficient (8 L/min) \\  & \(\bullet\) & Installation of shower flow regulator devices \\ \hline \hline \end{tabular}
\end{table}
Table 2: Proposed water-saving scenarios in both residences.  
\(\eta_{f}\): hydraulic filtering efficiency (dimensionless).  
The C value was set at 0.9, which is typical for ceramic tile roofs, and the \(\eta_{f}\) value at 0.9, representing the maximum hydraulic filtration efficiency [42]. The average monthly precipitation was statistically estimated using records from 2013 to 2023 [35].  
Concomitantly, the economic viability of each proposed water efficiency measure was assessed. This involved evaluating the initial investment required to implement the measures, the potential decrease in water bills due to improved water efficiency, and the timeframe for investment turnover. In the present study, the inflation rates were not considered. The cost of the associated works for the RWHS installation was not determined in the present approach, so in Scenario 2, the investment turnover was not estimated.