*3.3. Rainfall*

In order to carry out the study, daily rainfall data from the Blumenau rain station from February 1989 to January 2019 were considered, and the average annual precipitation in this period was 1770 mm. The maximum, minimum and average monthly rainfall for Blumenau are shown in Figure 1. From September to March, monthly rainfall was higher than the average (147 mm). In November 2008, rainfall was 1001 mm, the year with the most significant flood over the last 17 years.

**Figure 1.** Maximum, minimum and average monthly rainfall for Blumenau over 30 years.

### *3.4. Potential for Potable Water Savings*

Regarding an actual house, the upper rainwater tank capacity was estimated at 363.4 litres. In this way, an upper 500-litre tank was adopted. The ideal capacity for the lower tank was 5000 litres, and the corresponding potential for potable water savings was 50.32%. The house's water consumption was 612.8 litres/day, so that the rainwater system would provide 308.4 litres of rainwater per day. The average monthly consumption of 18 m<sup>3</sup> decreased to 9 m3. This way, the owners paid only the minimum monthly consumption fee, which is 10 m3.

Considering the different scenarios simulated, for a potable water demand of 100 litres/person/day or more and equal number of residents, the results were similar. The different roof areas showed little influence on the potential for potable water savings. The rainwater collected from the roof shows that the roof area meets the rainwater demand, so there is no need for a large roof area when the rainwater demand is low. However, as the rainwater demand increases, the roof area significantly influences the potential for potable water savings. The larger the roof area, the smaller the lower rainwater tank capacity. This occurs because the larger the roof area, the more rainwater is harvested, and the replenishment of rainwater in the tank is faster. Similar results were obtained in [1] and [15].

The potential for potable water savings ranged from 18.76% to 58.06%, with an average of 37.90%. As in the study of Lopes et al. [16], it was observed that the larger the rainwater demand and roof area, the greater the potential for potable water savings.

### *3.5. Economic Analysis*

The financial analysis of the implementation and operation of a rainwater harvesting system for the house resulted in the following indices: a net present value of BRL 4814.54, a payback period of 89 months and an internal rate of return of 1.44% per month.

From the 192 different scenarios analysed, 112 scenarios obtained positive net present values, indicating that the rainwater system would be economically feasible for 58.3% of the cases. Payback ranged from 221 to 60 months for economically feasible scenarios. The highest internal rate of return was 2.05% per month.

The scenarios with low water consumption proved to be economically unfeasible. Such infeasibility is due to the flat rate for monthly consumption of up to 10 m<sup>3</sup> of water. Once there is no charge reduction in the water bill but there is still an expenditure of energy for the operation of the pump, the net present value becomes higher than the initial cost. These results were also found by Berwanger and Ghisi [17].

The feasibility analysis showed that the greater the water consumption and the greater the rainwater demand, the more economically feasible the rainwater harvesting system. Figure 2a shows the number of scenarios in which the NPV was positive or negative as a function of the water demand. For water demand equal to 150 and 200 litres/person/day, the NPV was positive for 75% of the cases. For consumptions of 100 litres/person/day, only 25% of the cases had positive NPVs. Figure 2b, in turn, shows the number of scenarios in which the NPV was positive or negative as a function of the roof area. One observes that the scenario number does not vary as a function of the roof. Thus, the roof area showed no influence on the economic feasibility of rainwater harvesting systems.

The greater the number of residents, the more positive NPVs were obtained. This influence is directly related to water consumption. Figure 2c shows the number of scenarios in which the NPV was positive or negative as a function of the number of residents. For

scenarios with two residents, all NPVs were negative; however, for five residents, all NPVs were positive. In 66.6% of scenarios, the NPV was positive for scenarios with either three or four residents. The NPVs were shown to be equally distributed for each rainwater demand. For all demands, the NPV was positive for 58.3% of the scenarios. Figure 2d shows the number of scenarios in which the NPV was positive or negative as a function of the rainwater demand.

As in Ghisi and Schondermark [1]'s study, economic feasibility is directly related to the number of residents and water consumption per capita. Homes with a low number of residents and/or low water consumption should use the rainwater harvesting system only for environmental benefits, not economic ones. Figure 3 shows the NPV as a function of the rainwater demand (in litres/day) for all scenarios. For houses with rainwater demand equal to 60–120 litres/day, all scenarios proved to be economically unfeasible. In cases where the rainwater demand was greater than or equal to 250 litres/day, all scenarios proved to be economically feasible. For cases in which the rainwater demand ranged from 135 to 240 litres/day, it was found that economic feasibility does not have a trend. The absence of a tendency in such cases may occur because high water consumptions and low rainwater demands result in the same rainwater demand as a scenario with low water consumption and high rainwater demand, requiring analysis on a case-by-case basis.

**Figure 3.** NPV as a function of the rainwater demand for all scenarios.

The actual house obtained better economic rates than the scenarios with 140 and 180 m<sup>2</sup> of roof area and water demand equal to 150 litres/person/day and for the four residents and rainwater demand equal to 60% of the water demand. The comparison was made with these two scenarios, as they have the most similar characteristics to the house. For the scenario with a roof area of 140 m2, a payback period of 96 months was obtained, and for the 180 m2, a payback period of 93 months was obtained. The payback period for the actual house was 89 months, indicating better economic feasibility.
