**1. Introduction**

The storage and use of rainwater, while providing environmental benefits, can also be an investment to reduce potable water costs. The economic benefit of using rainwater has been addressed in several studies, varying the place of study, building and project type, among other characteristics. Ghisi and Schondermark [1] estimated the potential for potable water savings and performed an economic analysis for single-family homes in five cities in the state of Santa Catarina, Brazil. They obtained variable results depending on the water demand and found that, in most cases, the implementation of the system would be economically feasible.

Morales-Pinzón et al. [2] assessed the economic feasibility of a rainwater harvesting system in Spain. Several types of houses were chosen, covering most of the climates in the country. They observed that rainwater harvesting systems had shorter paybacks. In Italy, Liuzzo et al. [3] analysed the economic feasibility of a rainwater harvesting system in a house in Sicily, with a catchment area of 180 m2. Rainwater usage was considered only to flush the toilet and for irrigation. The system proved to not always be feasible, with a payback period ranging from 15 to 55 years.

Such studies show that the economic feasibility analysis must be conducted on a caseby-case basis, as it depends especially on water demand, rainfall, water tariff, costs, and catchment area. Blumenau is one of the most populous cities in Santa Catarina; and 80% of the households are single-family houses [4]. These factors, added to urbanisation and, sometimes, heavy rains, make the city prone to flooding [5]. Thus, the main objective of this work is to evaluate the potential for potable water savings and to perform an economic analysis considering rainwater usage in single-family houses in Blumenau.

**Citation:** Fugi, A.M.; Maykot, J.K.; Ghisi, E.; Thives, L.P. Economic Feasibility of Rainwater Harvesting in Houses in Blumenau, Brazil. *Environ. Sci. Proc.* **2023**, *25*, 56. https://doi.org/10.3390/ECWS-7- 14163

Academic Editor: Luis Garrote

Published: 14 March 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

### **2. Materials and Methods**

The study area was Blumenau, in Santa Catarina state, southern Brazil. A case study was performed in a three-storey single-family house with a roof area of 165 m<sup>2</sup> and four people living in the house. As it is a high-standard building, different scenarios of water end-uses were considered to represent the houses in Blumenau.

### *2.1. Water Consumption and End-Uses*

In order to estimate the water consumption and end-uses, questionnaires were given to the four residents. The questionnaires were left close to each fixture, allowing residents to write the frequency and duration of use of each fixture. For a washing machine, the water level was recorded; for a bowl-and-tank toilet, only the number of flushes per day was recorded. The water consumption measured in the water meter was also registered at the end of each monitoring for comparison purposes. This monitoring was performed over seven days (25–31 August 2019). More details, such as flow rate measurements, are presented by Fugi [6]. Based on the frequency and duration of use of each fixture and the corresponding water flow rate, each water end-use and total water consumption were calculated.

### *2.2. Computer Simulations*

The computer programme *Netuno*, version 4, is capable of performing simulations of rainwater harvesting systems [7]. In this study, the programme was used for sizing the rainwater storage tank, estimating the potential for potable water savings and performing the economic feasibility analysis of the three-storey house and the different scenarios.

Rainfall data for Blumenau were obtained from the Brazilian Water Agency [8]. A first flush equal to 2 mm was adopted as recommended in the Brazilian standard NBR 15527 [9]. Due to losses during rainwater harvesting, a runoff coefficient of 0.8 was adopted. The roof area of the house under study is approximately 165 m2. For the different scenarios, roof areas equal to 60, 100, 140, and 180 m<sup>2</sup> were adopted. Such values were based on the frequency of areas of Brazilian roofs indicated by Ghisi et al. [10].

The number of residents per household has a major influence on water consumption. For the scenarios considered, 2, 3, 4 and 5 persons were adopted per house; this represents 84.3% of households in Blumenau [11].

The upper tank was sized based on the daily rainwater consumption in each house and scenario, and the sizes were chosen according to availability on the local market. For sizing the lower tank, the minimum and maximum capacities were defined as 500 litres and 20,000 litres, respectively. The programme indicated the capacity to be chosen when the increase in the potential for potable water savings was lower than or equal to 3.5%/m3.

The total water demand was estimated based on the water consumption and number of residents in the house. Water consumptions equal to 100, 150 and 200 litres/person/day were adopted for the different scenarios. Finally, different rainwater demands were estimated based on the actual house's water end-uses and studies found in the literature: 30%, 40%, 50% and 60% of the total water demand were adopted. In the analysis for the actual house, the water end-use for non-drinking purposes was considered as the rainwater demand.

### *2.3. Economic Analysis*

To perform the economic analysis, the costs of implementing the rainwater harvesting system, water consumption and system operation were obtained. Then, the financial savings regarding the rainwater harvesting system were calculated, i.e., the difference between the water bill with no rainwater harvesting system and that with a rainwater harvesting system. Finally, discounted payback, net present value and internal rate of return were calculated.

The costs of the water tanks and motor pumps were obtained from stores in Blumenau, and the lowest prices found were considered. In order to estimate labour costs, the Brazilian

System of Research on Costs and Indices of Civil Construction was used [12]. The costs of pipes, connections and accessories represented 19% of the total cost related to labour, water tanks and motor pumps [13].

In turn, the energy cost for the pump operation was estimated based on the energy tariff—which was BRL 0.46978 per kWh, according to the local electric utility [14]—and the power and operation of the motor pump. All cash flows from the investment project were brought to day zero, considering the minimum attractive rate of return (MARR). A positive net present value (NPV) indicates that the system is economically feasible. The discounted payback represents the time when savings from using rainwater are equal to the initial investment. The IRR must be higher than the MARR to make the investment feasible.

The minimum attractive rate of return adopted was 0.5% per month, and the analysis period was 20 years. Once it became impossible to predict the future monthly inflation, a constant figure of 0.274% per month was considered.
