Technical and Economic Analysis of Strategies to Reduce Potable Water Consumption in a Library

Abstract

In Brazil, approximately 93 trillion litres of water are withdrawn annually from surface and groundwater sources, with urban human use being the second-largest water consumer. Therefore, reducing water consumption in buildings is crucial. This study performed a technical and economic analysis of isolated and combined water-saving strategies at the Central Library of the Federal University of Santa Catarina (UFSC). The strategies assessed included water-saving appliances, rainwater harvesting, and greywater and blackwater reuse, individually and in four combined scenarios. User surveys provided data on the frequency and duration of water appliance use and cleaning activities, while on-site water flow measurements enabled the estimation of water end uses. The potential for potable water savings was then determined for each strategy and scenario. The highest savings (77.96%) were achieved by combining water-saving appliances with blackwater reuse, followed by a combination of water-saving appliances, greywater reuse, and rainwater harvesting (65.73%). All strategies were economically viable, except the combination of water-saving appliances with greywater reuse, which showed a negative net present value. The scenario combining water-saving appliances and blackwater reuse generated the most significant financial savings (R$7782.48 per month), with a payback period of 50 months. Given its environmental and economic benefits, these scenarios were recommended for implementation. The study may be replicated worldwide, and one key conclusion is that libraries consume a significant amount of potable water for non-potable purposes, which should be supplemented with alternative sources. It is essential to consider whether the building is already built or under design, as some implementation processes require design modifications.

1. Introduction

Population growth in cities leads to a gradual increase in demand for water, accompanied by a corresponding reduction in urban water availability. Regions with abundant water resources also face restrictions on water consumption and use due to high demand, which impacts economic and social development, as well as the quality of life of the inhabitants [1]. In Brazil, approximately 93 trillion litres of water are withdrawn annually from surface and underground sources to meet various sectoral and multiple uses. Sectoral uses account for 70.0% of the country’s total annual water withdrawal, with irrigation being the sector that consumes the most water (49.8%), followed by urban human use (24.3%) [2].

In the context of urban human use, water consumption in buildings is a primary contributor and is linked to per capita water consumption and inappropriate use by users [3,4]. Leaks in the water supply system also account for a high share of the water produced, as highlighted by the National Sanitation Information System (SNIS). Water use in buildings is associated with high operational costs and corresponding environmental impact [5,6,7]. Studies have shown that up to 45% of the potable water consumed in households in Florianópolis is used for non-potable purposes. In contrast, for the public sector, non-potable uses account for 77% of potable water consumption, while for the commercial sector, the figure ranges from 56% to 86% [8,9,10].

Therefore, reducing water consumption in buildings is essential for both local public supply systems and users, ensuring a sustainable environment that is both environmentally and economically beneficial. Therefore, the goal is to ensure the preservation of the environment, as well as economic and social development, through the implementation of technological actions and alternative water source systems for non-potable use. In this sense, water-saving appliances, sectorised water consumption metering systems, rainwater harvesting, and greywater reuse are alternatives that can help save potable water in buildings [11].

Marinoski et al. [12], for example, conducted an analysis of isolated and combined strategies for saving drinking water in 20 single-family homes located in the metropolitan region of Florianópolis. The strategies analysed were the use of water-saving appliances, rainwater harvesting, and greywater reuse. The use of aerators on indoor and outdoor taps and a dual-flush system (3–6 L/application) resulted in a 28.9% reduction in water consumption. The use of rainwater and greywater resulted in reductions of 30.7% and 21.0%, respectively. Finally, the combined strategies achieved reductions of 42.9% for water-saving appliances and rainwater use, 36.8% for water-saving appliances and greywater reuse, 32.5% for rainwater use and greywater reuse, and 42.9% for the three combined strategies. Other Brazilian studies have also evaluated methods for conserving water in public, residential [13], and commercial [13,14] buildings.

Internationally, many researchers have evaluated ways to optimise water consumption, including structural measures such as greywater reuse [15,16,17], rainwater harvesting [18,19,20,21], and the use of water-saving appliances [22,23], as well as non-structural measures such as raising public awareness. Blackwater reuse has also been evaluated as a potential alternative for water-intensive buildings [24]. It is also vital to mention research on indirect consequences, such as the rebound effect [25,26], which studies the increase in water consumption based on users’ perception of efficiency. In such cases, as users become aware that the system is more efficient, they may become less cautious with water management, which ultimately leads to higher overall water consumption. In other words, several strategies are possible, and an integrated water management approach that addresses potential water-saving measures and utilises the best alternative for each location is of vital importance.

Therefore, given the need to save potable water in order to preserve water resources, it is pertinent to analyse the technical and economic feasibility of implementing various strategies to reduce water consumption in buildings. This study aims to apply a technical and economic evaluation of different strategies to a public building used as a central library at a Brazilian public university. The goal is to estimate the potable water saving potential from implementing structural strategies, including rainwater harvesting, greywater reuse, blackwater reuse, and the use of water-saving appliances.

The novelty of this work is the assessment of a little-studied typology, libraries, as well as a comparison of different methods for reducing water consumption. The aim is to provide a study that can serve as a reference and be replicated in other locations, offering methods for conserving water in libraries and achieving greater water efficiency, with consequent lower operational costs and reduced environmental impacts.

2. Literature Review

Public buildings, such as schools, hospitals, and government offices, often consume large volumes of water due to their large number of users and standardised usage routines [27]. Therefore, these facilities have great potential for water savings, particularly in restrooms, kitchens, and cleaning areas. Inefficiencies are commonly linked to outdated plumbing systems, leaks, and user behaviour, and implementing sectoral metering, water-saving appliances, and awareness campaigns are essential strategies for reducing waste and promoting a culture of responsible water use [28].

In this study, four strategies for library use were assessed: the use of water-saving appliances, rainwater harvesting, greywater reuse, and blackwater reuse. Other alternatives, such as the use of urban design to reduce water consumption, the refurbishment of buildings to prevent leakage losses, the use of technology for monitoring and control, as well as other possibilities, were not considered in this study. The alternatives were chosen based on similar strategies already applied in the city and the ease of finding suppliers. Future studies should explore alternatives beyond those evaluated in this research to identify optimal water conservation approaches for libraries.

Installing water-saving appliances is one of the most cost-effective approaches to reducing water consumption. Self-closing taps, dual-flush toilets, pressure-reducing valves, and dry or sensor-activated urinals are widely adopted technologies [29]. Their efficiency varies depending on usage patterns; however, reductions of between 30% and 60% have been reported in institutional settings. The selection of such appliances must strike a balance between efficiency, hygiene, and user comfort.

Rainwater harvesting systems are also a sustainable option for non-potable applications, such as toilet flushing, irrigation, and floor washing. These systems typically include gutters, filtration units, storage tanks, and pumps. The quality of rainwater can vary depending on the harvesting area and storage conditions, often requiring treatment with filtration and, in some cases, disinfection using chlorine or ultraviolet light [30]. The Brazilian standard NBR 15527 provides guidelines for the design and quality assurance of rainwater harvesting systems [31].

Greywater reuse—which involves recycling water from showers, sinks, and laundry—has gained traction in public facilities for applications such as toilet flushing and landscape irrigation [32,33]. This strategy requires separate plumbing systems, adequate treatment (physical, chemical, or biological), and ongoing monitoring to ensure compliance with water quality standards, such as NBR 13969 [34]. Parameters like turbidity, Biochemical Oxygen Demand (BOD), and coliform counts must be controlled to ensure user safety.

Both rainwater harvesting and greywater reuse have distinct advantages and challenges in terms of technical, economic, and social aspects. According to Rodrigues and Afonso [35], both systems are considered complementary measures for conserving potable water, with rainwater harvesting generally being simpler and less susceptible to health risks. At the same time, greywater requires greater technical control and care in terms of operation and maintenance. Leong et al. [36] point out that, from an environmental and economic perspective, domestic rainwater harvesting proved to be more financially attractive (USD 2.00/m³) and has a lower environmental impact, while greywater, although efficient, requires greater energy consumption and treatment to ensure the quality of the water. In terms of public perception, Stec [37] observed greater acceptance of rainwater use (54%) than treated greywater (39%), especially when it involves direct contact with the human body, highlighting substantial social barriers. Finally, Maskwa et al. [38] reinforced that greywater can meet up to 90% of non-potable demand in multi-family homes, surpassing rainwater harvesting, which varies between 50% and 70%.

The reuse of blackwater, which is recycled wastewater from toilets, poses significant challenges due to its high organic load and pathogen content. However, with appropriate treatment systems, such as anaerobic reactors followed by disinfection, blackwater can be reused for non-potable purposes such as restricted irrigation or flushing [24,39,40]. These systems require strict adherence to microbiological and chemical standards, often guided by environmental regulations (e.g., CONAMA—Brazilian National Environmental Council) and World Health Organization (WHO) recommendations. Although less common, some pilot projects have demonstrated the feasibility of safely reusing blackwater using advanced treatment technologies.

Previous studies in public institutions have demonstrated that combining strategies, such as installing water-saving appliances, reusing greywater or blackwater, and harvesting rainwater, can significantly reduce potable water consumption and operational costs. Case studies have reported savings of up to 70% [36,41]. These findings highlight the importance of integrating technological solutions with effective management and user education to maximise the sustainability and resilience of public infrastructure.

3. Methodology

The methodology focused on the study object, evaluated strategies, quantified potable water savings, and assessed the economic viability of the strategies. Figure 1 shows a flowchart of the methodology used, including the four main steps. First, the study object characterisation was presented, and then, the water-saving strategies were described, including the necessary systems. The necessary equations and simulations for assessing potable water savings potential were described, and the feasibility assessment and corresponding parameters were discussed.

3.1. The Library

The object of study is the central library of the Federal University of Santa Catarina, located in Florianópolis. It was constructed in 1968 to centralise the book collections of various faculties and improve information access. The library opened in 1976 and was expanded in 1996, serving around 3000 students and offering research guidance, digital inclusion, training, and netbook loans [42].

UFSC’s central library has two floors and a floor-plan area of 9134 m², featuring various administrative, academic, and support areas, including study rooms, an auditorium, laboratories, a canteen, restrooms, and service areas. The roof has a rainwater-drainage system with concrete gutters and PVC pipes connected to the rainwater gallery. To characterise the building, water meter readings were taken to estimate the water end uses, consumption was analysed, plumbing equipment was identified, and user habits were surveyed. Daily water meter readings were conducted between August 30 and 6 September 2023, and they were used to validate the average daily water consumption, which was then used to calculate the potable water savings potential by adopting alternative strategies.

The study area is located in a municipality between parallels 27°10′ and 27°50′ south and meridians 48°25′ and 48°35′ west. According to the Köppen-Geiger classification, the climate is humid mesothermal, characterised by hot summers and mild, windy winters. According to the Brazilian standard for bioclimatic zoning, the area is classified as 3A, with mixed and humid climates. The temperature ranges from 13 °C to 29 °C throughout the year, with an annual average of 20.8 °C. The rainy season runs from October to March, with the highest rainfall in January and February, and the dry season is from March to October, with August being the least rainy month. The average annual rainfall is 1506 mm/year.

Regarding possible interventions in the building, four isolated strategies and four combined scenarios were considered. The isolated strategies were the implementation of water-saving appliances, rainwater harvesting, and greywater and blackwater reuse, with the following combined scenarios:

(1)

water-saving appliances + rainwater harvesting;

(2)

water-saving appliances + greywater reuse;

(3)

water-saving appliances + blackwater reuse;

(4)

water-saving appliances + greywater reuse + rainwater harvesting.

The choice of combined systems took into account the use of water-saving appliances with the other alternatives (1, 2, and 3), as it is the least expensive measure and allows for easy installation with the other systems. This choice modifies the water end uses, and the end uses for the combined scenarios were re-evaluated. The last scenario (4) uses all strategies except for blackwater. This selection was made because the blackwater system is expensive and complex, and is not an attractive alternative in combination with the other alternatives. All systems were considered to have a useful life of 15 years, being the maximum time during which, with proper maintenance, the system can fulfil its purposes.

3.2. Water Flow Measurement, Questionnaires, and Field Research

The investigation of water consumption was conducted in detail, including direct measurements of water flow rates, a survey of usage habits through interviews and questionnaires, statistical analysis of samples, and comparison with real consumption data provided by the local water company (CASAN), from Florianópolis, Brazil. The main objective was to accurately estimate the daily and monthly water consumption of the building and identify the primary points of use, as well as opportunities for saving potable water by replacing or reusing it for non-potable purposes.

Water flow measurements were performed for each type of appliance. For both ordinary and self-closing taps, the water flow was estimated by collecting water in a graduated container over a period of time measured using a stopwatch. Three measurements were performed, and the average was taken into account. For appliances where direct measurement was not feasible, such as toilet bowls and urinals with an unidentified flush valve, reference values were based on NBR 5626 [43], with 1.7 L/s for flushing-valve toilets and 0.5 L/s for urinals.

Data on consumption habits were collected from three different groups of users: (1) students, (2) staff, and (3) cleaning staff. For groups 1 and 2, a sampling method was applied based on Equation (1), as proposed by Barbetta [44], considering sampling errors of less than 4% due to time limitations. Group 3, which was smaller, was interviewed in its entirety. The questionnaires covered the frequency of use, number of activations, and average time of use for each appliance, which were adapted according to the user’s role in the building. Supplementary S1 in the Supplementary Data shows the questionnaire.

$$n={\displaystyle \frac{n^{\prime }·N}{n^{\prime }+N}}, where n^{\prime }\ge {\displaystyle \frac{1}{{\epsilon }_0^2}}$$

(1)

where: $N$ is the number of people that use the building (people); ${\epsilon }_0$ is the desired sampling error (%); $n$ is the sample of people interviewed (people).

Based on these data, daily consumption estimates were made for each appliance using specific equations that considered the average volume per activation, frequency, and user population. The estimates included washbasin taps, urinals, toilets, drinking fountains (in common areas and the break room), sink taps, and floor and bathroom cleaning activities. Irrigation of the internal garden was not included due to low frequency and lack of data on the person responsible. Equations (2) to (8) were used for the estimates. Equations (2) to (5) were used to estimate the water consumption of each appliance, based on the responses to the questionnaires. Equations (6) to (8) were used to estimate the end-uses.

$$C_{fixed\_volume}=({\displaystyle \frac{{\sum }_{i=1}^n{f}_i·{na}_i·V}{n}})$$

(2)

$$C_{cont\_use}=({\displaystyle \frac{{\sum }_{i=1}^n{f}_i·t_i·Q}{n}})$$

(3)

$$C_{fixed\_use}=({\displaystyle \frac{{\sum }_{i=1}^n{f}_i·V_g}{n}})$$

(4)

$$C_{clean}={\sum }_{i=1}^n{f}_i·n_b·V_b$$

(5)

$$C_{total}=\sum C_{equip./activity}·P$$

(6)

$$C_{mensal}=C_{total}·d$$

(7)

$$P_{uso}={\displaystyle \frac{C_{uso}}{C_{total}}}·100$$

(8)

where: $f_i$ is the daily frequency of use (number of uses per day), ${na}_i$ is the number of activations per use, $V$ is the volume per activation (litres), $t_i$ is the time of use per activation (seconds), $Q$ is the water-flow rate of the appliance (litres per second), $V_g$ is the volume of the bottle or cup (litres), $n$ is the number of people interviewed, $P$ is the user population of the appliance or activity, $n_b$ is the number of buckets used per use, $V_b$ is the volume of the bucket (litres), $d$ is the number of days the building is in operation during the month (adopted 26 days).

During the user interviews, uncertainties were identified regarding the frequency and duration of the use of certain water appliances. Additionally, some urinals and toilets had faulty flushing valves that affected their operation. These issues can lead to inaccuracies in the water consumption estimates. Therefore, a sensitivity analysis was conducted by applying variations of −30% to +30% (in 10% increments) to the most water-consuming devices (washbasin taps, urinals, and toilets) to assess the impact of usage errors on the total water consumption and consequential non-potable demand estimation. The results indicate which appliances are most sensitive to errors. Subsequently, the estimated consumption of these appliances was proportionally adjusted to align with the actual consumption data from CASAN (the local water utility). The corrected end uses were then compared to real meter readings and monthly monitoring records, and adjustments were made to reflect more accurate percentages of water use across appliances and cleaning activities.

3.3. Potential for Potable Water Savings Assessment

This section describes the methodology used to estimate the potential reduction in potable water consumption using isolated and combined water-saving strategies. The process was divided into four main steps, each corresponding to a strategy: water-saving appliances, rainwater harvesting, greywater reuse, and blackwater reuse. Finally, the details of the combination of strategies into scenarios are presented.

3.3.1. Water-Saving Appliances

The first step involved replacing conventional appliances with water-saving appliances, following the performance standards of NBR 15575-6 [45]. The water flow rates of the washbasin taps, toilets, urinals, and kitchen taps were updated in the daily water-use equations. The break room taps and drinking fountains were not replaced, as the variation in water flow did not significantly impact their consumption patterns. The updated water consumption was calculated, and sensitivity-based correction factors were applied to account for user-reported uncertainties and measurement errors. The potential for potable water savings from this strategy was calculated as the percentage difference between the two water consumption patterns, with and without the water-saving appliances.

Table 1. Water-flow rates of present appliances and water-saving alternatives.

Appliance	Quantity	Flow Rate of Present Appliances	Flow Rate of Water-Saving Appliances	Reduction
Washbasin tap	26	0.58 L/use	0.18 L/use	0.40 L/use (68.96%)
Urinal	9	2.76 L/use	1.00 L/use	1.76 L/use (63.77%)
Flushing-valve toilet	27	1.70 L/s (average of 5.49 L/use)	4.90 L/use	0.59 L/use (10.75%)
Kitchen sink tap	1	0.12 L/use	0.08 L/s	0.04 L/s (33.33%)

shows a comparison between the current appliances and water-saving alternatives.

3.3.2. Rainwater Harvesting

Rainwater harvesting was evaluated as a strategy to reduce potable water consumption for toilet flushing and urinals only, and cleaning-related water consumption was not supplied with rainwater. Therefore, rainwater harvesting was simulated using the Netuno programme, version 4 [46]. The programme calculates potential potable water savings based on a daily mass-balance simulation, incorporating rainfall supply, user demand, and rainwater storage tank dynamics. The inputs required for the simulation included rainfall patterns, first flush, harvesting surface area, runoff coefficient, total water consumption, share of non-potable water consumption, upper rainwater tank capacity, and lower rainwater tank capacities. In the following paragraphs, each of these parameters is described in detail.

It is essential to emphasise that a system with two tanks, an upper and a lower tank, was chosen to ensure that the maximum amount of rainwater was available for use, acting as a buffer against rainfall variations. We also considered two tanks because it is customary in Brazil to have an upper tank that supplies water to appliances via gravity. It is also important to note that no qualitative study was conducted to differentiate between the evaluated scenarios. Therefore, a commercial rainwater treatment plant consisting of sand and activated carbon filters was used. The quantification of treatment implementation costs is discussed in the feasibility assessment. Figure 2 shows a schematic representation of the rainwater harvesting system. Supplementary S2 in the Supplementary Data shows the other schemes considered in this study.

The daily rainfall pattern was obtained from the Brazilian National Institute of Meteorology (INMET) based on data recorded at the Florianópolis Airport meteorological station (code 83899) from 1 January 2003 to 1 September 2023 [47]. These data provide daily precipitation values in millimetres and represent the long-term climatic conditions for the region, making them suitable for assessing system performance over time [48]. The first flush refers to the initial portion of rainfall, which typically contains a higher concentration of pollutants. To ensure water quality, this amount of water was discarded before collection began. In the simulations, a fixed value of 2 mm was adopted, following the recommendations of the Brazilian guidelines [31] and previous studies on rainwater quality [49].

The harvesting surface area was estimated using QGIS software, version 3.28, taking into account the building’s roof projection. Non-covered or non-usable roof sections (e.g., courtyards and open technical areas) were subtracted, and a 10% correction reduction factor was applied to account for geometric approximations. The resulting effective area represents the surface that contributes to runoff harvesting. The runoff coefficient is a dimensionless factor that accounts for the portion of rainfall that is effectively converted into runoff, depending on the roof material and slope. Based on the study by Klein [50], a coefficient of 0.92 was adopted for the roof type observed at the central library, which is consistent with the typical ceramic or metallic surfaces found in Florianópolis. Other studies have corroborated this value [51].

The total water consumption corresponds to the daily potable water demand of the building population. This value was determined based on survey responses and water appliance flow measurements, which were corrected through sensitivity analysis. Total consumption is used to proportionally allocate the share of water that could be replaced with rainwater. The non-potable water demand share comprises only uses that can be reliably served by rainwater, specifically, flushing toilets and urinals. Cleaning uses were excluded due to methodological uncertainties and low representativeness. This share was calculated as a percentage of the total daily consumption.

The upper tank stores the treated rainwater. According to NBR 16783 [52], its capacity should be at least one day and at most two days of non-potable water demand, preventing long storage times that may affect the water quality. A fixed capacity within this range was used in the simulations. The lower tank volume was the variable to be optimised in the simulation. Therefore, the lower rainwater tank capacity was simulated in increments of 5000 L, ranging from 0 to 100,000 L. The optimal capacity was identified when additional storage capacity produced a gain of less than 1.5%/m³ in potable water savings, indicating diminishing returns.

Table 2. Netuno parameters that were considered in this study.

Input Data	Value or Source
Daily rainfall data (mm/day)	Data from 1 January 2003 to 1 September 2023 from INMET
First flush (mm)	2.00
Harvesting area (m2)	4210
Daily water consumption per capita (L/capita/day)	Obtained from the water meter
Total population (people)	3090
Percentage of potable water demand that could be replaced with rainwater (%)	Obtained from questionnaires, plus a range from the sensitivity analysis
Roof runoff coefficient (non-dimensional)	0.92
Upper rainwater tank capacity (L)	20,000
Lower rainwater tank capacity (L)	0 to 100,000 with a 5000 L step

summarises the input data used in the computer simulations.

Netuno’s calculation model is a daily water balance model that has been validated in various studies for use in rainwater harvesting assessments [53,54]. The analysis was based on daily rainfall profiles, with the consequent consumption, storage, or drainage of all available water. It is known that using data on a longer or shorter time scale would lead to different results; therefore, the time scale was chosen as daily data were available. The recommendation by Geraldi and Ghisi [48] was also used, in which the authors recommended using at least 15 years of daily data to simulate rainwater harvesting.

It is essential to note that many international standards do not account for daily data when modelling rainwater harvesting systems, opting instead to use monthly or annual rainfall. This divergence can also be observed in installed systems, where designers often prefer to use monthly or even annual data for tank sizing, as it is usually easier to manage and may provide sufficient results. In any case, one chose to use Netuno because it is believed to be a robust and validated tool, which brings the simulation closer to the savings that can be achieved with the system being evaluated.

3.3.3. Greywater Reuse

To estimate the potential for water savings through the implementation of a greywater reuse system, it is necessary to verify the supply of greywater and the demand for non-potable water. The greywater supply of the central library corresponds to the wastewater generated from taps, as calculated using Equation (3). As previously stated, non-potable water demand was assumed to be used for urinals and toilet flushing. Consequently, the updated potable water consumption was determined by subtracting the greywater supply from the water demand. Therefore, the potential for potable water savings was obtained by dividing the economy generated by potable water consumption (with the greywater reuse system) by the total daily potable water consumption of the building without the greywater reuse system. The results are expressed as percentages. Equations (9) and (10) show how the potential potable water savings were calculated.

$${UWC}_{greywater}=C_w-S_{greywater}$$

(9)

$${PPWS}_{greywater}={\displaystyle \frac{S_{greywater}}{C_w}}·100$$

(10)

where: ${UWC}_{greywater}$ is the updated water consumption with greywater supply; $C_w$ is the total water consumption; $S_{greywater}$ is the supply of greywater; ${PPWS}_{greywater}$ is the potable water savings by using greywater.

3.3.4. Blackwater Reuse

To estimate potable water savings from blackwater reuse, the total wastewater generated by the building, comprising lavatories, toilets, urinals, kitchen sink taps, and break room taps, was considered the available supply. Thus, in this study, blackwater is considered to be all wastewater, including the previous greywater contributions. The non-potable destination was limited to toilet and urinal flushing. The savings potential was calculated by comparing the total daily potable water consumption after blackwater reuse implementation with the baseline consumption, following a similar approach as for greywater reuse. It is important to emphasise that the method considers all blackwater to be treated to non-potable water standards.

3.3.5. Scenario Modelling

Four scenarios were modelled to evaluate the combined effect of multiple water-saving strategies. Each scenario represents a progressive integration of the isolated interventions previously analysed, namely, the use of water-saving appliances, rainwater harvesting, and the reuse of greywater and blackwater. The calculation of potable water savings for each scenario followed a consistent methodology: first, the adjusted end-use consumption was obtained by applying the respective strategies; then, the remaining non-potable demand was matched with alternative water sources (rainwater or reused water), and finally, the total potable water consumption was recalculated. The percentage reduction compared to the baseline consumption (without any water-saving measures) was calculated using a standardised formula. Scenarios that included rainwater harvesting required further simulation in the Netuno program, with updated demands, thus adapting the inputs shown in Table 2. This approach enabled a consistent and comparative evaluation of the water savings potential.

Table 3. Scenarios considered in the assessment of water-saving strategies.

Scenario	Combined Strategies	Calculation Procedure
A	Water-saving appliances and rainwater harvesting	One considers the water-saving appliances in the water consumption and then simulates via the Netuno programme.
B	Water-saving appliances and greywater reuse	First, water-saving appliances were considered, and then greywater was assessed.
C	Water-saving appliances and blackwater reuse	First, water-saving appliances were considered, and then blackwater was assessed.
D	Water-saving appliances, greywater reuse and rainwater harvesting	First, water-saving appliances were considered, then greywater was assessed, and the results were considered in the Netuno simulation.

presents the scenarios assessed.

3.4. Feasibility Assessment

The feasibility assessment considered the implementation, operation, and maintenance costs of potable water reduction strategies, which were analysed both in isolation and in combination. Equipment and material costs were estimated using the lowest prices obtained from local suppliers in the Greater Florianópolis area and online stores. Compact treatment systems for rainwater harvesting, greywater reuse, and blackwater reuse were obtained directly from specialised manufacturers. Piping and labour costs were estimated as percentages of the total investment based on the methodology of Istchuk and Ghisi [55]. Electricity and maintenance costs were calculated using actual utility billing data from the UFSC and technical documentation provided by the system manufacturers.

The financial benefit of each strategy was determined by comparing the current potable water bill with the projected bill after strategy implementation, accounting for operational and maintenance expenses. The water tariff structure provided by CASAN includes a fixed charge and variable rates based on consumption ranges. Using this structure and the estimated savings potential, the adjusted monthly water cost was determined. The resulting monthly savings represent the net economic benefit generated by each strategy. Equations (11) and (12) show how the water economy was estimated.

$$C_{monthly}=t_f+V_1·t_1+V_2·t_2$$

(11)

$$E_m=C_{monthly,cur}-(C_{monthly,new}+C_{O\&M})$$

(12)

where: $C_{monthly}$ is the total monthly potable water cost (R$/month); $t_f$ is the fixed monthly water tariff charged by the utility (R$/month); $V_1$ is the volume of water consumed within the first tariff range (m³/month); $t_1$ is the tariff applied to potable water in the first consumption range (R$/m³); $V_2$ is the volume of water consumed within the second tariff range (m³/month); $t_2$ is the tariff applied to potable water in the second consumption range (R$/m³); $E_m$ is the monthly savings achieved by implementing the strategy, calculated as the difference between the conventional and new system costs, including operation and maintenance (R$/month); $C_{monthly,cur}$ is the current monthly cost of water consumption; $C_{monthly,new}$ is the monthly cost of water consumption after the implementation of the water saving strategies (R$/month); $C_{O\&M}$ is the monthly cost of operating and maintaining the water-saving systems (R$/month).

To determine the viability of each water-saving strategy, three financial indicators were calculated: Net Present Value (NPV), Payback Period, and Internal Rate of Return (IRR). The analysis horizon for this research was 15 years, which is in line with the usual useful life of hydraulic installations. The NPV reflects the present value of future savings, discounted using a Minimum Acceptable Rate of Return (MARR) of 0.60% per month, equivalent to the October 2023 Brazilian savings rate. Equation (13) shows the NPV calculations.

$$NPV=\sum _{j=1}^n{\displaystyle \frac{E_m}{{(1+MARR)}^j}}-I_0$$

(13)

where: $NPV$ is the Net Present Value (R$); $MARR$ is the Minimum Acceptable Rate of Return (%); $j$ is each assessed month; $n$ is the number of months in the assessment; $I_0$ is the initial investment for constructing the water-saving strategies (R$).

The payback period refers to the time required for accumulated savings to match the initial investment, resulting in an NPV equal to zero. The maximum payback period is the system’s useful life of 15 years. If the payback period exceeds this value, the system yields a negative NPV and is consequently infeasible. The IRRparameter indicates the project’s rate of return and must exceed the MARR for the system to be considered feasible. Equation (14) shows the IRR calculation.

$$IRR\stackrel{yields}{\to }\sum _{j=1}^n{\displaystyle \frac{E_m}{{(1+IRR)}^j}}-I_0=0$$

(14)

where: $NPV$ is the Net Present Value (R$); $MARR$ is the Minimum Acceptable Rate of Return (%); $j$ is each assessed month; $n$ is the number of months in the assessment; $I_0$ is the initial investment for constructing the water-saving strategies (R$); $IRR$ is the Internal Rate of Return (%).

Among the strategies analysed, the ideal scenario for the UFSC central library was selected based on the highest potable water savings and the shortest payback period. This integrated feasibility assessment allowed the identification of a technically and economically optimal solution tailored to the institution’s context. Finally, the cost per unit of water was calculated. The cost per unit of water is an interesting variable, as it allows a designer or manager to compare the cost of making water available between possible alternatives and the cost of water supplied by the local utility. Equation (15) shows the calculation of the cost per unit of water.

$$CUW={\displaystyle \frac{{\sum }_{j=1}^n-C_{O\&M}-I_0}{{\sum }_{j=1}^n{WP}_j}}$$

(15)

where: $CUW$ is the cost of unit of water (R$/m³); $j$ is each assessed month; $n$ is the number of months in the assessment; $I_0$ is the initial investment for constructing the water-saving strategies (R$); $C_{O\&M,j}$ is the cost of operation and maintenance in the month $j$ (R$); ${WP}_j$ is the water production in the month $j$ (m³/month).

For the initial investments, all the costs necessary to build and use the system were taken into account. Thus, commercial treatment plants for each alternative (rainwater, greywater, and blackwater) were included. In other words, the qualitative profiles of the non-potable water produced were not assessed, but the choice was made in a similar manner to that of a manager. Other costs include labour, pipes, fittings, and other products needed for installation.

4. Results and Discussions

4.1. Water Consumption Profile in the Building

This study assessed the water consumption of the UFSC central library based on monthly data from CASAN and daily water meter readings. Historical data from 2013 to 2023 showed stable consumption until 2019, with a drop in 2020 due to the global pandemic and a peak in 2022 due to leaks. For the comparative analysis, the average consumption between January and June 2023 was adopted, resulting in 678 m³/month, which is considered more representative of the period analysed. In March 2023, the consumption was much higher (950 m³) due to the cleaning of the water tank.

At the same time, daily monitoring was conducted between 30 August and 6 September 2023. The readings indicated an average daily consumption of 25.40 m³ on the working days. However, this figure should be interpreted with caution, as the measurement period was short and coincided with the start of the academic semester, when the flow of users may not yet have been fully established, which could distort the actual average consumption over time. Figure 3 shows all the water appliances in the building, which were evaluated to quantify water end-use.

Based on technical visits and interviews with users, the frequency of use and volume of water used by the main water appliances were estimated. Consumption estimates were refined based on interviews with 120 users divided into three groups: students (Group 1), staff (Group 2), and cleaning staff (Group 3). The minimum sample size required to guarantee a 10% error was met in Group 1 (n = 100) but not in Group 2, which had only 15 respondents compared to the recommended minimum sample size of 46. This limitation was due to the unavailability of interviews with the building’s other employees and constitutes a limitation of this study. All five members of Group 3 were interviewed.

Table 4. Characteristics of the water appliances.

Water Appliances	Location in the Building	Estimated Flow or Volume
Washbasin tap	Bathroom	0.578 L/use
Urinal	Bathroom	3.59 L/use
Flushing-valve toilet	Bathroom	1.7 L/s (NBR 5626)
Kitchen sink tap	Staff area	0.124 L/s
Break room tap	Staff	0.076 L/s
Drinking fountain	Common and staff area	500 mL/use
Mop cleaning equipment	Floor cleaning	20 L/use
Conventional bucket	Bathroom cleaning	5 L/use

lists the water appliances and the water flow or volume obtained.

After consolidating the data, it was found that the largest consumers of water were toilets, washbasin taps, and urinals, which accounted for approximately 88% of the estimated daily consumption before correction. However, when comparing this estimated consumption (36,175 L/day) with the average consumption measured using water meters (25,400 L/day), a 42% difference was found, which is considered high. To investigate this discrepancy, a sensitivity analysis of the primary end uses was conducted, as outlined in the methodology. This analysis showed that the use of toilets was the most sensitive to variations in frequency or activation time, followed by washbasin taps and urinals. Based on these results, a proportional correction was applied to the estimated consumption to bring it closer to the measured value. Supplementary S3 in the Supplementary Data presents the corrections obtained via the sensitivity analysis. Supplementary S4 in the Supplementary Data shows the measurements and questionnaires used.

After correction, the estimated total daily consumption was 26,681 L/day.

Table 5. Daily and monthly water end-uses for each appliance.

Water Appliances or Activity	Daily Water Consumption (L/day)	Monthly Water Consumption (L/month)	Water End-Use (%)
Toilet	12,696	330,096	47.6
Washbasin tap	5561	144,586	20.8
Urinal	4124	107,224	15.5
Common area drinking fountain	1789	46,514	6.7
Break room tap	1677	43,602	6.3
Kitchen sink tap	543	14,118	2.0
Cleaning toilets	130	3380	0.5
Washing floors	100	2600	0.4
Drinking fountain in the staff break room	61	1586	0.2
Total	26,681	693,706	100%
Total non-potable	16,820	437,320	63%

shows the corrected daily and monthly consumption values for each appliance and activity. The proportion of non-potable uses (urinals and toilets) represents 63% of the total consumption, a figure close to those reported in similar studies of public buildings, which vary between 63.5% and 82% [9,56]. The details of the calculations, coefficients used, and spreadsheets with the estimates are included as Supplementary Data (Supplementary S4) to ensure the transparency and reproducibility of the methodology.

4.2. Potable Water Savings Potential for Each Strategy

4.2.1. Water-Saving Appliances

Water-saving appliances were selected to minimise the volume of water used without compromising the functionality of the appliance. Therefore, the decision was made to replace the following existing water appliances in the building: washbasin taps, urinals, toilets, and kitchen sink taps. There was a reduction of 0.40 L/application (68.96%) for washbasin taps, 1.76 L/application (63.77%) for urinals, 0.59 L/application (10.75%) for toilets, and 0.04 L/s (33.33%) for kitchen sink taps. This resulted in a daily potable water consumption of 17.757 L/day and a non-potable water consumption of 11.877 L/day, that is, a total reduction of 33.45% in the total water consumption.

Washbasin taps achieved the most significant reduction in daily water consumption, equivalent to 3812 L/day, as they also showed the most significant reduction in water flow when compared to the other water-saving appliances. Toilets showed the second highest reduction (2533 L/day), followed by urinals (2, 410 L/day) and kitchen sink taps (169 L/day). Thus, the potential saving in potable water obtained by implementing water-saving appliances was 33.45%.

4.2.2. Rainwater Harvesting

To estimate the potential savings from using rainwater, historical rainfall data, the roof area of the central library, and end uses compatible with non-potable water, such as flushing toilets and urinals, were considered. As shown in Table 5, it is essential to emphasise that the value of 63% of non-potable uses was used for the individual assessment of the strategy. Water consumption was also based on the total consumption and estimated number of users, which resulted in 8.64 l/capita/day. In the case of scenarios with combined use with other strategies, both values were recalculated.

The capacity of the upper rainwater tank is a program input that must be adopted. The central library’s non-potable water demand is 16,820 L/day; therefore, a volume of 20,000 L was adopted for the upper tank. Figure 4 shows the potential for potable water savings as a function of rainwater tank capacity. The study determined that the ideal capacity of the lower tank for the rainwater harvesting system is 40,000 L, as above this capacity, the potential gains in potable water savings are less than 1.5%/m³. To meet the demand, two 20,000-L polyethylene tanks were used for the lower tank and one 20,000-L tank for the upper tank, totalling 60,000 L of storage capacity. The potable water savings potential obtained with this configuration was 25.22%, considering a 53% use of non-potable water, which is the worst-case scenario. The system also provides for the use of potable water from the utility as a backup during periods of water shortage.

4.2.3. Greywater Reuse

The greywater supply and demand for non-potable water were assessed to estimate the potential water savings through the implementation of a greywater reuse system. The greywater supply of the central library is 7238 L/day, corresponding to the consumption of washbasin taps and break room taps. The non-potable water demand is 16,820 L/day, corresponding to the consumption of urinals and toilet flushing. Therefore, all greywater can be used, provided it is treated for this purpose.

The volume of the lower greywater storage tank must be greater than the greywater supply of 7238 L/day. Therefore, a 7500-L polyethylene tank was adopted. The upper tank containing treated greywater feeds the toilets and urinals of the building through pipes that are separate from the potable water pipes. Therefore, the volume of the upper tank must be greater than the daily demand for non-potable water, which is 16,820 L/day. A 20,000 L polyethylene tank was used as the upper tank. Finally, the upper tank was equipped with a backup water supply (potable water from the utility) in the case of a greywater shortage.

4.2.4. Blackwater Reuse

The blackwater reuse system obtains water from washbasin taps, sink taps, urinals, and toilet flushing, yielding a non-potable source of 24,601 L/day, which exceeds the non-potable water demand of 16,820 L/day (from toilets and urinals). In this way, the blackwater reuse system can supply all of the central library’s daily non-potable water demand, reducing potable water consumption by 16,820 L/d, equivalent to 63.04% of the total water consumption. The potential for reducing potable water consumption by implementing the blackwater reuse system was 63.04%.

The capacity of the lower blackwater storage tank must be greater than the blackwater supply of 24,601 L/day. Therefore, 7500 L and 20,000 L polyethylene tanks were used. The upper treated blackwater tank feeds the building toilets and urinals through pipes independent of the potable water pipes. Therefore, the capacity of the upper tank must be greater than the daily demand for non-potable water, which is 16,820 L/day. Therefore, a 20,000 L polyethylene tank was adopted for the upper tank, similar to that of the greywater system. Finally, the upper tank was supplied with backup water (potable water from the utility) in case of a shortage of greywater.

4.2.5. Scenarios

The four scenarios analysed in this study combined water-saving strategies to significantly reduce potable water consumption in the building. Scenario 1 integrated the use of water-saving appliances with rainwater harvesting, resulting in a daily water consumption of 11,112 L/day and a savings rate of 58.35%. For this purpose, two 20,000 L lower tanks and one 20,000 L upper tank were used. The system was sized based on the demand for non-potable water, and the savings potential was estimated using the Netuno program, considering rainwater harvesting and potable water supply as a backup.

In Scenario 2, the combination was between water-saving appliances and greywater reuse (from washbasin and break room taps), with a daily supply of 3426 L of greywater. This reduced the consumption to 14,331 L/day, representing a 46.29% saving in potable water. The system used a lower tank of 5000 L for greywater and another of 20,000 L for the upper tank, with a provision for supplying potable water in the event of a shortage. Reuse was limited by the lower greywater supply compared with the non-potable water demand.

Scenario 3 involved the reuse of all blackwater generated in the building, together with the use of water-saving appliances. The supply of blackwater was 15,677 L/day, exceeding the demand, and allowing all non-potable needs to be met in full. Potable water savings were the most significant among the scenarios, at 77.96%, with a final consumption of only 5880 L/day. Lower and upper tanks, each with a capacity of 20,000 L, were used to store treated blackwater and supply non-potable demand.

In Scenario 4, greywater reuse and rainwater harvesting strategies were combined with water-saving appliances. Consumption was reduced to 9143 L/day, representing a 65.73% savings compared with the initial estimated consumption. The demand for non-potable water was met by 3426 L/day of greywater and 8451 L/day of rainwater. For this, two 20,000 L tanks for rainwater and a 5000 L tank for greywater were adopted, with a single 20,000 L upper tank for distribution, due to the common use of treated water.

In all scenarios, the upper supply was planned to receive backup water from the water company if alternative sources were unavailable. The results show that the more comprehensive the strategy adopted, the greater the potential for saving potable water. However, there is also greater complexity in the storage and treatment systems, as in scenarios involving grey and blackwater reuse. The choice between scenarios must consider both the percentage of savings and the technical and operational feasibility of their implementation. Figure 5 presents a comparative summary of the evaluated scenarios and isolated strategies.

A comparison with the literature is also important for validating the results obtained. The water-saving appliance strategy demonstrated a potential for potable water savings comparable to those obtained in studies by Santos et al. [57] and Lombardi [58]. The isolated rainwater utilisation system had a lower water-saving potential than that found in the studies by Marinoski and Ghisi [59] and Rainmap [56]. Regarding the greywater system, the study by Fasola et al. [60] resulted in a potential water savings of only 5.10% due to the low supply of greywater in the building. The combination of water-saving appliances and rainwater harvesting in the study by Fasola et al. [60] had a savings potential of 27.80%, which is less than half of the potential obtained in this study. The difference is possibly due to the low savings potential obtained for the water-saving appliances of Fasola et al. [60], since the rainwater utilisation systems resulted in savings potentials that were close to ours. It can be seen that the Brazilian literature lacks studies analysing blackwater reuse in educational institutions, as well as combined strategies.

4.3. Economic Feasibility Assessment

The economic analysis showed that of all the strategies evaluated separately, water-saving appliances showed the best performance, with an IRR of 4.46% per month and a payback of only 20 months. This advantage is due to the low cost of implementation and the significant reduction in the monthly water bill, even taking into account recurring maintenance costs, which were reliably estimated considering corrective maintenance standards and no preventive maintenance.

The blackwater reuse system also proved to be highly viable, with an IRR of 2.50% per month and monthly savings of more than R$6000, even outperforming the greywater and rainwater systems alone. The good performance was due to the significant reduction in the volume of potable water used for toilet flushing, which is the main non-potable water consumption of the building. In public buildings with high water consumption, there is a high potential for savings through non-potable use strategies, as demonstrated in this research.

Finally, the strategies for using rainwater and greywater individually yielded the worst results, due to the lower percentage of reduction in potable water use and the higher cost of implementing the systems compared to water-saving appliances. However, they also obtained a positive NPV over the analysis horizon (15 years), demonstrating the economic viability of the system.

Table 6. Summary of the economic analysis for the four main strategies.

Strategy	Implementation Costs (R$)	Operation and Maintenance Costs (R$/year)	Monthly Savings (R$/Month)	IRR (%)	Discounted Payback (Months)	Cost per unit of Water (R$/m3)
Water-saving appliances	43,486.99	20,729.13	3501.77	4.46	20	8.66
Rainwater harvesting	127,060.20	220.69	2475.75	1.46	73	5.30
Greywater reuse	168,676.03	401.87	2505.72	0.94	114	7.10
Blackwater reuse	210,324.59	559.27	6159.80	2.50	42	3.96

presents a summary of the economic results of the four strategies in isolation.

Among the combined scenarios, Scenario 3, which integrates water-saving appliances and blackwater reuse, proved to be the most advantageous strategy, combining the highest monthly financial savings (R$7782.48) and a robust internal rate of return (2.05% per month). This confirms the synergistic potential of combined strategies targeting the largest water consumers. It is also vital to align cheaper strategies, such as the use of water-saving appliances, with those that generate greater potable water savings, such as blackwater reuse.

In contrast, Scenario 2 (water-saving appliances + greywater reuse) showed a negative IRR (−5.56% per month), making it economically unviable within 15 years. This result shows that the high cost of the greywater treatment plant, combined with the limited savings due to the end uses served, compromises the attractiveness of the strategy. In other words, it is important to balance the strategies according to the initial investments of each site analysed to ensure that the installation and operating costs do not burden the system evaluated to the point of making it economically unviable.

Table 7. Summary of the economic analysis for the four scenarios combining strategies.

Scenario	Implementation Costs (R$)	Operation and Maintenance Costs (R$)	Monthly Savings (R$)	IRR (%)	Discounted Payback (Months)	Cost per Unit of Water (R$/m3)
1 (Water-saving appliances and rainwater harvesting)	188,045.60	220.69 [*]	6025.60 [**]	1.73	57	7.38
2 (Water-saving appliances and greywater reuse)	209,340.63	401.87 [*]	4553.90 [**]	−5.56	- [***]	10.24
3 (Water-saving appliances and blackwater reuse)	246,324.59	599.27 [*]	7782.48 [**]	2.05	50	6.75
4 (Water-saving appliances, greywater reuse and rainwater harvesting)	316,896.66	694.24 [*]	6340.14 [**]	0.73	138	9.16

Note(s): [*] The cost of operation and maintenance of the combined scenarios also includes the annual cost of operation and maintenance of the water-saving appliances (R$20,729.13/year). [**] The monthly financial savings from the combined strategy scenarios do not take into account the annual cost of operating and maintaining the water-saving appliances. This cost was discounted from the cash flow in multiple periods of 12. [***] The system was not feasible; therefore, it does not present a discounted payback of less than 180 months.

presents the economic viability results for the four scenarios.

By incorporating complete systems and contextualising costs to the reality of a public institution, this study increases methodological robustness and reinforces the importance of local analyses in defining the most effective strategies. It is essential to emphasise that the blackwater reuse strategy entails the use of a more complex treatment plant, which necessitates technical support for operation and control. In other words, simpler strategies have advantages in this context for public buildings, as they require less technical support for their operation.

The current average cost of water at the site is 15.32 R$/m³, which is well above all the costs estimated in the scenarios and alternatives. Even with a cost per m³ below the utility’s cost of water, the financial return was not achieved in Scenario 2 due to the high initial investment and the lower reduction in potable water consumption, as shown in Figure 5. It is interesting to note the high price paid per cubic meter of water in Brazil, which is approximately 3.0 USD/m³ at the current exchange rate. This amount is doubled because the sewerage billing method is 100% of the cost of the water bill. Alternatives, such as those evaluated in this study, can reduce these costs, thereby improving the economic sustainability of buildings. Other studies could replicate the methodology for tariffs in other countries.

4.4. Critical Discussion, Limitations, and Future Studies

4.4.1. Decision-Making by Managers

Firstly, it is important to emphasise that this study is particularly interesting because it focuses on public buildings and libraries. In such cases, there is a significant water consumption profile, with a large proportion being non-potable. For this specific typology, the use of blackwater has proven beneficial, producing a large quantity of non-potable water, although at a high implementation cost. For other typologies, such as residential, the cost-benefit order of the alternatives should change, favouring less expensive implementations such as rainwater harvesting.

The use of rainwater and water-saving appliances has the advantage of requiring less destructive work and can often be built without the need to demolish or break down walls. Greywater and blackwater systems, on the other hand, require the use of specific sewage installations, with consequent major structural interventions for implementation. Therefore, it is important to note that the choice of strategy must also take into account whether the building is already built or is in the design phase. For retrofits, measures with less demolition are generally chosen, while projects in the initial phase have greater freedom to choose strategies for saving potable water.

Decision-making should be driven by both structural and non-structural constraints, including the feasibility and technical aspects of alternatives, which are studied here in terms of the share of potable water savings. For example, one possible outcome of this work is the suggestion that public libraries in Florianópolis consider implementing blackwater reuse, due to its high non-potable demand and expensive water bills. Other public buildings that follow the same considerations may also be recommended to adopt this strategy. Nevertheless, it is essential to consider water-saving techniques, especially in regions with expensive water supply and sewage management tariffs, such as Florianópolis.

4.4.2. Urban Consequences of Adopting a Strategy

Regarding the rainwater utilisation system, it is worth noting that the system is connected to the building’s drainage management and can therefore be linked to other advantages in the urban water context. For example, the attenuation of peak flows through temporary storage in public buildings may be an advantage of the system that has not been explored in this work. This factor is directly related to the rainfall regime, which is also variable and influenced by the climate-change profile.

Regarding greywater and blackwater systems, there is also the value of reducing the environmental impact associated with the treatment and final disposal of these effluents. In Brazil, the treatment of waste for final disposal is often not carried out properly. By adopting the reuse of greywater and blackwater, complete treatment can be achieved through a private treatment plant, thereby mitigating the negative implications of improper wastewater management. More robust analyses, such as Life Cycle Cost Assessment and Life Cycle Assessment, incorporating environmental costs and fluxes, can be carried out in future studies, broadening the horizon of analysis when comparing strategies.

4.4.3. Uncertainties

It is essential to contextualise the results obtained in this research regarding potential public decision-making. For example, uncertainties in the simulation variables and the quantification of costs can modify the results in such a way as to change the ranking of the most attractive investments and even render them economically unviable. There are also uncertainties related to the use of non-potable water in buildings that can lead to lower user acceptance. For example, if the quality of the water is not sufficient to eliminate colour or odour, the use of non-potable water can lead to a deterioration in service. These aspects must be taken into account during the operation and maintenance of the system to ensure that potable water savings are not linked to a worsening of the system’s performance. It is also important to maintain awareness campaigns so that saving potable water does not lead to a rebound effect, increasing water consumption based on users’ awareness of saving.

Additionally, uncertainties exist in the application of the questionnaires and in the measurement of water consumption, which affect the building’s water consumption and the percentage of non-potable uses. Although a sensitivity analysis was conducted, future studies could further explore consumption in university libraries by utilising more comprehensive historical data and leveraging the support of other technologies. For example, the use of Internet of Things (IOT) devices and smart meters. Thus, there could be greater reliability in the results obtained for the percentage of non-potable uses and potable water consumption, which are highly influential in the potential for potable water savings and consequent economic viability. In any case, this study presents significant findings for the public management of university libraries, demonstrating the potential for sustainable water management as a means of reducing water consumption and achieving financial savings.

4.4.4. Limitations and Future Studies

This study has several limitations that warrant further exploration in future work. Firstly, the research was limited to the geographical region where it was conducted and could be replicated in other locations. The use of the local rainfall profile also yielded results that differed from those at other locations, depending on the availability of rainfall. Characteristics such as seasonality and annual rainfall amounts influence the results and should be further explored. There are also geographical limitations related to the financial analysis, such as the use of national and local tariffs and fees, as well as the installation and operating values quantified for the evaluated city. Therefore, future studies could build upon the work presented in this research, aiming to contextualise it better and help decision-makers consider possible interventions to save potable water in libraries.

5. Conclusions

Water consumption worldwide is a subject of constant study, and measures to reduce consumption and increase efficiency are crucial globally. This study analysed the technical and economic feasibility of different strategies for reducing potable water consumption in the central library of the Federal University of Santa Catarina. Isolated strategies were evaluated, including the use of water-saving appliances, rainwater harvesting, and greywater and blackwater reuse, as well as four combined scenarios. The building had a daily water consumption of 26,861 L, 63% of which was used for non-potable purposes and could be supplied by alternative sources. The most efficient scenario was the combination of water-saving appliances with blackwater reuse (Scenario 3), which resulted in a 77.96% reduction, or 20,801 L/day. From an economic perspective, this strategy also yielded the highest monthly financial savings (R$7782.48) and a discounted payback of 50 months. In contrast, the isolated rainwater utilisation system had the lowest potential for potable water savings (25.22%).

All the scenarios analysed were considered economically viable, except for Scenario 2 (water-saving appliances + greywater reuse), which showed negative results in terms of net present value and internal rate of return. The analysis showed that strategies based on water-saving appliances are more advantageous in terms of cost-benefit, with the highest internal rate of return (4.46% per month) and the lowest payback period (20 months). Based on the technical and economic results, Scenario 3 was recommended as the ideal solution. This research also highlighted the need for greater implementation of rainwater harvesting and grey/blackwater reuse systems in Brazilian buildings, despite the existence of favourable legislation, underscoring the importance of public policies and sustainable actions aimed at water conservation.

The importance of this study lies in the potential application of these measures in public buildings to reduce the operating costs and water consumption. This could have a positive impact on both economic and environmental termsand serve as a benchmark for local public management. It is suggested that the university evaluate the possibility of using alternatives, depending on the availability of the budget to build the systems. The importance of this study is also reflected in the teaching content, serving as an example for other local structures. The limitations include the use of deterministic values in the simulations, as well as the consideration of local rainfall characteristics and financial analyses. Future studies could replicate the models used in this research at other universities to validate and expand the discussion on water efficiency in libraries.