Rainwater Harvesting at Universities. Case study: Valle de las Palmas

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Introduction
The scenario for the management of water resources is limited, especially at places with a semi-arid climate where this issue has been related to health problems, limitations for economic and social development (Vargas-Parra et al., 2013), it also has become a public safety problem (Gebru et al., 2021), thus all these conditions, together with the accelerated growth of the population in cities (Rahman et al., 2020) has led to the expansion of urban areas, with a higher demand of water resources and, in consequence, exerting pressure on water supplies, therefore affecting the hydrological cycle in cities (Teston et al., 2018).
The United Nations Environmental Program estimates that by 2050, more than two billion people will live in a condition of high water stress, and this will limit the development of countries around the world (Ammar et al., 2016).
The Arid and Semi-arid Regions (ASARs) are zones which are characterized by short rainfall periods during the season in which they occur (Tamagnone et al., 2020), these regions represent 35% of the Earth's surface, which equals about 50 million square kilometers (Ammar et al., 2016) and, due to climate change. Márquez et al. (2021), point out that the frequency and intensity of droughts in ASARs restricts the availability and the usage of water resources (Ammar et al., 2016), therefore there is international interest in the implementation of sustainable measures for optimal water management, which can generally be categorized into actions such as: Re-use of treated wastewater (Dos Santos & de Farias, 2017), desalination (Imteaz et al., 2013), reduction in water consumption, and use of alternative means of supply, such as rainwater harvesting (Almeida et al., 2021), the latter being a topic that has received attention since 1980 (Ammar et al., 2016).
It is precisely in the ASARs where rain harvesting is used to optimize water supply sources (Bitterman et al., 2016), an alternate and decentralized option that contributes to the resilience of urban systems (Silva et al., 2022) since some authors point to it as a specific strategy to combat climate change (Youn et al., 2012), as it is considered as low-impact (Guo & Guo, 2018) and also known as a green infrastructure practice (Li et al., 2021) with the potential to reduce greenhouse gas emissions from storage tanks and water treatment processes which contribute to climate change.
REB&S 2023, 5(2), 1-11. https://doi.org/10.56845/rebs.v5i2.82 (Aladenola & Adeboye, 2010;Imteaz et al., 2012) in addition they help to reduce the volume of water runoff as well as becoming a flood prevention measure (Sepehri et al., 2018). Sangave et al., (2019) define the concept of rainwater harvesting (RWH) as the natural collection and storage of rainwater in order to increase its availability for human consumption, either being for domestic, industrial and commercial use (Sangave et al., 2019). Other authors integrate into this concept its use in agriculture, for the ecosystem's conservation (Gebru et al., 2021), as well as being used for the maintenance of landscape areas (Nop et al., 2021).
It has been estimated that with the implementation of water harvesting strategies, savings between 12% and 100% can be generated (Şahin & Manioğlu, 2019), since reports show that, for residential buildings, the savings range between 41% and 100% in countries such as Brazil and China, while the save ranges from 38% to 69% when the use of harvested rainwater is for toilets, laundry and irrigation of a housing unit (Rahman et al., 2020), thus the savings potential will depend on the amount of precipitation, catchment area, size of the container for its harvest and the demand for drinking water for domestic use (Teston et al., 2018).
Universities are institutions that also can join these strategies to contribute significantly to reduce water resources demand, with estimated savings between 10% and 100% (see Table 1), due to the potential use of water collection from the roofs of buildings (Anchan & Prasad, 2021). Cardoso et al., (2020) point out that, in Japan, 30% of schools have facilities for water harvesting. In this regard, Almeida et al. (2021) show that, in the case of schools, there is variability in water consumption patterns because university buildings have demand profiles that will be dependant on the periods of use, type of consumer, teaching activities, and school periods; likewise, the practice of sustainability in higher education institutions can contribute to the efficient use of water, reduce its management costs, and its implementation can have an investment return by 4.8 years (Teston et al., 2018). Furthermore, few studies have analyzed the feasibility of RWH systems for university buildings and, consequently, there is still a knowledge gap regarding the performance of RWH systems applied to this type of building. Regarding the quality of rainwater, it is considered mostly clean, although its final quality will depend on the environmental context, catchment area size and rain season details (Almeida et al., 2021).

Location of the study area
The study area is the Faculty of Engineering and Technology Sciences (FCITEC by its acronym in Spanish), an Academic Unit of the Autonomous University of Baja California (UABC by its acronym in Spanish), located in the area known as Valle de las Palmas, southeast of the municipality of Tijuana, Baja California, Mexico, and it is an area with a climate classified as BSKs Arid, Temperate, with an average annual temperature between 12º C and 18º C, with average rainfall of 237.1 mm during winter, concentrated between the months of January and March (XIX Ayuntamiento de Tijuana, 2008) and a precipitation range between 150 to 400 mm. The site's topography features mountain formations such as

Information on the study area and inventory of fixed assets
The FCITEC is an Academic Unit which belongs to the Tijuana campus of the Autonomous University of Baja California.  FCITEC's existing infrastructure 1. "Prisma" (Spanish for "prism"), is the main building divided into 4 blocks (buildings "A", "B", "C", and "D"), each of these has three levels and a ground floor, it has 68 classrooms, 20 cubicles for teachers, 17 laboratories, and it has 54 restrooms.

STUDENT POPULATION
Undergraduate students Graduate students 2. Administrative building, besides Dean's office, teachers' room, meeting room, cubicles and two small conference rooms on the ground floor, the first floor has the library and the second floor includes five computer labs, and a main hall for conferences, the whole building has a total of 10 restrooms. 3. Cafeteria building, it also includes university's sweepstakes administrative offices, sports office, and the whole building has 8 restrooms. 4. Workshop Building for Civil Engineering, Renewable Energies Engineering and Mechanical Engineering laboratories, it has a total of 6 restrooms. 5. Buildings "E", "F" and "G", with a total of 22 classrooms, 3 computer laboratories, 15 cubicles for teachers, a meeting room, and 10 restrooms. 6. Sports fields and courts. 1. FCITEC also has 2 parking lots, a waste warehouse and a drinking water storage cistern with a capacity of 650 cubic meters, which is fed from the municipal's water network. The hydraulic and sanitary systems at the academic unit work based on a pressure driven supply system.

Project Engineering
The potential for the use of rainwater will be evaluated by means of a mathematical model and the initial process will support the subsequent stages of development, such as the storage capacity and the demand for water to be used for a rainwater harvesting system. In this sense, the Storm Water Management Model (SWMM, EPA, USA) was developed for the planning, analysis and design of stormwater runoff, and reduce rain water infiltration and, as well as to simulate hydrology and water quality in agriculture and urban areas (Li et al., 2021;Ammar et al., 2016).
Rainwater collection and use technique is understood as the practice (work or technical procedure) capable of, individually or combined with others, increasing the availability of water on the farm, for domestic, animal or vegetable use. Generally speaking, they are improved soil and water management techniques, crop management and animal handling, as well as the construction and operation of hydraulic works that allow collecting, diverting, conducting, storing and/or distributing rainwater. According to Siegert, the classification of rainwater harvesting techniques can be grouped into the following (FAO, 2013). The calculation elements for the harvesting of rainfall events can be classified into: floor and roofs (Dos Santos & de Farias, 2017) in this study, the method based on the roof catchment will be used, which depends on the surface, depth of the rainfall, storage and runoff coefficient. To calculate the amount of water that can be used, the World Meteorological Organization recommends having a database of precipitation records for 30 or at least 25 years, regardless of the method used for the calculation (Geraldi & Ghisi, 2017), however, studies such as Mitchell's (Mitchell, 2007) that the use of 10-year periods can be representative of a long-term precipitation pattern. For the purpose of this study, the sustainable methodology is used to capture rainwater in buildings.
The elements that set up a rainwater harvesting system are those that can be seen in the following figure: Figure 5. Elements that integrate a rainwater harvesting system (Anaya-Garduño, 2017).

Potential catchment area
In this study, two potential catchment areas at FCITEC are considered, being those the Prisma building with an area of 3,244.60 m² and the Administrative building, with 1,624.63 m².
REB&S 2023, 5(2), 1-11. https://doi.org/10.56845/rebs.v5i2.82 The demand or endowment per person is the amount of water that a person needs daily to fulfill the physical and biological functions of his/her body and it also considers the number of inhabitants to be benefited and domestic use. To calculate the amount of rainwater to be captured on the roof of the building, we will use the following formula, which is total maximum rainfall. where: TPpm (m) is the total water from pluvial precipitation; and Ppm (mm) is the maximum rainfall of the rainfall record.
The maximum monthly precipitation was obtained from the Automatic Meteorological Station (EMA by its acronym in Spanish) at FCITEC, considering the rain data for the year 2019, a period with the heaviest rainfall since 2010. This year was selected since rain harvesting studies do not require analyzing such long periods of rain data as other types of UNESCO Recommendati on National Average in Mexico research work like, for example, hydrological studies, which may need over 10 years of complete weather records. In Baja California the rainy season lasts 5.7 months, starting in October and ending in April, the average monthly precipitation for the year 2019 was 45 mm and the annual maximum is 291 mm, which occurred in the month of December. When the average monthly rainfall is less than 25 to 30 mm and of low intensity, it is not recommended to consider them and even if they are during dry months (Anaya-Garduño, 2017). To determine the total volume of rainwater collected on the roof.

Vtpp = *
(2) where Vtpp (m 3 ) is the total volume of rainfall; and Az (m 2 ) is the surface area of the building's roof that will capture rainwater.
The following equation is used to calculate the actual total volume of water collected on the roof.
where Vr (m 3 ) is the actual volume of total rainwater harvested; and 0.85 is a constant for calculation to establish the actual stored volume of water equivalent to 85% of the total volume of water collected on the roof.
With the following equation, the number of rainwater downspouts is calculated from the actual total volume of water collected on the roof. The following equation is used, considering a 4¨ pipe when the area is less than 100 m² and, if it is greater than 150 m², then 6¨ pipes must be considered, in accordance with the Complementary Technical Standards for Architectural Projects (Tapia-Vargas, 2016).

Bap = Az/Abap
where Bap (pieces) is the number of downspouts needed; and Abap is the coverage area on the roof for each Bap installed.
The rainwater infiltration tank is determined with the following expression. where Tf (m 3 ) is the actual volume of rainwater collected in the filter tank; and 0.95 is a calculation constant to determine the percentage of rainwater that is actually stored in the tank. For the calculation of the design capacity of the filtration tank, the following equation is used.
where Tff (m 3 ) is the design volume of the filtration tank in cubic meters; and D (days) is the water retention time inside the filtration tank, the recommendation is from 2 to 5 days.
To determine the total volume of rainwater in the cistern, we calculate it with the following expression.
Vt = Tf * Bap * 5 where Vt (m 3 ) is the total volume of water for the design cistern; and 5 is a calculation constant to store the volume of rainwater for 5 days.

Results and Discussion
Applying the previous equations and following the proposed methodology, we obtained the following results (see Table  2), as we present a summary of the calculations made for each building. In this case, building 1 (labeled as "Prisma"), and building 2 (named "Administrative"). In this study, two potential catchment areas are considered in the FCITEC, which are the Prisma building with a roof surface area of 3,244.60 m², and the Administrative building, with an area of 1,624.63 m². To start the calculations, it was established that the maximum annual precipitation value to be used would be 291 mm, according to data obtained from the Automatic Meteorological Station (EMA) located in-site at FCITEC. With the Academic Unit being founded on 2009, the EMA's records date back to 2010 and, after reviewing the precipitation data, the most complete and higher yearly records belonged to 2019.  Table 2 shows that, over the course of a year, the Prisma building can capture up to 3,810 m 3 of rainwater, while the Administrative building could collect a volume of 1,908 m 3 . The collection volume is a function of the roof surface area and the amount of rain that occurs during the year. It is important to acknowledge that, for the analysis of rain, there needs to be at least a minimum rainfall record of 10 continuous years to have reliable results for the calculations and sizing of the cistern. For rainwater drains, PVC pipes can be used, for this exercise they are proposed with a 6 inch diameter.

Standards Compliance.
In accordance with the NOM-127-SSA1-2021 standards, which establishes the permissible limits of quality and the purification treatments of water for human use and consumption, which must be met by public and private supply systems or any natural or legal person who may distribute it, throughout the Mexican national territory and NOM-12-SSA1-1994, which establishes the sanitary requirements that must be met by public and private water supply systems for human use and consumption to preserve its quality, rainwater that shall be captured, must be submitted to a disinfection process, to guarantee its bacteriological quality, ensuring the absence of pathogenic microorganisms; there are three classes of pathogenic microorganisms in water that could cause various diseases on human beings, such risks are viruses, bacteria and protozoa.
The disinfection can be carried out through different processes, such as the physical ones that include ultraviolet radiation, gamma radiation, X-rays, ultrasonic waves and direct application of thermal energy, while there are also chemical processes in which a variety of agents could be used such as chlorine, ozone, bromine, iodine, hydrogen peroxide, potassium permanganate and metal ions, such as silver and copper. Nevertheless, the most widely used disinfection method is chlorination due to its broad, low-cost germicidal spectrum.

Conclusions
Rainwater harvesting is a convenient way to obtain water for human consumption and/or use in areas where this liquid is scarce. Among the main advantages of using these systems there is the good quality of rainwater, the system's design scheme is independent, which is highly recommended for schools located in arid and semi-arid regions, especially useful at those which do not have potable water service available. Rainwater harvesting is an alternative, moreover due to the fact that this technique helps to avoid overexploitation of aquifers and, at the same time, prevents erosion and degradation of basins. However, it also has certain disadvantages, for example the economic aspects, such as the issue of depending on the project requirements, it can turn out to be expensive, and the fact that the amount of collected rainwater depends entirely on the precipitation pattern of the location and the available catchment area, which varies by region.
The purpose of this project is to implement such system at the FCITEC Academic Unit, since it is an area with a water deficit. Although it may not become a substitute for the current water supply system at the academic unit, it will help mitigate the site's water consumptions as well as achieving another goal sought with this project which is to promote the cultural value of rainwater harvesting at the community, starting on a municipal level and eventually reaching statewide relevance. Since the projected 5,718 m 3 total volume of rainwater collected from the two studied buildings is a yearly value that is highly dependent on the precipitation rates of the region, it is recommended to use this technique for the irrigation of landscape garden areas of the FCITEC, as an alternative to face the deficit of water in the vicinity area and the long term prospect to implement these systems at other campuses of the Autonomous University of Baja California.
It should also be considered that this alternative will have the following impacts: (1) Economic impact: savings, return on investment, catchment surface area, contribution to energy savings in drinking water and sewage management systems, where it is estimated that the world's energy consumption for this concept is close to 3% (Teston et al., 2018); and (2) Environmental impact: reduction in the flow of water that runs off roads and sanitary drainage. Reduction in the use of potable water for non-potable uses (landscape area irrigation, sidewalk cleaning, car washing), as well as the conservation of water resources, the contribution to water safety and the promotion of environmentally sustainable actions.