Rainwater harvesting as an alternative for water supply in regions with high water stress

In this study, the reliability of using rainwater harvesting to cover the water demand of a transportation logistics company located in Mexico City was assessed. Water consumption in facilities and buildings of the company was determined. Rainwater potentially harvestable from the roofs and maneuvering yard of the company was estimated based on a statistical analysis of the rainfall. Based on these data, potential water saving was determined. Characterization of rainwater was carried out to determine the treatment necessities for each water source. Additionally, the capacity of water storage tanks was estimated. For the selected treatment systems, an economic assessment was conducted to determine the viability of the alternative proposed. Results showed that current water demand of the company can be totally covered by using rainwater. The scenario where roof and maneuvering yard rainwater was collected and treated together resulted in being more economic than the scenarios where roof and maneuvering yard rainwater was collected and treated separately. Implementation of the rainwater harvesting system will generate important economic benefits for the company. The investment will be amortized in only 5 years and the NPV will be on the order of US$ 5,048.3, the IRR of 5.7%, and the B/I of 1.9. This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY 4.0), which permits copying, adaptation and redistribution, provided the original work is properly cited (http://creativecommons.org/licenses/by/4.0/). doi: 10.2166/ws.2018.018 om https://iwaponline.com/ws/article-pdf/18/6/1946/473620/ws018061946.pdf er 2019 Miguel Ángel López Zavala (corresponding author) Mónica José Cruz Prieto Cristina Alejandra Rojas Rojas Tecnológico de Monterrey, Water Center for Latin America and the Caribbean, Av. Eugenio Garza Sada Sur No. 2501, Col. Tecnológico, Monterrey, NL C.P. 64849, Mexico E-mail: miganloza@itesm.mx


INTRODUCTION
Water scarcity and stress are reaching worryingly high levels worldwide due to the intensive exploitation and pollution of water resources. Furthermore, climate change is intensifying this pressure in some regions of the world, including but, reports on rainwater harvesting to meet water demands for uses similar to that addressed in this paper are seldom found in the literature.
Thus, in this study, the reliability of implementing rainwater harvesting to meet 100% of water demand of a transportation logistics company located in Mexico City was assessed. The importance of this study relies on (1) the fact that worldwide there exist emergent economies of highly dense population that are facing rapid industrialization, water scarcity and high pressure on water resources; thus, (2) the approach and results presented in this work can be replicated in regions with high water stress to achieve feasible, reliable, and economically viable solutions for water supply based on rainwater harvesting.

MATERIALS AND METHODS
Rainwater potentially harvestable from roofs and maneuvering yard where V i,j is the volume of month i for the roof or maneuvering yard j; P i the precipitation of month i; A j the surface area of the roof or maneuvering yard j; and C r is the runoff coefficient, and for concrete and asphalt surfaces, a value of 0.85 is recommended (Chow et al. ).

Water consumption from the public network
Water from the public network consumed in facilities and buildings was estimated based on the consumptions registered on the last 4-year water bills paid by the authorities of the company.

Characterization of rainwater and determination of treatment necessities
Necessities of rainwater treatment were assessed based on its physical, chemical and bacteriological characteristics.  Determination of volumes to be treated and sizing of water storage tanks Rainwater flowrates to be treated and dimensions of the storage tanks were determined regarding three scenarios: (1) when only rainwater from roofs is collected, (2) when rainwater from the maneuvering yard is stored alone, and (3) when both rainwater sources are collected together. The flowrate to be treated and the size of the storage tank for each scenario were estimated by conducting monthly water balances, where rainwater contributions were the inflows, and water demands from the public network for building services and vehicle washing were the outflows. The equation reported by Khastagir & Jayasuriya () was used for this purpose: where S tþ1 is the storage volume in the tank at the end of the month; S t the storage value at the beginning of the month; Q t the runoff from the catching surface in the month; D t the total demand of water in the month; and C is the active tank capacity. Overflows of the storage tanks in times of high water production were taken into consideration for the water balances, but overflows were not considered in the volumes to be treated. Dimensioning of the storage tank was conducted with regard to 100% of the water demand being guaranteed.
Selection and design of treatment processes for rainwater Selection of operations, processes, and systems needed to achieve the level of treatment required was conducted according to the following criteria: final use of the treated rainwater, efficiency in removing contaminants and cost effectiveness. Once selected, the treatment systems were designed. Additionally, modifications of the current water distribution network and the pumping system were conducted to incorporate and distribute the treated rainwater into the buildings and facilities of the company. Because the design of the systems was not part of the scope of this paper, details of the design process are not presented; only the main results, such as the dimensions of the systems and selected equipment, are included.

Economic assessment
Determination of the investment and operation costs of the selected treatment systems Based on the design, the investment needed to implement the treatment systems for rainwater was estimated. Thus, the costs of equipment, construction, and implementation of the treatment systems were calculated. Additionally, operation and maintenance costs associated with those systems were also estimated.

Determination of benefits
The economic benefits derived from the implementation of the rainwater harvesting system were determined by multiplying the saved volume of water from the public network per the corresponding tariff.

Cash flows and metrics
The cash flows were prepared based on the investments, operation and maintenance costs, and previously estimated benefits. The metrics used to conduct the economic assessment of the technological alternatives proposed in this study to replace the water consumption from the public network were the net present value (NPV), the internal rate of return (IRR), and the benefits-investment ratio (B/I). The minimum acceptable rate of return (MARR) used in this evaluation was estimated as follows: The inflation rate was set at 5.76%, based on the information provided by the Bank of Mexico (BANXICO ), and the risk was fixed at 3% due to the low risk associated with this type of investment project (Urbina ). The NPV was estimated using Equation (4): where I 0 is the initial investment; NCF the net cash flow of year n, corresponding to the net benefits after taxes of year n; and i the reference interest rate, set as MARR (Urbina ).
The IRR is the interest rate that makes the NPV zero, and was estimated using Equation (5):

Rainwater potentially harvestable from roofs and maneuvering yards
Due to the median result being more representative than the average, the median was used to determine the monthly rainfall for a confidence interval with a significance level of 95%. The rainwater volumes were calculated using Equation (1) for the three types of catchment surfaces. The catchment area without grease and oil contribution (roofs) was estimated at 6,602.0 m 2 and the areas with grease and oil contribution were estimated at 10,093.8 m 2 for the maneuvering yard and 365.1 m 2 for the administrative building (with air conditioning equipment). Table 1 summarizes the results of these calculations. As seen, the highest rainwater volume was estimated for August; meanwhile, December resulted in the lowest rainwater volume. The annual rainwater volume potentially harvestable from the roofs and maneuvering yard of the company was about 10,586.3 m 3 .

Water consumption from the public network
The annual water consumption was estimated at 2,608.8 m 3 and the greatest water demand occurred in the period May-June with a consumption of 543.7 m 3 . Figure 2 shows the bimonthly water consumption from the public network.   Determination of volumes to be treated and sizing of water storage tanks The water balance results are presented in Table 2. As can be seen, the storage capacity of the tank to meet 100% of the water demand, when only rainwater from roofs is collected,  maximum volume to be treated is also June with 875.3 m 3 .
Meanwhile, when both rainwater sources are collected together, the storage tank capacity needed is 582.0 m 3 and the month with the greatest volume to be treated is May with 797.6 m 3 . Even though there are differences in the monthly volumes to be treated among the three scenarios, the total volume of rainwater to be treated is the same, 2,608.8 m 3 . It is important to remark that in dry months all the rainwater captured is treated; meanwhile, in rainy months only the volume needed to cover the demand is treated.
The dimensions of the storage tanks resulting from the monthly water balances are presented in Table 3. To minimize the costs, the storage tanks were conceived as lining water reservoirs made of linear low-density polyethylene (LLDPE) geomembranes with a formulated sheet density of 0.939 g/ml and 1 mm thickness, and covered by concrete slabs. The cross-section of the reservoirs was trapezoidal, with 1:1 sloped embankments.
As can be seen in Tables 1 and 3, it is clear that greater rainwater harvesting contributes to having a smaller size of storage tank. This result is interesting to analyze because a small size of the storage tank will imply lower investment; but, greater water volume harvested will contribute to having high operating and maintenance costs linked to treatment necessities. Further discussion is conducted in the section below on economic assessment.   NOM--SEMARNAT-) and achieve the treatment level required to remove suspended solids, organic constituents, grease and oils and eliminate fecal coliforms from the rainwater.
In Figure 3, ① represents a first-flush diverter of rainwater. A volume of 2.5 L for every square metre was considered to be diverted as a first flush (Brown et al.

Economic assessment
Determination of the investment and operation costs of the selected treatment systems Table 4  Costs associated with the modification of the water distribution system (WDS) included concepts such as excavation, pipes and accessories. They were calculated based on the unit prices included in the costs catalog of the Mexican Chamber of Construction Industry. As can be Annual operation and maintenance (O&M) costs are also shown in Table 4

Determination of benefits
The benefit expected from the implementation of the rainwater harvesting system was estimated based on the economic savings obtained from the replacement of water consumption from the public network (

Cash flows and metrics
Based on Tables 4 and 5, the cash flow was prepared and it is presented in Table 6. A MARR of 8.76% was set in this study. It was calculated using Equation (3) regarding an inflation rate of 5.76% and a risk of 3.0%, based on the information provided by the Bank of Mexico (BANXICO ) and Urbina (). As can be seen, the investment will be amortized in 5 years and the NPV will be on the order of US$ 5,048.3, the IRR of 5.7%, and the B/I of 1.9. The project will present an IRR greater than the MARR from the sixth year. In a decade, the IRR, NPV and the B/I will be 19.7%, more than twice the MARR, US$ 38,851.4 and 4.3, respectively, denoting economic feasibility. Based on these results, it is clear that the implementation of the rainwater harvesting system resulted in being feasible and reliable to meet the company's total demand of water; furthermore, the investment can be amortized in a short period.

CONCLUSIONS
Rainwater harvesting is presented in this study as a potential alternative to cover the total water demand (100%) of a transportation logistics company. Implementation of the rainwater harvesting system will contribute not only to reducing water consumption from the public network, but also to achieving important economic savings for the company and for the public water system operator, denoting that rainwater harvesting is a feasible and reliable strategy for other uses different to the conventional urban and commercial uses. Such a scheme becomes economically viable and the investment can be amortized in a short period, only 5 years.
The water storage tank represented more than half of the total investment cost of the rainwater harvesting system. The results obtained in this study show that, despite the high cost of the water storage tank, the approach is feasible, reliable, and economically viable when rainwater is used for other uses different to the conventional urban and commercial uses.