rainwater harvesting system research paper

Alternative Water Sources for Producing Potable Water pp 11–29 Cite as

Rainwater Harvesting for Potable Water Supply: Opportunities and Challenges

• Aysha Akter   nAff19  
• First Online: 14 September 2023

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Part of the book series: The Handbook of Environmental Chemistry ((HEC,volume 124))

Rainwater harvesting systems are used worldwide for potable water supply, stormwater reduction, and groundwater recharge. This chapter is dedicated to urban rainwater harvesting for potable water supply, with a focus on the new/advanced system. This chapter presents an overview of worldwide urban rainwater harvesting systems, factors that influence rainwater harvesting system selection, basic design principles, water quality assessment of harvested rainwater, positive and negative aspects of the system, reliability and economic analysis, modeling application, and moving toward a smart water city. Then, it summarizes the roles of city planners, architects, and engineers and contributes to the decision support system.

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Aysha Akter

Present address: Department of Civil Engineering, Chittagong University of Engineering and Technology (CUET), Chittagong, Bangladesh

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Akter, A. (2023). Rainwater Harvesting for Potable Water Supply: Opportunities and Challenges. In: Younos, T., Lee, J., Parece, T.E. (eds) Alternative Water Sources for Producing Potable Water. The Handbook of Environmental Chemistry, vol 124. Springer, Cham. https://doi.org/10.1007/698_2023_1018

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Urban rainwater harvesting systems: Research, implementation and future perspectives

Affiliations.

• 1 Department of Civil Engineering and Architecture, University of Catania, Viale A. Doria, 6, 95125, Catania, Italy. Electronic address: [email protected].
• 2 Centre for Water Systems, College of Engineering, Mathematics and Physical Sciences, University of Exeter, Exeter, EX4 4QF, UK.
• 3 Waterway Ecosystem Research Group, School of Ecosystem and Forest Sciences, University of Melbourne, Burnley, Australia.
• 4 Department of Environmental, Water & Agricultural Engineering, Faculty of Civil and Environmental Engineering, Technion-Israel Institute of Technology, Haifa, 32000, Israel.
• 5 Biological and Agricultural Engineering, North Carolina State University, Campus Box 7625, Raleigh, NC 27695, USA.
• 6 Department of Civil Engineering, University of Cape Town, Private Bag X3, Rondebosch, South Africa.
• 7 Federal University of Santa Catarina, Department of Civil Engineering, Laboratory of Energy Efficiency in Buildings, Florianópoli, SC, Brazil.
• 8 School of Computing, Engineering and Mathematics, University of Western Sydney, Sydney, Australia.
• 9 Research Center for Water Environment Technology, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan.
• 10 Department of Civil and Environmental Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul, South Korea.
• PMID: 28279940
• DOI: 10.1016/j.watres.2017.02.056

While the practice of rainwater harvesting (RWH) can be traced back millennia, the degree of its modern implementation varies greatly across the world, often with systems that do not maximize potential benefits. With a global focus, the pertinent practical, theoretical and social aspects of RWH are reviewed in order to ascertain the state of the art. Avenues for future research are also identified. A major finding is that the degree of RWH systems implementation and the technology selection are strongly influenced by economic constraints and local regulations. Moreover, despite design protocols having been set up in many countries, recommendations are still often organized only with the objective of conserving water without considering other potential benefits associated with the multiple-purpose nature of RWH. It is suggested that future work on RWH addresses three priority challenges. Firstly, more empirical data on system operation is needed to allow improved modelling by taking into account multiple objectives of RWH systems. Secondly, maintenance aspects and how they may impact the quality of collected rainwater should be explored in the future as a way to increase confidence on rainwater use. Finally, research should be devoted to the understanding of how institutional and socio-political support can be best targeted to improve system efficacy and community acceptance.

Keywords: Rainwater harvesting; Stormwater management; Sustainable urban water systems; Water conservation; Water efficiency.

Copyright © 2017 Elsevier Ltd. All rights reserved.

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• Conservation of Natural Resources
• Water Supply / economics*

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Evaluation of rainwater harvesting systems for drinking water quality in iraq.

© 2024 The authors. This article is published by IIETA and is licensed under the CC BY 4.0 license ( http://creativecommons.org/licenses/by/4.0/ ).

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Rainwater harvesting is one of the solutions to avoid water loss in the future because it provides sufficient supply and is more economical when compared to other conventional types. The shortage of water supply become a concern due to the growing population as well as the environmental pollution. Rainwater harvesting is seen as the most accessible and easy-to-use resource for drinking and other domestic uses. The current study consists of two main parts; the first part is a hydrological study that includes studying the possibility of benefiting from the amount of rainfall and developing future plans to benefit from this collected water and how to manage it through the implementation of water harvesting technology by collecting rainwater from the roofs to provide part of the population's water needs instead of its wastage and loss. The second part is an environmental study that includes a study of evaluating the quality of water collected through the harvesting of rainwater technology and comparing it with World Health Organization (WHO) specifications for water-drinking purposes. Rainwater samples were analyzed in the environmental laboratory to compare with (WHO) World Health Organization specifications. Samples were obtained at, (28.3, 84.9, and 33.96) liters, respectively, where the average is (49.05) liters. The depths of rain were recorded in the measuring cylinder (5, 14.6, and 9) mm, respectively, where the average is (9.53) mm; the measurement is a negative indicator compared to the expected (26.15) mm. The variables identified (total hardness, calcium, nitrates, sulfates, chlorides, and dissolved substances). Furthermore, 33.33, 0.80, 35.67, 8.83, and 95.0) mg/L, respectively, while the pH (7.97) and conductivity (µs/cm 170.13) were within the specification and the Temperature (23.60℃) and turbidity 10.47 NTU)) It was not in conformity with the specification, as the specification refers to placing in heat (20℃) and turbidity (5 NTU). The current study discloses that the overall quality of water is quite satisfactory as per WHO specifications. The harvesting of rainwater system offers an adequate amount of water and energy savings through lower consumption. Furthermore, considering the cost of fixing and maintenance expenditures, the system is effective and economical. This current study provides the environmental benefits of rainwater harvesting and identifies its probable boundaries and its role in developing a more sustainable water resource management under climate change. This study contributes to improving adaptability strategies of rainwater harvesting for sustainable water resources management under changing climate.

rainwater harvesting, roof rainwater, storage tank, water quality, water resources, water reuse

Rainwater harvesting is a multipurpose way of providing drinkable during a drought period, recharging the groundwater and finally reducing the runoff and water logging during the season of heavy rainfall [1]. Rooftop rainwater harvesting (RTRWH) is the most common method of harvesting rainwater (RWH) for domestic consumption. Water is covered on the roofs of buildings that are used for its consumption, as it is used to recharge groundwater, or the tank saves water when it is consumed. It rains in the evening. Affected by rainwater, animals or birds, insects, dirt, and organic matter. Therefore, it is important to carry out regular maintenance (cleaning, repairs, etc.), as well as treatment of the water used (such as filtration and/or disinfection). Rainwater harvesting systems gradually become an integral part of the toolkit for sustainable rainwater management. However, there is a need to design more carefully the environmental and life cycle impacts of these systems [2].

The quality of rainwater often exceeds surface water and may be comparable to groundwater because it does not pass through soil and rocks and is not in contact with them, where it can dissolve salts and minerals that are in the atmosphere, which harms potable uses [3]. On the other hand, rainwater is valued for its purity and softness. The quality of rainwater is usually affected by geographical location, human activity in the area, and the storage tank. However, with minimal treatment and adequate care of the water harvesting system, rainwater can be used as potable water as well as for irrigation [4, 5].

A large percentage of the world's population does not have access to safe sources of water. WHO/UNICEF reports that 1.1 billion people do not have access to "safe drinking water sources" (WHO/UNICEF 2000). Although great efforts are being made to deliver safe piped water to the world's population, the reality is that water supplies that provide safe drinking water will not be available to all people in the near future [6]. The World Health Organization's Millennium Declaration set a goal of halving the proportion of the world's population without access to safe drinking water by the year 2015. It is clear that all possible methods must be tried to reduce the drinking water problem and help families obtain drinking water more safely [7].

The quality of any water is determined by identifying the quality of the source water, the extent of its exposure to pollutants during its collection and treatment, and the method of storing it until it reaches the consumer. The rooftop rainwater collection system consists of a collection system (Roof), a transportation system (pipes), and a storage system (tank). Water pollution may occur at any of these stages. Rainwater is generally uncontaminated or at least of fairly good purity. However, it may be acidic and contain traces of lead, pesticides, and some pollutants, depending on the area and prevailing winds [8, 9].

The first flow phenomenon is known to be distinctive because its concentrations are very high at the beginning of the rainfall in the first minutes and later decrease towards a constant value. In general, these dynamic effects are observed within the first 2 mm of runoff height. The first flow effect occurs as a result of one or a combination of the following three processes:

• Materials deposited on the surface during the previous dry period (the period is washed away by rainfall).
• Weathering and corrosion residues of the roof covering.
• Concentrations in the falling rain itself decrease with increasing depth of rainfall due to melting particles and gases by raindrops.

The quality of rainwater collected through first flow diversion can be significantly improved. However, most of the time, this interest is not considered due to a variety of reasons. Proper maintenance of the first flush alone can greatly improve the quality of the collected rainwater [10].

Contrary to popular belief that rainwater collected from rooftops is as clean as pure water, there are a number of pollutants in such water - and many previous studies have confirmed this. Recognizing this fact, it has become possible to control the quality of water and reach acceptable levels using simple and inexpensive devices [11, 12].

Current research addresses these issues in addition to reviewing the quality of harvested rainwater due to its exposure to pollutants. In this background, the study also shows water quality management strategies in the post-harvest stages.

2.1 Area of the study

The current study was conducted in Erbil city north of Iraq. The house roof area is a hundred square meters (with dimensions of 5 × 20 meters), as shown in Figure 1. The rain period is usually between October to May, and the average annual rainfall is between 45.8 mm and 0.4 mm (2020), with an average of between 180-210 days annually. The study area was selected due to the lack of a river in the city, which is the main source of water for domestic use from wells. Unfortunately, most wells dry out in the dry season when the rain stops. The people depend heavily on available rainwater because it is believed to be economical, affordable, and drinkable during the rainy season.

For each rainfall intensity and each other, the height of the water in the collection tank is measured to determine the volume of water collected from the rainfall intensity.

Figure 1. Iraq map and roof plan of the house

The system consists of the following items, as shown in Figure 2.

• Roof: The area through which rainwater is collected, where work is done to connect the tank to the Roof through the drainage pipes of the house, and conditions such as cleaning the Roof from soil to prevent sediment are considered. Materials in the water are collected, as well as ensuring water quality and no pollution.
• Collection pipe: The pipe transports water from the Roof of the house to the collection tank. It is preferable to take into account that the diameter of the pipe is proportional to the surface area and the amount of rain expected to be collected.
• Collection tank: The tank to collect runoff. The tank with a capacity of 200 liters and a diameter of 60 cm.

As for measuring the intensity of rain, a measuring cylinder with a 10 cm diameter was placed above the Roof of the house, and between one rain and another, the height of the water in the measuring cylinder was taken to calculate and estimate the type of rain intensity.

Figure 2 . Harvesting of rainwater system

2.2 Method of sampling and collecting

2.2.1 Harvesting of rainwater

The samples of rainwater were collected from the selected house roof. In the year 2020, during January-April, the rainwater was collected from the study area. The tank was washed with clean water and drained before using to gather the rainwater samples from the system. The tank with a pipe rainwater was connected to the derange of the house (system), and then collected the water from the rainwater; after the process of water collecting, water quality was tested. The Temperature and pH of the rainwater sample were measured immediately after gathering. The samples were kept in a sterilized capped bottle and then taken to the laboratory for microbial and chemical analysis. Mean annual rainfall was around 26.15 (mm/month) in 2020, as shown in Figures 3 and 4. The figures show the amounts of rainfall this year (dark blue) and the average amounts for the previous years (20 years, 1994-2013) in light blue. The figures show rainfall in the region and its distribution over the months, especially for periods when rainfall is consistently above or below average, during the early stages of the season, and during times of need [13].

Figure 3 . Rainfall hytograph for the study area

Figure 4 . Rainfall anomaly variation

2.2.2 Water quality testing

Water quality exams must be regularly carrying out by a relevant in-country agency, such as the Ministry of Health. The parameters, total hardness, calcium, nitrates, sulfates, chlorides, and dissolved substances pH, Temperature, Electrical Conductivity, and turbidity must be examined and compared to WHO specifications. According to the World Health Organization (WHO), water use purposes of drinking have an important impact on health. Therefore, preserving the quality of water is very important. World health organization and other national and international organizations have put specifications of water quality to be used as references for preserving water quality [14]. Table 1 shows the parameters from World Health Organization Specifications:

Table 1. World Health Organization specifications [7]

3.1 Harvesting of rainwater

For the harvesting of rainwater, the results as shown in Table 2:

Table 2. Water harvesting variables

The volume of water harvested in the second sample was (84.9) liters, which is greater than the first and third samples, which were (28.3 and 33.96) liters, respectively, with an average of (49.05) liters less than the expected (200) liters due to the dry season and lack of rain, as shown in Figure 5. As for measuring the height of rainwater with the measuring cylinder, where the reading was 14.6 mm, compared to the first and third samples, it was (5 and 9) mm, respectively. As for the rate of measuring the height of rainwater with the measuring cylinder is (9.53) mm, it is also considered a negative indicator because it is much less than the expected 26.8 mm, as shown in Figure 6.

Figure 5 . The volume of rainwater

Figure 6 . The depth of water in cylindrical measurement

3.2 Water quality analysis

For the water quality, samples of three different rainfall intensities were examined in the laboratory, as shown in Table 3, and compared with the determinants of the water quality indicator according to the quality of the variable.

Table 3. The sample of parameters

Figure 7 shows the relationship between the samples of total hardness (206, 79.2 and 79.2) mg/l and its average (121.47) mg/l compared with WHO-specific (300) mg/l.

Figure 7 . The total hardness for samples

Figure 8 shows the relationship between the samples of calcium (64, 20 and 16) mg/l and its average (33.33) mg/l compared with WHO-specific (75) mg/l.

Figure 8 . The Calcium value for samples

Figure 9 shows the relationship between the samples of Nitrate (0.7, 0.9 and 0.8) mg/l and its average (0.8) mg/l compared with WHO-specific (10) mg/l.

Figure 9. The Nitrate for samples

Figure 10 shows the relationship between the samples of Sulfate (45, 29 and 33) mg/l and its average (35.67) mg/l compared with WHO-specific (400) mg/l.

Figure 10 . The Sulfate value for samples

Figure 11 . The Chloride for samples

Figure 11 shows the relationship between the samples of Chloride (8.5, 8.5 and 9.5) mg/l and its average (8.83) mg/l compared with WHO-specific (250) mg/l.

Figure 12 shows the relationship between the samples of total dissolved solids (199, 37 and 49) mg/l and its average (95) mg/l compared with WHO-specific (500) mg/l.

Figure 12 . Total dissolved solids

Figure 13. The pH value for the collected samples

Figure 14 . The temperature of the samples

Figure 13 shows the relationship between the samples of pH (8.6, 7.8, and 7.5) and its average (7.97) compared with the upper and lower WHO-specific (6.5-8.5).

Figure 14 shows the relationship between the sample's Temperature (23.6, 23.6, and 23.6)℃ and its average (23.6)℃ compared with WHO-specific (20)℃.

Figure 15 shows the relationship between the samples of electrical Conductivity (333.5, 77, and 99.9) s/cmµ and its average (170.13) µs/cm compared with WHO-specific (1000) µs/cm.

Figure 15. Electrical conductivity for the collected samples

Figure 16 shows the relationship between the samples of turbidity (27.5, 3.4, and 0.5) NTU and its average (10.47) NTU compared with WHO-specific (5) NTU.

Figure 16. The turbidity of the collected samples

The parameters were identified as total hardness (121.47 mg/l), calcium (33.33 mg/l), nitrates (0.8 mg/l), sulfates (35.67), chlorides (8.83 mg/l), and total dissolved substances (95.0 mg/l), while the pH (7.97) and conductivity (µs/cm 170.13) were within the specification, and the Temperature (23.60℃) and turbidity 10.47 NTU It was not in conformity with the specification, as the specification refers to placing in heat (20℃) and turbidity (5 NTU).

• This year (2020), there was little rain compared to previous years.
• The results of the tests of variables were (total hardness, calcium, nitrates, sulfates, chlorides, and dissolved substances) respectively, conform to the specified specification.
• Many gases are dissolved in the air when rainfall, such as acidic CO 2 gas, which reduces the PH value; then, the solubility of the basic salts increases, which raises the pH value, so it remains constant to a certain range, where the determinant of the water quality index indicates a value between (6.5 and 8.5).
• The high-water temperature does not have a direct impact on the water quality. However, it increases the proliferation of bacteria in the water, reduces the dissolved oxygen in the water, increases the solubility of substances such as salts, and increases the speed of chemical reactions between organic substances. The reading was less than the specified standard.
• The difference in the conductivity values and the reason for this is due to the presence of dirt on the surface where the pollution was as much as possible. The general average of samples for conductivity values is (170.13 µs/cm), and it is within the specified specification (1000 µs/cm).
• The difference in turbidity is due to the presence of dirt on the surface where the contamination was as much as possible. The general average of the samples for turbidity values is (10.47 NTU), which does not conform to the specified specification (5 NTU).

It is possible to say that there is ample scope to study water harvesting from an economic perspective, especially in cities that depend on marketing water to users at high costs.

[1] Rahman, S., Khan, M.T.R., Akib, S., Bin Che Din, N., Biswas, S.K., Shirazi, S.M. (2014). Sustainability of rainwater harvesting system in terms of water quality. The Scientific World Journal, 2014: 721357. https://doi.org/10.1155/2014/721357 [2] Che-Ani, A.I., Shaari, N., Sairi, A. (2009). Rainwater harvesting as an alternative water supply in the future. European Journal of Scientific Research, 34(1): 132-140.  [3] Payus, C., Meng, K.J. (2015). Consumption of rainwater harvesting in terms of water quality. International Journal of GEOMATE, 9(2): 1515-1522. https://doi.org/10.21660/2015.18.95782 [4] Olaoye, R.A., Olaniyan, O.S. (2012). Quality of rainwater from different roof material. International Journal of Engineering and Technology, 2(8): 1413-1421.  [5] Khanoosh, A.A., Khaleel, E.H., Mohammed-Ali, W.S. (2023). The resilience of numerical applications to design drinking water networks. International Journal of Design & Nature and Ecodynamics, 18(5): 1069-1075. https://doi.org/10.18280/ijdne.180507 [6] Meera, V., Ahammed, M.M. (2006). Water quality of rooftop rainwater harvesting systems: A review. Journal of Water Supply: Research and Technology-Aqua, 55(4): 257-268. https://doi.org/10.2166/aqua.2006.0010 [7] Guidelines for drinking-water quality: First addendum to the fourth edition. (2017). Journal AWWA, 109(7): 44-51. https://doi.org/10.5942/jawwa.2017.109.0087 [8] Ganoulis, J. (2009). Risk Analysis of Water Pollution. John Wiley & Sons.  [9] Mohammed-Ali, W.S., Khairallah, R.S. (2022). Review for some applications of riverbanks flood models. IOP Conference Series: Earth and Environmental Science, 1120: 012039. https://doi.org/10.1088/1755-1315/1120/1/012039 [10] Gwenzi, W., Dunjana, N., Pisa, C., Tauro, T., Nyamadzawo, G. (2015). Water quality and public health risks associated with roof rainwater harvesting systems for potable supply: Review and perspectives. Sustainability of Water Quality and Ecology, 6: 107-118. https://doi.org/10.1016/j.swaqe.2015.01.006 [11] Tebbutt, T.H.Y. (1997). Principles of Water Quality Control. Elsevier.  [12] Mohamme-Ali, W.S., Khaleel, E.H. (2023). Assessing the feasibility of an explicit numerical model for simulating water surface profiles over weirs. Mathematical Modelling of Engineering Problems, 10(3): 1025-1030. https://doi.org/10.18280/mmep.100337 [13] Climate explorer. https://dataviz.vam.wfp.org/climate-explorer, accessed on Feb. 22, 2024. [14] Yidana, S.M., Yidana, A. (2010). Assessing water quality using water quality index and multivariate analysis. Environmental Earth Sciences, 59: 1461-1473. https://doi.org/10.1007/s12665-009-0132-3

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• v.2014; 2014

Sustainability of Rainwater Harvesting System in terms of Water Quality

Sadia rahman.

1 Department of Civil Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia

M. T. R. Khan

2 Department of Architecture, Faculty of Built Environment, University of Malaya, 50603 Kuala Lumpur, Malaysia

Shatirah Akib

Nazli bin che din, s. k. biswas.

3 Department of Civil Engineering, Bangladesh University of Engineering & Technology, Dhaka 1000, Bangladesh

S. M. Shirazi

4 Institute of Environmental and Water Resources Management (IPASA), Faculty of Engineering, Universiti Teknologi Malaysia, 81310 Johor, Malaysia

Water is considered an everlasting free source that can be acquired naturally. Demand for processed supply water is growing higher due to an increasing population. Sustainable use of water could maintain a balance between its demand and supply. Rainwater harvesting (RWH) is the most traditional and sustainable method, which could be easily used for potable and nonpotable purposes both in residential and commercial buildings. This could reduce the pressure on processed supply water which enhances the green living. This paper ensures the sustainability of this system through assessing several water-quality parameters of collected rainwater with respect to allowable limits. A number of parameters were included in the analysis: pH, fecal coliform, total coliform, total dissolved solids, turbidity, NH 3 –N, lead, BOD 5 , and so forth. The study reveals that the overall quality of water is quite satisfactory as per Bangladesh standards. RWH system offers sufficient amount of water and energy savings through lower consumption. Moreover, considering the cost for installation and maintenance expenses, the system is effective and economical.

1. Introduction

Dhaka is a densely populated city with an area of 1425 km 2 [ 1 ] which is already labelled as a mega city [ 2 – 4 ]. This significant population craves a larger amount of water for different purposes. Therefore, there is always a shortcoming of supplied water due to an imbalance between demand and supply. Dhaka Water Supply and Sewerage Authority (DWASA) is the only authoritative organization available to deliver consumable water to Dhaka City dwellers. DWASA [ 1 ] provides 75% of total demand of water in which about 87% is accumulated from groundwater sources, and the remaining 13% is collected from different treatment plants. Dhaka presently relies heavily on groundwater, with approximately 80 to 90% of demand coming from this source. Overreliance on groundwater sources is depressing the water level. Every year the groundwater table is dropping down around 1 to 3 m due to the extreme amount of withdrawal. Figure 1 shows the groundwater level depletion trend for Dhaka City. Moreover, scientific studies on the groundwater revealed that excessive exploitation has been lowering the aquifer level, thus limiting natural recharge [ 5 , 6 ]. Additionally, overexploitation for longer periods may account for several natural hazards such as unexpected landslides, sustained water logging, reduction in soil moisture, and changes in natural vegetation [ 2 , 7 – 9 ].

Groundwater depletion in Dhaka City [ 1 ].

Conjunctive use of groundwater and surface water would be one potential solution to reduce heavy reliance on groundwater. Surface water treatment plants are treating polluted water before delivering it to a supply pipeline. But the level of pollution of surface water has limited the applicability of the treatment process. DWASA supplies 2092.69 million liters of water daily against the current demand for 2815.7 million liters [ 1 ], which indicates that the city is facing a huge shortage of water daily. All the scenarios between water demand and supply prevail the immediate need for adopting alternative solutions to release the pressure on water sources. Moreover, current water practices have limited attention to the climate change impacts on water availability [ 10 ]. Surveys on climate projections provide evidence on critical impacts of climate on natural water sources that eventually affect human societies and ecosystems [ 11 ].

Rainwater harvesting (RWH) could be the most sustainable solution to be included in the urban water management system. It could mitigate the water crisis problem, reduce the burden on traditional water sources, alleviate nonpoint source pollutant loads, control water logging problems, prevent flooding, help in controlling climate change impacts, contribute to the storm water management, and so forth [ 12 – 16 ]. Water scarcity and the limited capacity of conventional sources in urban areas promote RWH as an easily accessible source [ 17 ]. The system could be utilized locally and commercially for securing water demand in water-scarce areas all around the world. Harvested rainwater could be idealized and used like supply water if the water-quality parameters satisfy the desired level. The monitoring of collected rainwater is of great concern as it is the potential for health risk because of the presence of chemical and microbiological contaminants [ 18 ]. Therefore quality assessment of collected water is essential before use. This paper is mainly focused on scrutinizing and assessing water-quality parameters as per allowable limit and also on the financial benefit acquired by using this technique. Finally this paper suggests a rainwater harvesting system as a potential source of water supply in Dhaka City.

2. Water Scenario in Dhaka City

About 75% of total demand of water in Dhaka is supplied by DWASA, and the rest comes from privately owned tube wells. At present DWASA can yield about 2092.69 million liters (ML) [ 1 ] per day in which about 1840.04 MLD is collected from 586 deep tube wells (DTW), and the remaining 252.65 MLD is supplied by two surface water treatment plants [ 1 ]. More details are given in Figure 2 .

Water production per day in Dhaka city [ 1 ].

Buriganga, Balu, Turag, and Tongi Khal are the main four water bodies surrounding the city and could be an ideal sources of water supply [ 19 , 20 ]. But these water bodies already lost their potentiality as sources of supply due to the huge pollutions. Untreated municipal and industrial wastes make the river water so contaminated that most of the water quality parameters surpassed their allowable level. However, the water supply authority mainly relies on groundwater sources and needs to install more tube wells to fulfill demand [ 21 , 22 ]. Installation of more tube wells must lower the groundwater level. Therefore it is urgent to find a sustainable solution that could alter the usage of groundwater. Rainwater harvesting would be one of the most conceivable and viable solutions to release the pressure on the groundwater table as the system utilizes natural rainwater without affecting groundwater sources.

3. Water Supply and Demand Variation

In order to understand the variation between demand and supply, the total demand needs to be known. That could be calculated through population data and per capita demand. According to Bro [ 23 ], per capita demand for 2006 was about 200 liters, including 10% provisions for commercial use and 40% due to system loss during supply. As per capita demand will be assumed to be decreased in the future by proper inspection and management, for 2015 the total per capita demand will stand at 180 liters per day and for 2025 and 2030 at 160 liters per day. According to DWASA, 2011 [ 24 ], the water supply is about 1356.67 MLD (considering service flow with 40% leakages), and the total demand is 2200 MLD (assuming 85% service area). So the deficit is about 843.33 MLD. As demand is more than just supplied water, deficit prevails, which is increasing every day. Therefore the water crisis becomes a normal issue due to this huge deficit in Dhaka City during the dry period. The trend of deficit is due to difference in demand and supply as shown in Figure 3 . In 1963 the total demand was 150 million liters (ML), which turned into 2240 million liters in 2011 due to the augmentation of the population. Within 48 years demand became 15 times more than expected. In a similar way, the deficit also crosses predicted values. In 1963 the deficit was 20 ML, and in 2010 it became 190 ML, which was more than calculated. But after that, the shortage became something better than in the previous year. This indicates that supply capacity is improving, and authorities are trying to reduce the shortages. The overall deficiency of supplied water triggers the need for augmentation and improvement of the water supply system to meet the increased demand in future [ 5 ].

Relation among water demand, supply, and deficit in Dhaka City [ 1 ].

Figure 4 shows the variation of the water deficit with the present supply and variation of the population for the projected years. If the present supply prevails for the coming years, the deficit of water will be increasing to a high amount that could not be alleviated within the allowable limit.

Present water supply, shortage, and population variation for projected years.

Dhaka is located in a hot and humid country, and its annual temperature (25°C) categorizes the city as monsoon climate zone. The city is blessed by a huge amount of rainfall during the monsoon period, which poses ample opportunity to use this rainwater in a sustainable manner [ 25 ]. Figures ​ Figures5, 5 , ​ ,6, 6 , and ​ and7 7 show the monthly rainfall pattern, monthly average relative humidity, and the maximum and the minimum monthly temperature trend, respectively, for Dhaka City.

Monthly average rainfall in mm in Dhaka City.

Monthly average relative humidity (%) in Dhaka City.

Maximum and minimum temperature (°C) trend in Dhaka City.

The common practices of recharging natural aquifers are by direct rainfall, river water, and direct infiltration and percolation during floods [ 26 ]. Overpopulation makes these options inappropriate by reducing the recharge area. Covering the vertical recharge inlets with pavement materials or other construction materials can cause water logging for even small duration heavy rainfall in most areas of Dhaka City. Inadequate storm water management infrastructures and improper maintenance of storm sewer systems further aggravates the scale of this problem. Harvesting of this storm water in a systematic way thus prevents water logging. Furthermore, utilization of collected rainwater highly releases the dependency on groundwater sources.

4. Rainwater Harvesting

Rainwater harvesting is a multipurpose way of supplying usable water to consumers during a crisis period, recharging the groundwater and finally reducing the runoff and water logging during the season of heavy rainfall. Traditional knowledge, skills, and materials can be used for this system. During the rainy season, an individual can collect water on his rooftop and manage it on his own. Reserved rainwater on rooftops can be used for self-purposes or domestic use. Water from different rooftops of a lane can also be collected through a piped network and stored for some time. This water can be then channeled to deep wells to recharge groundwater directly, to ponds to replenish groundwater slowly, and to reservoirs to dilute reclaimed water for nonpotable use. Figure 8 shows the schematic view of a rainwater harvesting system.

Schematic of a rainwater harvesting system.

Unless it comes into contact with a surface or collection system, the quality of rainwater meets Environmental Protection Agency standards [ 27 ], and the independent characteristic of its harvesting system has made it suitable for scattered settlement and individual operation. If needed, a chemical treatment such as chlorination can be used to purify the water. The acceptance of rainwater harvesting will expand rapidly if methods are treated such as building services and if designed into the structure instead of being retrofitted [ 28 ].

5. Benefits of Rainwater Harvesting

Rainwater harvesting is a simple and primary technique of collecting water from natural rainfall. At the time of a water crisis, it would be the most easily adaptable method of mitigating water scarcity. The system is applicable for both critical and normal situations. It is an environmentally friendly technique that includes efficient collection and storage that greatly helps local people. The associated advantages of rainwater harvesting are that

• it can curtail the burden on the public water supply, which is the main source of city water;
• it can be used in case of an emergency (i.e., fire);
• it is solely cost effective as installation cost is low, and it can reduce expense that one has to pay for water bills;
• it extends soil moisture levels for development of vegetation;
• groundwater level is highly recharged during rainfall.

6. Quality of Rainwater

The quality of harvested rainwater is an important issue, as it could be utilized for drinking purposes. Quality of captured water from roof top depends on both roof top quality and surrounding environmental conditions, that is, local climate, atmospheric pollution, and so forth [ 11 ]. Tests must be performed to check its viability and applicability before using as drinking water. Previous researches [ 29 – 31 ] showed that water quality of collected water did not always meet standard limits due to unprotected collection. Local treatment of harvested water could easily make water potable. Again rainwater could be also identified as non-potable sources for the purpose of washing, toilet flushing, gardening, and so forth, where quality is not a great concern. In this respect, treatment of collected water is of no such importance; rather it is used for household purposes. In this paper an assessment has been made on the quality of rainwater collected through a well-maintained catchment system.

7. Methodology

Rainwater harvesting is a more effective technology that could be easily undertaken through normal equipment during a water crisis. Qualitative assessment is important before introducing collected rainwater as potable water. In this paper, a case study has been made to check rainwater quality to identify its acceptability and suitability as household water. Water samples were collected from the selected residential building where a rainwater harvesting system was introduced successfully using laboratory prepared plastic bottles to collect samples. The samples were bottled carefully, so that no air bubble is entrained in the bottle. All parameters were measured in the environment laboratory of Bangladesh University of Engineering Technology (BUET).

The maximum amount of rainwater that could be encountered from a roof top is

where V is the amount of harvestable water, A is catchment area, R is total amount of rainfall, and C is the runoff coefficient.

Equation ( 1 ) was used to calculate the amount of harvested water from a residential building located at Dhaka, Bangladesh. The system was designed for meeting water requirements of 60 persons living in the entire building. Total area was about 3600 sq. ft. (square feet). Maximum ground coverage would be around 2250 sq. ft. (considering the floor area rule of RAJUK, the city development authority), and within this area 1850 sq. ft was used as catchment area where rainwater was collected. Per capita water consumption is about 135 lpcd for conservative use. The total demand for this building stands at about 8100 liter per day and 243,000 liters per month. In a practical case, the size of the catchment area is taken from maximum ground coverage. To get an overview of the amount of collected rainwater, monthly average rainfall data from January to December has been considered, including the dry and monsoon periods. The runoff coefficient value was taken as 0.85. For analysis purpose, a one-year rainfall data were considered. Volume of collected rainwater was also an important aspect in introducing rainwater for domestic purposes. In the selected time frame, maximum volume of water was collected during June, 2012, which was about 4.5 m 3 and a minimum was collected during October, 2011. Significant amount of water could be collected during heavy rainfall. From this point of view, it could be said that, with larger catchment area, amount of harvested water would be significant to be used in household works.

8. Results and Discussion

The main focus of this paper relies on several aspects, such as examining the quality of water with respect to standard values, analyzing associated financial benefits in terms of cost, and considering water and energy conservation and lastly suggesting the system as a potential source of water both in normal and critical situations.

In this section, the quality of harvestable water was checked considering several parameters such as pH, fecal coliform, total coliform, total dissolved solids, turbidity, NH 3 –N, lead, and BOD 5 . The time period for analysis was from October 2010 to October 2011. Two different collecting points were considered: water collected before entering into the storage tank (called first flush water) and water collected from the storage tank (tank water). Figure 9 shows the variation of pH over time. According to Bangladesh standards for drinking water [ 32 ], the allowable limit for pH is 6.5 to 8.5. Results showed that pH value for both flash and tank water was very near to this range during the tested time period. Therefore, the pH level of collected water did not pose any threat to water quality and conformed to the standard limit.

Variation of pH over time.

Figure 10 shows the variation of total coliform over time. The number of total coliforms present in the water was quite low until June 2011. After that a large number of total coliform grew in both flash and tank water. Figure 11 shows the variation of fecal coliform over time. In the case of drinking water, it is expected that water should be free from all types of fecal and total coliforms. In the present case, at first in October 2010, few fecal coliforms were found in water. It remains zero until March 2011. But after that there was an increasing trend in the number of fecal coliform. In October 2011, there was huge number of fecal coliform, which is not expectable for drinking water. In both cases (fecal and total coliform), at first when rainwater was harvested, growth of coliform was lower but with time those increased to a large quantity. From June 2011, rainfall was not adequate and maintenance was not proper, which is why coliform grew to a huge quantity in the stored unused water. As pure water should be free from all kinds of coliforms, proper maintenance of tank and catchment areas could minimize coliform level and make rainwater safe for household purposes.

Variation of total coliform over time.

Variation on fecal coliform with time.

Figure 12 shows the variation of total dissolved solids over time. The allowable limit for total dissolved solids (TDS) in drinking water is about 1000 (mg/L) according to Bangladesh standards for drinking water [ 32 ]. For all the selected periods, the total dissolved solids in collected water were quite lower than the standard limit. Therefore total dissolved solids did not pose any threat to water used for drinking purposes. Figure 13 shows the variation of turbidity over time. The standard limit for turbidity is 10 NTU. The measured turbidity level in collected water was below this standard limit. Therefore rainwater could be considered satisfactory from an aesthetic point of view. In a similar way, the NH 3 –N level was quite below the standard limit (0.5 mg/L) during the collection period ( Figure 14 ).

Variation of total dissolved solids over time.

Variation of turbidity over time.

Variation of NH 3 –N over time.

Figure 15 shows the variation of BOD 5 in the collected flash and tank water. In all of the selected time period, BOD 5 is less than the Bangladesh standard for drinking water [ 32 ]. Another thing, BOD 5 became less in flash water than in tank water. Due to the lack of proper maintenance, BOD 5 increased in the tank water. Further treatment may make water more usable for household work. In order to analyze the water quality in terms of lead concentration in collected water, tests were performed, which found that lead concentration always remained below the allowable limit according to the Bangladesh standards for drinking water [ 32 ]. Figure 16 shows the variations of lead concentrations with time.

Variation of BOD 5 over time.

Variation of lead over time.

9. Cost Effectiveness Analysis

Thefinancial benefit associated with a rainwater harvesting system is solely connected with cost. The associated costs of a rainwater harvesting system are for installation, operation, and maintenance. Of the costs for installation, the storage tank represents the largest investment, which can vary between 30% and 45% of the total cost of the system dependent on system size. A pump, pressure controller, and fittings in addition to the plumber's labor represent other major costs of the investment. A practical survey showed that (in Dhaka) the total cost related to construction and yearly maintenance of a rainwater harvesting system for 20 years' economic life is about 30000 BDT. This cost includes construction cost of tanks, gutters, and flushing devices and labor cost [ 33 ]. In the present case study, about 313.80 thousands liter water can be harvested from rain over one year. This amount of water could be collected within 1850 sq. ft catchment area and considering monthly rainfall data. The yearly consumption of this selected building stands at 2916 thousands liters. Therefore utilizing harvested rainwater for this building can save up to 11% of the public water supply annually. This volume of rainwater can serve a building with 60 members for about 1.5 months in a year without the help of traditional water supply. Figure 17 shows the month-wise harvestable amount of rainwater and the associated amount of cost savings. Furthermore, considering DWASA current water bill, about 8359.70 BDT can be saved per year, and about 125395.30 BDT can be saved in 15 years if rainwater is used for daily consumption. So, within three to four years, the installation cost of a rainwater harvesting system can be easily returned. Moreover, the building owner would be exempted from paying large amount of water bill as well as additional taxes and fees charged by the city authority with the water bill if rainwater is utilized for daily consumption. Cost comparison and associated benefit between a rainwater harvesting system and traditional water supply system encountered and revealed a rainwater harvesting system as a cost-effective technology.

Month-wise harvestable amount of rainwater and the associated cost savings.

10. Water Savings Strategy

Rainwater harvesting system plays an important role in developing sustainable urban future [ 24 ]. Availability of water of serviceable quality from conservative sources is becoming limited day by day due to huge demand. Rainwater provides sufficient quantity of water with small cost. Hence, the system can promote significant water saving in residential buildings in many countries. Herrmann and Schmida [ 35 ] studied that potential saving of roof captures water was about 30–60% of potable water demand in a house depending on the demand and catchment area. Coombes et al. [ 36 ] analyzed 27 houses in Australia with rainwater harvesting system and found that about 60% of potable water could be saved. Ghisi et al. [ 37 ] performed investigation on collected rainwater in Brazil and found that about 12–79% of potable water could be saved depending on the size of roof tank. Most of the researches on rainwater harvesting systems (RWHS) revealed that water conservation achieved through RWHS is quite significant especially in places where water is not easily available to consumers.

11. Energy and Climate

Conventional use of water imparts critical impacts on natural resources. Water collection from ground and surface sources, treatment, and distribution are closely associated with energy consumption, however, being related to climate consequences. The extraction of water from the sources, the treatment of raw water up to the drinking standards and the delivery of water to the consumers require high energy. Moreover, there should be some energy losses during performing extracting, treating, and delivering of water. Therefore, the water sector consumes a huge amount of electricity from local and national grid. Approximately 300 billion kilowatt hours of energy could be saved if potable water demand could be reduced by 10% [ 38 ]. Adoption of RWHS is one of the most potential solutions that could save energy directly by reducing potable water demand. Table 1 represents the estimated energy required to deliver potable water to consumers. Reduction of water demand by 1 million gallons can result in savings of electricity use by 1,500 kWh. In the present case study, with an 1850 sq. ft. catchment area, about 69,026 gallons (313.8 thousands liters) could be harvested over one year. However, this amount could reduce potable water demand and approximately 100 kWh electricity could be saved in the selected residential building by introducing rainwater capturing system. Integrating rainwater harvesting system with the conventional water collection and distribution approach in residential as well as large scale, nonresidential applications suggest a potential method of reducing energy use. However, limiting energy demand has critical impact on carbon dioxide emissions, as release of carbon dioxide is closely associated with electricity generation. There should have sufficient reduction in carbon dioxide emissions when fossil fuel is used for power generation. Hence, limited contribution is to be expected from lower carbon release in climate change concept. Table 2 showed the carbon dioxide emissions from electric power generation.

Energy consumption in conventional water resources system [ 34 ].

Carbon dioxide emission from water treatment and distribution system [ 39 ].

However, water use should be critically judged from availability, safety, and sustainability of natural resources. Energy conservation is a critical component in sustainability concern. Decreased use of conventional potable water reduces energy demand that in turn reduces emission of carbon dioxide. Integrated water management approach with rainwater harvesting along with gray water and reclaimed water reuse could limit contributions to climate change and conserve limited water and energy resources.

12. Future Action Plan

Rainwater is one of the advantageous methods of using natural water in a sustainable manner. Rain is a blessing of nature. Densely populated cities with a water crisis and adequate rainfall should adopt this technology. Cities like Dhaka, where water is a major concern during dry periods, should introduce this system along with its traditional water supply system. Pressure on groundwater tables thus could be prevented, and natural recharging would also be proceeded through this system. Regular maintenance of harvested water might make it suitable for daily consumption. Water shortages will become the most concerned issue all around the world in the future. Therefore city planners should rethink of the possibilities, outcome, and benefits of a rainwater harvesting system and should create policies to make the system easily available to everyone. The following research could be made in future.

• This study focused only on rainwater harvesting system on a small scale basis. Further research could be performed on large scale residential, commercial or industrial sector.
• Comparisons could be made with rainwater harvesting systems to conventional ground water system on the basis of quality, quantity, environmental impacts, energy saving, water conservation, economy, and so forth.
• Case studies could be investigated to evaluate energy consumption in rainwater system with ground water system in a large scale. In a more applied setting, energy efficiencies of large scale rainwater harvesting systems should be analyzed to help determine the future of rainwater harvesting as a valuable technology for providing water, a crucial resource that is becoming more depleted with the ever increasing population and water demand.
• A comprehensive cost-benefit analysis should be performed on different climate regions to get essential insight on the economic viability of rainwater harvesting system (RWHS).
• More detailed and advanced research on impacts on climate factors, human health risk, and potential ecological aspects should be performed in a large scale.
• More comprehensive studies for better quantification of energy and climate factors should be made for proper development of the system.
• Rainwater could be highly polluted by pesticides in any agricultural region. Hence, biological and chemical analysis should be done before adopting harvested rainwater as a source of daily water.

13. Conclusion

Water shortage is one of the critical problems in Dhaka City. This problem is not new one, and it cannot be solved overnight. As DWASA relies on groundwater abstraction through deep tube wells to overcome the excessive demand, the water table is lowering day by day, and the recharge of groundwater table is facing difficulties. Rainwater harvesting is an effective option not only to recharge the groundwater aquifer but also to provide adequate storage of water for future use. This paper tried to focus on the sustainability and effectiveness of a rainwater harvesting system in terms of quality. Water was collected in a well maintained catchment system from rain events over one year and chemical analysis was performed regularly to observe the quality of collected water. The overall quality of rainwater was quite satisfactory and implies that the system could be sustained during critical periods as well as normal periods. Additionally, the system is cost effective as large amounts of money can be saved per year. Energy conservation and related reduced emissions are crucial parts of this system. Moreover, increased awareness on water crisis has led rainwater harvesting to be proposed as a community facility. The small and medium residential and commercial construction can adopt this system as sustainable option of providing water. It is almost the only way to upgrade one's household water supply without waiting for the development of community system. The system could become a good alternative source of water supply in Dhaka City to cope up with the ever-increasing demand and should be accepted and utilized by the respective authorities as well as by the city dwellers.

Acknowledgments

The authors gratefully acknowledge the support of Bangladesh University of Engineering and Technology (BUET). This research is financially supported by University Malaya Research Grant (UMRG) RP009/2012 and High Impact Research Fund, Project no. UM.C/625/1/HIR/MOHE/ENG/61.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

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Materials and methods, results and discussion, conclusions, acknowledgements, rainwater harvesting as an alternative for water supply in regions with high water stress.

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Miguel Ángel López Zavala , Mónica José Cruz Prieto , Cristina Alejandra Rojas Rojas; Rainwater harvesting as an alternative for water supply in regions with high water stress. Water Supply 1 December 2018; 18 (6): 1946–1955. doi: https://doi.org/10.2166/ws.2018.018

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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.

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 Mexico, resulting in an infallible decrease in water resources in the coming years ( Bates et al. 2008 ). On the other hand, the growing population, rapid urbanization, industrialization and intensive agriculture of Mexico are putting remarkable pressure on water resources availability to the extent that Mexico City faces an extremely low water availability, on the order of 150 m 3 /person/day ( CONAGUA 2015 ). Consequently, the interest in and necessity of the use of alternative water sources such as rainwater are also growing ( Antunes et al. 2016 ). Rainwater harvesting may be an effective supplementary water source because of its many benefits and affordable costs ( Imteaz et al. 2011 , 2012 ; Cook et al. 2014 ; Gurung & Sharma 2014 ; Liuzzo et al. 2016 ). Information related to the use of rainwater to meet the water supply demands of urban dwellings is somewhat available; 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.

Rainwater potentially harvestable from roofs and maneuvering yard

Figure 1 shows a view of catchment surfaces of the buildings and maneuvering yard considered in this study. The surfaces were grouped into two categories, areas with and areas without grease and oil contribution. The areas of the catchment surfaces were estimated by using AutoCAD ® (Autodesk, San Rafael, CA, USA).

Catchment surfaces of buildings and maneuvering yard.

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. Quality parameters were selected from Mexican Official Standards for wastewater discharge into water bodies and soil ( Ministry of Environment 1996 ) and wastewater reuse with direct and indirect contact ( Ministry of Environment 1997 ). Each parameter was measured based on the protocols described in Standard Methods for the Examination of Water and Wastewater ( APHA/AWWA/WEF 2005 ). Rainwater samples were taken from discharges of three representative surfaces, two with grease and oil contribution (administrative building roof and maneuvering yard) and the other free of them (building roofs).

Determination of volumes to be treated and sizing of water storage tanks

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

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 .

Monthly rainfall and rainwater volumes for a confidence interval with a significance level of 95%, n = 30 years

a Roof of the administrative building.

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.

Water consumption of the company.

Physical, chemical and bacteriological characteristics of rainwater are presented as supplementary material (available with the online version of this paper). Concentrations of fecal coliforms in rainwater from roofs were much greater than the permissible values; rainwater from the mechanical workshop presented the greatest concentration. On the maneuvering yard, the fecal coliform contribution was negligible.

Suspended solids for all rainwater samples did not meet the standard values; especially the rainwater from the administrative building roof, which was the most concentrated in suspended solids with 122.0 mg/L. On the other hand, the concentration of suspended solids in rainwater samples from the maneuvering yard was surprisingly low; dust and other dirtiness adhering to the tires of vehicles was expected to contribute more importantly to this parameter.

All rainwater samples did not meet the permissible limits for biological oxygen demand (BOD 5 ). Organic matter in rainwater samples from the roofs might be the result of biological activity. In the case of the administrative building roof, the concentration of grease and oils in rainwater samples was greater than the limit established by the standard. Greater concentration of grease and oils was associated with the leakage of these compounds from the air conditioning equipment. In the case of maneuvering yard rainwater, the presence of organic load and grease and oils might be linked to leakage of organic compounds from vehicle engines. Elimination of fecal coliforms and removal of organic load, suspended solids, and grease and oils from the rainwater of some surfaces is needed to avoid nuisance problems and health risks to workers when rainwater is used in the buildings and facilities of the company.

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, is 925.0 m 3 and the month with the maximum volume to be treated in this scenario is June with 827.3 m 3 . When only rainwater from the maneuvering yard is collected, the storage capacity of the tank is 804.0 m 3 and the month with the 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.

Volumes of rainwater to be treated and storage tank capacity (m 3 )

Note: Capacity of the storage tank for each scenario is written in bold and italic fonts.

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.

Dimensions of the storage tanks for rainwater

Note: The cross-section of the reservoirs is 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.

The end rainwater uses were vehicle washing, floor cleaning and building services such as toilet flushing. Grease and oil traps, anthracite-sand filter and disinfection, represented schematically in Figure 3 , were the systems proposed in this study to meet the maximum permissible limits set by the Mexican Standards ( NOM-001-SEMARNAT-1996 ; NOM-003-SEMARNAT-1997 ) and achieve the treatment level required to remove suspended solids, organic constituents, grease and oils and eliminate fecal coliforms from the rainwater.

Schematic representation of the treatment systems for 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. 2005 ). Fifteen first-flush diverters were selected. ② is the grease and oil traps for the administrative building roof rainwater. Three traps of galvanized steel were selected with 90 L/min capacity each. ③ is an anthracite-sand filter that includes a dual lateral Leopold underdrain. Backwash was set every 4 days. Additionally, a drying bed was designed to dry out the impurities contained in the backwash effluent. Characteristics of the anthracite-sand filter and drying bed are presented as supplementary material (available online). ④ is the chlorination system. Sodium hypochlorite pentahydrate (NaOCl·5H 2 O) was selected as the disinfectant with doses of 4.4 mg/L, ensuring a free residual chlorine concentration of 1.5 mg/L. A dosing pump (Chem-Tech 120 GPD 80 psi, AquaQuality, Acapulco, Mexico) was selected to apply the disinfectant. ⑤ represents the storage tanks for rainwater (WSTs) after disinfection.

Modifications of the current water distribution network and the pumping system were conducted to distribute the treated rainwater into the different buildings and facilities of the company. Details of such modifications are not presented in this paper because it is not within the scope; however, the costs associated with such modifications were considered in estimating the investment and operation costs.

Table 4 presents the investment needed to implement the treatment systems. The cost of first-flush diverters was estimated based on the prices provided by RainHarvest Systems (Cumming, GA, USA). The investment needed for grease and oil traps was determined based on the prices provided by Helvex (Mexico DF, Mexico). The cost of the anthracite-sand filters (ASF), including the drying bed for the backwash effluent, was calculated based on the prices provided by a local company (Aquamex SA de CV, Monterrey, Mexico). The cost of chlorine dosifiers was determined based on the prices provided by AquaQuality (Acapulco, Mexico). The investment linked to WSTs included costs associated with soil excavation, construction of embankments, placement of the LLDPE geomembrane, concrete structures (slabs and columns), and others. Such costs were calculated based on the unit prices included in the costs catalog of the Mexican Chamber of Construction Industry.

Investment and annual operation and maintenance cost for the implementation of the treatment systems for rainwater (US$)

a First-flush diverters.

b Grease and oil traps.

c Anthracite-sand filter.

d Chlorine dosifiers.

e Water storage tanks.

f Pumping and water distribution system.

The pumping systems cost was determined based on the prices provided by a local company (Hidroservicios Ambientales-Sistemas de Bombeo, Monterrey, Mexico). 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 seen, the lowest investment corresponded to the scenario where roofs and maneuvering yard rainwater is collected together, with a total investment of US$ 35,302; meanwhile, the most expensive alternative was when only rainwater from roofs is collected, with an investment of US$ 42,569. The most costly component was the water storage tank, representing 62.5%, 63.0% and 52.7% of the total investment when the roofs and maneuvering yard rainwater are treated separated and when both sources are treated together, respectively. Meanwhile, the anthracite-sand filter (ASF) and the pumping and modifications of the water distribution system (PWDS) varied from 22.4% to 29.4% and from 7.0% to 8.1%, respectively. These results are in accordance with those reported in the literature, where the storage tank represents 50% to 70% of the total cost of the rainwater-harvesting systems ( Li et al. 2010 ).

Annual operation and maintenance (O&M) costs are also shown in Table 4 . Costs associated with chlorination, pumping of treated rainwater to the buildings and facilities and backwash are included. A chlorination cost of approximately US¢ 1.3/m 3 was estimated based on the local price of sodium hypochlorite pentahydrate (AquaQuality, Acapulco, Mexico). The pumping cost of treated rainwater was calculated based on the electricity consumption linked to the pumping and the local electricity tariff, resulting in approximately US$ 0.13/m 3 . The backwash cost was determined regarding the pumping cost of backwash water, the air supply cost and the treatment cost of backwash water, resulting in about US$ 0.31/m 3 . Other maintenance costs were determined based on the unit prices included in the costs catalog of the Mexican Chamber of Construction Industry. As can be seen, the greatest annual O&M costs were those linked to the scenario where roof and maneuvering yard rainwater was collected and treated together; meanwhile, the lowest O&M costs were obtained for the scenario where roof rainwater was collected and treated separately. This result was because backwash costs increased with the size of the anthracite-sand filter. On the other hand, regarding both the investment and the O&M costs, the cheapest alternative corresponded to the scenario where roof and maneuvering yard rainwater was collected and treated together. This scenario was used for conducting the cash flows and metrics.

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 ( Table 5 ). The cost of public network water was estimated based on the tariffs set by the authorities of Mexico City; for 2017 the tariff was integrated from a basic tariff of US$ 301.5 per the first 120 m 3 consumed and an additional tariff of US$ 4.4 per each additional cubic metre consumed. As can be seen, replacing 100% of the public water supply by rainwater in the buildings and facilities of the company will signify a saving of approximately US$ 8,787.2/year. The water savings considered in this study (100%) are much greater than those reported in the literature for individual houses (36%) and multi-story residential buildings (42%) ( Ghisi & de Oliveira 2007 ; Ghisi & Ferreira 2007 ).

Economic benefits expected after implementing the systems for rainwater harvesting

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 2017 ) and Urbina (2010) . 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.

Cash flow for implementing the systems for rainwater harvesting (US$)

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.

This work was supported by the Tecnológico de Monterrey.

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